View source for History of the periodic table

{{short description|Development of the table of chemical elements}}
{{pp-pc|small=yes}}
[[File:Seaborg in lab.jpeg|thumb| The American chemist [[Glenn T. Seaborg]]—after whom the element [[seaborgium]] is named—standing in front of a [[periodic table]], May 19, 1950]]
{{sidebar periodic table|history}}

The [[periodic table]] is an arrangement of the [[chemical element]]s, structured by their [[atomic number]], [[electron configuration]] and recurring [[Chemical property|chemical properties]]. In the basic form, elements are presented in order of increasing atomic number, in the reading sequence. Then, rows and columns are created by starting new rows and inserting blank cells, so that rows ([[Periodic table period|periods]]) and columns ([[Periodic table group|groups]]) show elements with recurring properties (called periodicity). For example, all elements in group (column) 18 are [[noble gases]] that are largely—though not completely—unreactive.

The '''history of the periodic table''' reflects over two centuries of growth in the understanding of the chemical and physical properties of the elements, with major contributions made by [[Antoine-Laurent de Lavoisier]], [[Johann Wolfgang Döbereiner]], [[John Newlands (chemist)|John Newlands]], [[Julius Lothar Meyer]], [[Dmitri Mendeleev]], [[Glenn T. Seaborg]], and others.<ref>[http://www.iupac.org/didac/Didac%20Eng/Didac01/Content/S01.htm IUPAC article on periodic table] {{webarchive|url=https://web.archive.org/web/20080213082719/http://www.iupac.org/didac/Didac%20Eng/Didac01/Content/S01.htm |date=2008-02-13 }}</ref><ref name="NYT-20190827" />

== Early history ==
{{Further|Classical element}}

In the 5th century BCE, [[Leucippus]] and his pupil [[Democritus]] proposed that all matter was composed of small indivisible particles which they called "atoms". Atoms, Democritus believed, are too small to be detected by the senses; they are infinite in numbers and come in infinitely many varieties, and they have existed forever and that these atoms are in constant motion in the void or vacuum. What we perceive as water, fire, plants, or humans are merely combinations of atoms in the void.

Around 330 BCE, the [[Ancient Greek philosophy|Greek philosopher]] [[Aristotle]] proposed that everything is made up of a mixture of one or more ''roots'', an idea originally suggested by the [[Sicily|Sicilian]] philosopher [[Empedocles]]. The four roots, which the Athenian philosopher [[Plato]] called ''elements'', were ''[[earth (classical element)|earth]]'', ''[[water (classical element)|water]]'', ''[[air (classical element)|air]]'' and ''[[fire (classical element)|fire]]''. Similar ideas existed in other ancient traditions, such as [[Indian philosophy]] with five elements: Earth, water, fire, air and aether collectively called 'pañca bhūta'.<ref>{{cite book|title=India through the ages|url=https://archive.org/details/indiathroughages00mada|last=Gopal|first=Madan|year= 1990| page= [https://archive.org/details/indiathroughages00mada/page/79 79]|editor=K.S. Gautam|publisher=Publication Division, Ministry of Information and Broadcasting, Government of India}}</ref> 

Of the [[chemical element]]s shown on the periodic table, nine – [[carbon]], [[sulfur]], [[iron]], [[copper]], [[silver]], [[tin]], [[gold]], [[mercury (element)|mercury]], and [[lead]] – have been known since [[Ancient history|antiquity]], as they are found in their native form and are relatively simple to mine with primitive tools.<ref name=scerri06>Scerri, E. R. (2006). ''The Periodic Table: Its Story ad Its Significance''; New York City, New York; Oxford University Press.</ref> Five more elements were known in the age of alchemy: [[zinc]], [[arsenic]], [[antimony]], and [[bismuth]].{{citation needed|date=June 2024}} [[Platinum]] was known to [[pre-Columbian]] South Americans, but knowledge of it did not reach Europe until the 16th century.{{citation needed|date=June 2024}}

== First classification ==
[[File:Joseph Wright of Derby The Alchemist.jpg|thumb|right|upright|alt=Hennig Brand |[[Hennig Brand]], as shown in ''[[The Alchemist Discovering Phosphorus]]'']]

The history of the periodic table is also a history of the [[Timeline of chemical element discoveries|discovery of the chemical elements]]. 
In 1661, Boyle defined elements as "those primitive and simple Bodies of which the mixt ones are said to be composed, and into which they are ultimately resolved."<ref>{{cite book|url=https://archive.org/stream/scepticalchymis00BoylA#page/16/mode/2up|title=The Skeptical Chymist|last1=Boyle|first1=Robert|date=1661|publisher=J. Crooke|location=London, England|page=16}}</ref>

The first person in recorded history to discover a new element was [[Hennig Brand]], a [[Bankruptcy|bankrupt]] [[Germans|German]] merchant. Brand tried to discover the [[philosopher's stone]]—a mythical object that was supposed to turn inexpensive base [[metal]]s into gold. In 1669, or later, his experiments with [[Distillation|distilled]] human [[urine]] resulted in the production of a glowing white substance, which he called "cold fire" (''kaltes Feuer'').<ref>{{cite book|url=https://archive.org/stream/discoveryoftheel002045mbp#page/n135/mode/2up|title=Discovery of the Elements|last1=Weeks|first1=Mary|date=1956|publisher=Journal of Chemical Education|edition=6th|location=Easton, Pennsylvania, USA|page=122}}</ref> He kept his discovery secret until 1680, when [[Anglo-Irish]]<ref>{{cite encyclopedia|url=https://www.britannica.com/EBchecked/topic/76496/Robert-Boyle|title=Robert Boyle|encyclopedia=Encyclopædia Britannica|access-date=24 February 2016}}</ref> chemist [[Robert Boyle]] rediscovered [[phosphorus]] and published his findings.

The discovery of phosphorus helped to raise the question of what it meant for a substance (any given variety of [[matter]]) to be an [[chemical element|element]], in a world where versions of [[atomic theory]] were only [[wikt:speculative#Adjective|speculative]] and [[chemical substance|later understandings of the nature of substances]] were only beginning to become possible.

In 1718, [[Étienne François Geoffroy]]'s Affinity Table made use of several aspects — (1) tabular grouping and (2) correlation with [[chemical affinity]] — that would later be reprised. 

[[File:Affinity-table.jpg|300px|right|thumb|[[Étienne François Geoffroy|Geoffroy's]] 1718 Affinity Table: at the head of each column is a [[chemical species]] with which all the species below can combine. Some historians have defined this table as being the start of the [[chemical revolution]].<ref name="Kim-2003">{{Cite book| publisher = MIT Press| isbn = 978-0-262-11273-4| last = Kim| first = Mi Gyung| title = Affinity, That Elusive Dream: A Genealogy of the Chemical Revolution| year = 2003}}</ref>]]

In 1789, French chemist [[Antoine Lavoisier]] wrote ''[[Traité Élémentaire de Chimie]]'' (''Elementary Treatise of Chemistry''), which is considered to be the first modern [[textbook]] about [[chemistry]]. Lavoisier defined an element as a substance whose smallest units cannot be broken down into a simpler substance.<ref>Lavoisier with Robert Kerr, trans. (1790) ''Elements of Chemistry''. Edinburgh, Scotland: William Creech. [https://babel.hathitrust.org/cgi/pt?id=gri.ark:/13960/t5v735k9h;view=1up;seq=30 From p. xxiv:] "I shall therefore only add upon this subject, that if, by the term ''elements'', we mean to express those simple and indivisible atoms of which matter is composed, it is extremely probable we know nothing at all about them; but, if we apply the term ''elements'', or ''principles of bodies'', to express our idea of the last point which analysis is capable of reaching, we must admit, as elements, all substances into which we are capable, by any means, to reduce bodies by decomposition. Not that we are entitled to affirm, that these substances we consider as simple may not be compounded of two, or even of a greater number of principles; but, since these principles cannot be separated, or rather since we have not hitherto discovered means of separating them, they act with regard to us as simple substances, and we ought never to suppose them compounded until experiment and observation has proved them to be so."</ref> Lavoisier's book contained a list of "simple substances" that Lavoisier believed could not be broken down further, which included [[oxygen]], [[nitrogen]], [[hydrogen]], [[phosphorus]], [[Mercury (element)|mercury]], [[zinc]] and [[sulfur]], which formed the basis for the modern list of elements. Lavoisier's list also included "[[light]]" and "[[Caloric theory|caloric]]", which at the time were believed to be material substances. He classified these substances into metals and nonmetals. While many leading [[chemist]]s refused to believe Lavoisier's new revelations, the ''Elementary Treatise'' was written well enough to convince the younger generation. However, Lavoisier's descriptions of his elements lack completeness, as he only classified them as metals and non-metals.

[[File:Dalton's symbols of the elements. 1806 Wellcome M0004592.jpg|right|thumb|upright|[[John Dalton|Dalton]]'s 1806 list of known elements by atomic weight]]

In 1808–10, British natural philosopher [[John Dalton]] published a method by which to arrive at provisional [[standard atomic weight|atomic weights]] for the elements known in his day, from [[stoichiometry|stoichiometric]] measurements and reasonable inferences. Dalton's [[atomic theory]] was adopted by many chemists during the 1810s and 1820s.

In 1815, British physician and chemist [[William Prout]] noticed that atomic weights seemed to be multiples of that of hydrogen.<ref>{{cite journal|last1=Prout|first1=William|date=November 1815|title=On the relation between the specific gravities of bodies in their gaseous state and the weights of their atoms|url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433062744812;view=1up;seq=351|journal=Annals of Philosophy|volume=6|pages=321–330}}</ref><ref>{{cite journal|last1=Prout|first1=William|date=February 1816|title=Correction of a mistake in the essay on the relation between the specific gravities of bodies in their gaseous state and the weights of their atoms|url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433062744820;view=1up;seq=135|journal=Annals of Philosophy|volume=7|pages=111–113}}</ref>

In 1817, German physicist [[Johann Wolfgang Döbereiner]] began to formulate one of the earliest attempts to classify the elements.<ref>{{cite journal|last1=Wurzer|first1=Ferdinand|title=Auszug eines Briefes vom Hofrath Wurzer, Prof. der Chemie zu Marburg|journal=Annalen der Physik|date=1817|volume=56|issue=7|pages=331–334|url=https://babel.hathitrust.org/cgi/pt?id=chi.096071138;view=1up;seq=351|trans-title=Excerpt of a letter from Court Advisor Wurzer, Professor of Chemistry at Marburg|language=de|doi=10.1002/andp.18170560709|bibcode=1817AnP....56..331.|url-access=subscription}} Here, Döbereiner found that strontium's properties were intermediate to those of calcium and barium.</ref> In 1829, he found that he could form some of the elements into groups of three, with the members of each group having related properties. He termed these groups ''[[Döbereiner's triads|triads]]''.<ref>{{cite journal|last1=Döbereiner|first1=J. W.|title=Versuch zu einer Gruppirung der elementaren Stoffe nach ihrer Analogie|journal=Annalen der Physik und Chemie|date=1829|volume=15|issue=2|pages=301–307|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015065410634;view=1up;seq=315|series=2nd series|trans-title=An attempt to group elementary substances according to their analogies|language=de|bibcode=1829AnP....91..301D|doi=10.1002/andp.18290910217}} For an English translation of this article, see: [http://web.lemoyne.edu/~giunta/dobereiner.html Johann Wolfgang Döbereiner: "An Attempt to Group Elementary Substances according to Their Analogies" (Lemoyne College (Syracuse, New York, USA))]</ref> In 1843, building on work done by Döbereiner, [[Leopold Gmelin]] developed a forerunner of the modern periodic table that listed 55 chemical elements grouped by common characteristics.<ref>[https://www.rsc.org/news-events/features/2019/jan/finding-the-periodic-table/ Finding the periodic table], [[Royal Society of Chemistry]]. Accessed April 24, 2025. "In 1843, Leopold Gmelin worked with the Dobereiner’s system and developed a table of 55 elements that contained many of the similar relationships later found in the modern periodic table."</ref>

'''Definition of Triad law'''

"Chemically analogous elements arranged in increasing order of their atomic weights formed well marked groups of three called Triads in which the atomic weight of the middle element was found to be generally the arithmetic mean of the atomic weight of the other two elements in the triad.

#[[chlorine]], [[bromine]], and [[iodine]]
#[[calcium]], [[strontium]], and [[barium]]
#[[sulfur]], [[selenium]], and [[tellurium]]
#[[lithium]], [[sodium]], and [[potassium]]

All those attempts to sort elements by atomic weights were inhibited by the inaccurate determination of weights, and not just slightly: carbon, oxygen and many other elements were believed to be half their actual masses (cf. the illustration by Dalton above), because only [[monatomic gas]]es were believed to exist.<ref>{{cite web |title=Assembling the Modern Periodic Table |url=https://inchemistry.acs.org/atomic-news/assembling-the-periodic-table.html |date=2019 |first=Julianna |last=Poole-Sawyer |publisher=American Chemical Society |access-date=2023-01-21}}</ref> Even though [[Amedeo Avogadro]] and, independently of him, [[André-Marie Ampère]], proposed the solution in the form of [[diatomic molecule]]s and [[Avogadro's law]] already in the 1810s, it was not until after [[Stanislao Cannizzaro]]'s publications in late 1850s when the theory began to be widely considered.

In 1860, the modern [[scientific consensus]] emerged at the first international chemical conference, the [[Karlsruhe Congress]], and a revised list of elements and atomic masses was adopted. It helped spur creation of more extensive systems. The first such system emerged in two years.<ref name=":0">{{Cite web|url=http://www.rsc.org/periodic-table/history/about|title=Development of the periodic table|website=www.rsc.org|access-date=2019-07-12}}</ref>

== Comprehensive formalizations ==

{{blockquote|Properties of the elements, and thus properties of light and heavy bodies formed by them, are in a periodic dependence on their atomic weight.| source = Russian chemist Dmitri Mendeleev, formulating the periodic law for the first time in his 1871 article "Periodic regularity of the chemical elements"{{sfn|Mendeleev|1871|p=111}} }}

French geologist [[Alexandre-Émile Béguyer de Chancourtois]] noticed that the elements, when ordered by their atomic weights, displayed similar properties at regular intervals. In 1862, he devised a three-dimensional chart, named the "telluric helix", after the element [[tellurium]], which fell near the center of his diagram.<ref>{{cite journal|last1=Béguyer de Chancourtois|title=Tableau du classement naturel des corps simples, dit vis tellurique |journal=Comptes rendus de l'Académie des Sciences|date=1862|volume=55 |pages=600–601 |url=https://archive.org/stream/ComptesRendusAcademieDesSciences0055/ComptesRendusAcadmieDesSciences-Tome055-Juillet-dcembre1862#page/n597/mode/2up |trans-title=Table of the natural classification of elements, called the "telluric helix"|language=fr}}</ref><ref name=ley196610>{{Cite magazine|last=Ley|first=Willy |date=October 1966|title=The Delayed Discovery|department=For Your Information|magazine=Galaxy Science Fiction|pages=116–127 |url=https://archive.org/stream/Galaxy_v25n01_1966-10#page/n115/mode/2up}}</ref> With the elements arranged in a spiral on a cylinder by order of increasing atomic weight, de Chancourtois saw that elements with similar properties lined up vertically. The original paper from Chancourtois in ''[[Comptes rendus de l'Académie des Sciences]]'' did not include a chart and used geological rather than chemical terms. In 1863, he extended his work by including a chart and adding [[ion]]s and [[Chemical compound|compounds]].<ref>{{cite book|last1=Chancourtois|first1=Alexandre-Émile Béguyer de |title=Vis tellurique. Classement des corps simples ou radicaux, obtenu au moyen d'un système de classification hélicoïdal et numérique |date=1863 |publisher=Mallet-Bachelier |location=Paris, France |language=fr}} 21 pages.</ref>

The next attempt was made in 1864. British chemist [[John Newlands (chemist)|John Newlands]] presented in ''Chemical News''<ref name=EB1911>{{cite EB1911 |wstitle=Newlands, John Alexander Reina |volume=19 |page=515}}</ref> a classification of the 62 known elements. Newlands noticed recurring trends in physical properties of the elements at recurring intervals of multiples of eight in order of mass number;<ref name="ChemReview">John Newlands, ''Chemistry Review'', November 2003, pp. 15-16.{{full citation needed|date=November 2020}}</ref> based on this observation, he produced a classification of these elements into eight groups. Each group displayed a similar progression; Newlands likened these progressions to the progression of notes within a musical scale.<ref name=ley196610/><ref>See:
*{{cite journal|last1=Newlands|first1=John A. R.|title=On relations among the equivalents|journal=The Chemical News|date=7 February 1863|volume=7|pages=70–72|url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433062748920;view=1up;seq=78}}
*{{cite journal|last1=Newlands|first1=John A. R.|title=Relations between equivalents|journal=The Chemical News|date=30 July 1864|volume=10|pages=59–60|url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433062749290;view=1up;seq=73}}
*{{cite journal|last1=Newlands|first1=John A. R.|title=On relations among the equivalents|journal=The Chemical News|date=20 August 1864|volume=10|pages=94–95|url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433062749290;view=1up;seq=108}}
*{{cite journal|last1=Newlands|first1=John A. R.|title=On the law of octaves|journal=The Chemical News|date=18 August 1865|volume=12|page=83|url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433062749274;view=1up;seq=97}}
*{{cite journal|last1=(Editorial staff)|title=Proceedings of Societies: Chemical Society: Thursday, March 1.|journal=The Chemical News|date=9 March 1866|volume=13|pages=113–114|url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433062749266;view=1up;seq=121}}
*{{cite book|last1=Newlands|first1=John A.R.|title=On the Discovery of the Periodic Law and on Relations among the Atomic Weights|date=1884|publisher=London, England|location=E. & F.N. Spon |url=https://archive.org/stream/ondiscoveryperi02newlgoog#page/n4/mode/2up}}</ref><ref>in a letter published in ''Chemistry News'' in February 1863, according to the [http://www.nndb.com/people/480/000103171/ Notable Names Data Base]</ref><ref>{{Cite web|url=http://web.lemoyne.edu/~giunta/EA/NEWLANDSann.HTML|title=An Unsystematic Foreshadowing: J. A. R. Newlands|website=web.lemoyne.edu|access-date=2019-07-13}}
</ref> Newlands's table left no gaps for possible future elements, and in some cases had two elements at the same position in the same octave. Newlands's table was ignored or ridiculed by some of his contemporaries.<ref name=EB1911/> The [[Chemical Society]] refused to publish his work. The president of the Society, [[William Odling]], defended the Society's decision by saying that such "theoretical" topics might be controversial;<ref name="Shaviv">{{cite book|last1=Shaviv|first1=Giora |title=The Synthesis of the Elements|date=2012|publisher=Springer-Verlag |location=Berlin, Germany|page=38|url=https://books.google.com/books?id=sXdQuJddBpYC&pg=PA38 |isbn=9783642283857}} From p. 38: "The reason [for rejecting Newlands's paper, which was] given by Odling, then the president of the Chemical Society, was that they made a rule not to publish theoretical papers, and this on the quite astonishing grounds that ''such papers lead to a correspondence of controversial character''."</ref> there was even harsher opposition from within the Society, suggesting the elements could have been just as well listed alphabetically.<ref name=":0" /> Later that year, Odling suggested a table of his own<ref>See:

* {{cite journal|last1=Odling|first1=William|date=June 1857|title=On the natural groupings of the elements. Part 1|url=https://babel.hathitrust.org/cgi/pt?id=pst.000068485702;view=1up;seq=421|journal=Philosophical Magazine|series=4th series|volume=13|issue=88|pages=423–440|doi=10.1080/14786445708642323|url-access=subscription}}
* {{cite journal|last1=Odling|first1=William|date=1857|title=On the natural groupings of the elements. Part 2|url=https://babel.hathitrust.org/cgi/pt?id=pst.000068485702;view=1up;seq=478|journal=Philosophical Magazine|series=4th series|volume=13|issue=89|pages=480–497|doi=10.1080/14786445708642334|url-access=subscription}}
* {{cite journal|last1=Odling|first1=William|date=1864|title=On the hexatomicity of ferricum and aluminium|url=https://babel.hathitrust.org/cgi/pt?id=pst.000068485924;view=1up;seq=129|journal=Philosophical Magazine|series=4th series|volume=27|issue=180|pages=115–119|doi=10.1080/14786446408643634|url-access=subscription}}
* {{cite journal|last1=Odling|first1=William|date=1864|title=On the proportional numbers of the elements|url=https://www.biodiversitylibrary.org/page/15275808#page/674/mode/1up|journal=Quarterly Journal of Science|volume=1|pages=642–648}}</ref> but failed to get recognition following his role in opposing Newlands's table.<ref name="Shaviv" />

German chemist [[Lothar Meyer]] also noted the sequences of similar chemical and physical properties repeated at periodic intervals. According to him, if the atomic weights were plotted as ordinates (i.e. vertically) and the atomic volumes as abscissas (i.e. horizontally)—the curve obtained  is a series of maximums and minimums—the most [[electropositive]] elements would appear at the peaks of the curve in the order of their atomic weights. In 1864, a book of his was published; it contained an early version of the periodic table containing 28 elements, and classified elements into six families by their [[valence (chemistry)|valence]]—for the first time, elements had been grouped according to their valence. Works on organizing the elements by atomic weight had until then been stymied by inaccurate measurements of the atomic weights.<ref name="Meyer table" /> In 1868, he revised his table, but this revision was published as a draft only after his death.<ref name=scerri06/> In a paper dated December 1869 which appeared early in 1870, Meyer published a new periodic table of 55 elements, in which the series of periods are ended by an element of the alkaline earth metal group. The paper also included a line chart of relative atomic volumes, which illustrated periodic relationships of physical characteristics of the elements, and which assisted Meyer in deciding where elements should appear in his periodic table. By this time he had already seen the publication of Mendeleev's first periodic table, but his work appears to have been largely independent.{{Citation needed|date=September 2021}}

In 1869, Russian chemist [[Dmitri Mendeleev]] arranged 63 elements by increasing [[standard atomic weight|atomic weight]] in several columns, noting recurring chemical properties across them. It is sometimes said that he played "chemical solitaire" on long train journeys,<ref>''Physical Science'', Holt Rinehart & Winston (January 2004), page 302 {{ISBN|0-03-073168-2}}</ref> using cards containing the symbols, atomic weights, and chemical properties of the known elements.<ref>{{cite journal |title=Mendeleev's matrix |first=Martin |last=Kemp |journal=Nature |volume=393 |page=527 |date=1998 |issue=6685 |doi=10.1038/31124|bibcode=1998Natur.393..527K }}</ref> Another possibility is that he was inspired in part by the periodicity of the [[Sanskrit]] alphabet, which was pointed out to him by his friend and linguist [[Otto von Böhtlingk]].<ref name=":2">{{Cite journal|last1=Ghosh|first1=Abhik|last2=Kiparsky|first2=Paul|date=2019|title=The Grammar of the Elements|url=https://www.americanscientist.org/article/the-grammar-of-the-elements|journal=American Scientist|volume=107|issue=6|pages=350|doi=10.1511/2019.107.6.350|s2cid=209975833|issn=0003-0996|url-access=subscription|doi-access=free}}</ref> Mendeleev used the trends he saw to suggest that atomic weights of some elements were incorrect, and accordingly changed their placements: for instance, he figured there was no place for a [[trivalent]] beryllium with the atomic weight of 14 in his work, and he cut both the atomic weight and valency of beryllium by a third, suggesting it was a divalent element with the atomic weight of 9.4. Mendeleev widely distributed printed broadsheets of the table to various chemists in Russia and abroad.<ref>{{cite journal|last=Mendeleev|first=Dmitri|title=Versuche eines Systems der Elemente nach ihren Atomgewichten und chemischen Functionen |trans-title= System of Elements according to their Atomic Weights and Chemical Functions |journal=Journal für Praktische Chemie|year=1869|volume=106|pages=251|url=https://books.google.com/books?id=qZBTAAAAcAAJ}}</ref><ref>{{cite journal|last1=Менделеев|first1=Д.|title=Соотношение свойств с атомным весом элементов|journal=Журнал Русского Химического Общества (Journal of the Russian Chemical Society)|date=1869|volume=1|pages=60–77|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015065536586;view=1up;seq=70|trans-title=Relationship of properties of the elements to their atomic weights|language=ru}}</ref><ref>{{cite journal|last=Mendeleev|first=Dmitri|title=Ueber die Beziehungen der Eigenschaften zu den Atomgewichten der Elemente|trans-title=On the relations of properties of the elements to their atomic weights|journal=Zeitschrift für Chemie|year=1869|volume=12|pages=405–406|url=https://archive.org/stream/zeitschriftfrch12unkngoog#page/n414/mode/2up}}</ref> Mendeleev argued in 1869 there were seven types of highest oxides.{{Sfn|Petrov|1981|p=65}}{{Efn|They were R<sub>2</sub>O, R<sub>2</sub>O<sub>2</sub>, R<sub>2</sub>O<sub>3</sub>, R<sub>2</sub>O<sub>4</sub>, R<sub>2</sub>O<sub>5</sub>, R<sub>2</sub>O<sub>6</sub>, and R<sub>2</sub>O<sub>7</sub>. The list was later appended with R<sub>2</sub>O<sub>8</sub>.|name=|group=}} Mendeleev continued to improve his ordering; in 1870, it gained a tabular shape, and each column was given its own highest oxide,{{Sfn|Mendeleev|1870|p=76}} and in 1871, he further developed it and formulated what he termed the "law of periodicity".{{Sfn|Mendeleev|1871|p=111}} Some changes also occurred with new revisions, with some elements changing positions.

<gallery class="center" caption="Various attempts to construct a comprehensive formalization" widths="200">
File:Periodic table Meyer 1864.png|Meyer's periodic table, published in "Die modernen Theorien der Chemie", 1864<ref name = "Meyer table">Meyer, Julius Lothar; Die modernen Theorien der Chemie (1864); [https://web.archive.org/web/20190102050414/https://reader.digitale-sammlungen.de/de/fs1/object/goToPage/bsb10073411.html?pageNo=147 table on page 137].</ref>
File:Newlands periodiska system 1866.png|Newlands's law of octaves, 1866
File:1869-periodic-table.jpg|Mendeleev's first ''Attempt at a system of elements'', 1869
File:Dmitry Mendeleyev Osnovy Khimii 1869-1871 first periodic table.jpg|Mendeleev's ''Natural system of the elements'', 1870
File:Mendelejevs periodiska system 1871.png|Mendeleev's periodic table, 1871
</gallery>

==Priority dispute and recognition==
{{blockquote|That person is rightly regarded as the creator of a particular scientific idea who perceives not merely its philosophical, but its real aspect, and who understands so to illustrate the matter so that everyone can become convinced of its truth. Then alone the idea, like matter, becomes indestructible.| author=Mendeleev in his 1881 article in British journal ''Chemical News'' in a correspondence debate with Meyer over priority of the periodic table invention{{sfn|Scerri|2019|p=147}} |title=|source=}}

=== Mendeleev's predictions and inability to incorporate the rare-earth metals ===
<div style="float:right; margin-left:1em; font-size:85%">
{| class="wikitable"
|+ Mendeleev's predictions{{sfn|Scerri|2019|p=142}}{{efn|Scerri notes that this table "does not include elements such as astatine and actinium, which he [Mendeleev] predicted successfully but did not name. Neither does it include predictions that were represented just by dashes in Mendeleev’s periodic systems. Among some other failures, not included in the table, is an inert gas element between barium and tantalum, which would have been called ekaxenon, although Mendeleev did not refer to it as such."{{sfn|Scerri|2019|p=143}} }}
! rowspan=2|Name
! colspan=2|Atomic weight
! rowspan=2|Modern name<br/>(year of discovery)
|-
!Mendeleev
!Modern
|-
|Ether || 0.17 || — || —
|-
|Coronium || 0.4 || — || —
|-
|Eka-boron || 44 || 44.6 || Scandium
|-
|Eka-cerium || 54 || — || —
|-
|Eka-aluminum || 68 || 69.2 || Gallium
|-
|Eka-silicon || 72 || 72.0 || Germanium
|-
|Eka-manganese || 100 || 99 || Technetium (1937)
|-
|Eka-niobium || 142 || — || —
|-
|Eka-molybdenum || 146 || — || —
|-
|Eka-cadmium || 155 || — || —
|-
|Eka-iodine || 170 || — || —
|-
|Tri-manganese || 190 || 186 || Rhenium (1925)
|-
|Eka-caesium || 175 || — || —
|-
|Dvi-tellurium || 212 || 210 || Polonium (1898)
|-
|Dvi-caesium || 220 || 223 || Francium (1939)
|-
|Eka-tantalum || 235 || 231 || Protactinium (1913)
|}
</div>

Even as Mendeleev corrected positions of some elements, he thought that some relationships that he could find in his grand scheme of periodicity could not be found because some elements were still undiscovered, and that the properties of such undiscovered elements could be deduced from their expected relationships with other elements. In 1870, he first tried to characterize the yet undiscovered elements, and he gave detailed [[Mendeleev's predicted elements|predictions]] for three elements, which he termed ''eka-boron'', ''eka-aluminium'', and ''eka-silicium'';{{sfn|Mendeleev|1870|pp=90–98}} he also more briefly noted a few other expectations.{{sfn|Mendeleev|1870|pp=98–101}} It has been proposed that the prefixes ''eka'', ''dvi'', and ''tri'', Sanskrit for one, two, and three, respectively, are a tribute to [[Pāṇini]] and other ancient [[Sanskrit grammar]]ians for their invention of a periodic alphabet.<ref name=":2" /> In 1871, Mendeleev expanded his predictions further.

Compared to the rest of the work, Mendeleev's 1869 list misplaces seven then known elements: [[indium]], [[thorium]], and five [[rare-earth element|rare-earth metals]]: [[yttrium]], [[cerium]], [[lanthanum]], [[erbium]], and [[didymium]]. The last two were later found to be mixtures of two different elements; ignoring those would allow him to restore the logic of increasing atomic weight. These elements (all thought to be divalent at the time) puzzled Mendeleev as they did not show a regular increase in valency despite their seemingly consequential atomic weights.{{sfn|Thyssen|Binnemans|2015|p=159}} Mendeleev grouped them together, thinking of them as of a particular kind of series.{{Efn|He noted similarity despite sequential atomic weights; he termed such sequences as primary groups (as opposed to regular secondary groups, such as the halogens and the alkali metals). Other primary groups were rhodium, ruthenium, and palladium; and iridium, osmium, and platinum.}} In early 1870, he decided that the weights for these elements must be wrong and that the rare-earth metals should be trivalent (which accordingly increased their predicted atomic weights by half). He measured the [[heat capacity]] of indium, uranium, and cerium to demonstrate their higher assumed valency (which was soon confirmed by Prussian chemist [[Robert Bunsen]]).{{sfn|Thyssen|Binnemans|2015|pp=174–175}} Mendeleev treated the change by assessing each element to an individual place in his system of the elements rather than continuing to treat them as a series.

Mendeleev noticed that there was a significant difference in atomic mass between cerium and [[tantalum]] with no element between them; his consideration was that between them, there was a row of yet undiscovered elements, which would display similar properties to those elements which were to be found above and below them: for instance, an eka-molybdenum would behave as a heavier homolog of molybdenum and a lighter homolog of wolfram (the name under which Mendeleev knew [[tungsten]]).<ref name="scimag19">{{cite journal|last1=Cheisson|first1=T.|last2=Schelter|first2=E. J.|date=2019|title=Rare earth elements: Mendeleev's bane, modern marvels|journal=Science|volume=363|issue=6426|pages=489–493|doi=10.1126/science.aau7628|pmid=30705185|bibcode=2019Sci...363..489C|s2cid=59564667|doi-access=free}}</ref> This row would begin with a trivalent lanthanum, a tetravalent cerium, and a pentavalent didymium. However, the higher valency for didymium had not been established, and Mendeleev tried to do so himself.{{sfn|Thyssen|Binnemans|2015|p=177}} Having had no success in that, he abandoned his attempts to incorporate the rare-earth metals in late 1871 and embarked on his grand idea of [[luminiferous ether]]. His idea was carried on by Austrian-Hungarian chemist [[Bohuslav Brauner]], who sought to find a place in the periodic table for the rare-earth metals;{{sfn|Thyssen|Binnemans|2015|pp=179–181}} Mendeleev later referred to him as to "one of the true consolidators of the periodic law".{{Efn|Mendeleev referred to Brauner in this manner after Brauner measured the atomic weight of tellurium and obtained the value 125. Mendeleev had thought that due to the properties tellurium and iodine display, the latter should be the heavier one while the contemporary data pointed otherwise (tellurium was assessed with the value of 128, and iodine 127). Later measurements by Brauner himself, however, showed the correctness of the original measurement; Mendeleev doubted it for the rest of his life.{{sfn|Scerri|2019|pp=130-131}}}}

In addition to the predictions of scandium, gallium, and germanium that were quickly realized, Mendeleev's 1871 table left many more spaces for undiscovered elements, though he did not provide detailed predictions of their properties. In total, he predicted eighteen elements, though only half corresponded to elements that were later discovered.<ref name="pastfuture" />

=== Priority of discovery ===
None of the proposals were accepted immediately, and many contemporary chemists found it too abstract to have any meaningful value. Of those chemists that proposed their categorizations, Mendeleev strove to back his work and promote his vision of periodicity, Meyer did not promote his work very actively, and Newlands did not make a single attempt to gain recognition abroad.{{citation needed|date=April 2020}}

Both Mendeleev and Meyer created their respective tables for their pedagogical needs; the difference between their tables is well explained by the fact that the two chemists sought to use a formalized system to solve different problems.{{sfn|Gordin|2012|pp=75–76}} Mendeleev's intent was to aid composition of his textbook, ''Foundations of Chemistry'', whereas Meyer was rather concerned with presentation of theories.{{sfn|Gordin|2012|pp=75–76}} Mendeleev's predictions emerged outside of the pedagogical scope in the realm of journal science,{{sfn|Gordin|2012|p=76}} while Meyer made no predictions at all and explicitly stated his table and his textbook it was contained in, ''Modern Theories'', should not be used for prediction in order to make the point to his students to not make too many purely theoretically constructed projections.{{sfn|Gordin|2012|pp=71–74}}

Mendeleev and Meyer differed in temperament, at least when it came to promotion of their respective works. Boldness of Mendeleev's predictions was noted by some contemporary chemists, however skeptical they may have been.{{sfn|Gordin|2012|p=75}} Meyer referred to Mendeleev's "boldness" in an edition of ''Modern Theories'', whereas Mendeleev mocked Meyer's indecisiveness to predict in an edition of ''Foundations of Chemistry''.{{sfn|Gordin|2012|p=75}}

=== Recognition of Mendeleev's table ===
Eventually, the periodic table was appreciated for its descriptive power and for finally systematizing the relationship between the elements,<ref name=":1">{{Cite journal|last=Scerri|first=Eric R.|date=1998|title=The Evolution of the Periodic System|journal=Scientific American|volume=279|issue=3|pages=78–83|doi=10.1038/scientificamerican0998-78|jstor=26057945|bibcode=1998SciAm.279c..78S|issn=0036-8733}}</ref> although such appreciation was not universal.{{Sfn|Scerri|2019|pp=170–172}} In 1881, Mendeleev and Meyer had an argument via an exchange of articles in British journal ''Chemical News'' over priority of the periodic table, which included an article from Mendeleev, one from Meyer, one of critique of the notion of periodicity, and many more.{{sfn|Scerri|2019|pp=147–149}} In 1882, the [[Royal Society]] in London awarded the [[Davy Medal]] to both Mendeleev and Meyer for their work to classify the elements; although two of Mendeleev's predicted elements had been discovered by then, Mendeleev's predictions were not at all mentioned in the prize rationale.
[[File:St Andrews Periodic Table.jpg|left|thumb|The oldest surviving periodic table for educational purpose, between 1879 and 1886. Wardlaw Museum, [[the University of St Andrews]]. It includes gallium and scandium, but not germanium. ]]
Mendeleev's ''eka-aluminium'' was discovered in 1875 and became known as [[gallium]]; ''eka-boron'' and ''eka-silicium'' were discovered in 1879 and 1886, respectively, and were named [[scandium]] and [[germanium]].<ref name="ley196610"/> Mendeleev was even able to correct some initial measurements with his predictions, including the first prediction of gallium, which matched ''eka-aluminium'' fairly closely but had a different density. Mendeleev advised the discoverer, French chemist [[Paul-Émile Lecoq de Boisbaudran]], to measure the density again; de Boisbaudran was initially skeptical (not least because he thought Mendeleev was trying to take credit from him) but eventually admitted the correctness of the prediction. Mendeleev contacted all three discoverers; all three noted the close similarity of their discovered elements with Mendeleev's predictions, with the last of them, German chemist [[Clemens Winkler]], admitting this suggestion was not first made by Mendeleev or himself after the correspondence with him, but by a different person, German chemist [[Hieronymous Theodor Richter]].{{efn|Notably, Mendeleev did not immediately identify germanium as ''eka-silicium''. Winkler explained, "The present case, however, shows quite clearly how deceptive it can be to use analogies, because the tetradic value of germanium has meanwhile become an irrefutable fact, and there can be no doubt that the new element is nothing other than "eka-silicium" predicted by Mendeleev fifteen years ago. This identification comes from the short and still very imperfect characteristic of germanium that I gave at the beginning and was first decisively pronounced by V. v. Richter. Almost at the same time, Mendeleev, the deserving creator of the periodic system, commented that although several of the properties of germanium I mentioned reminded of those of eka-silicium, the observed liquidity of the element indicated the possibility of placing it elsewhere in the periodic system. Lothar Meyer declared the germanium to be eka-silicium from the beginning, adding that according to the atomic volume curve produced by it, contrary to Mendeleev's assumption, it had to be easily meltable and probably also easy to vaporize. At that time the germanium had not yet been presented in the reguline state; it is all the more remarkable that, as will be shown below, Lothar Meyer's condition has, to some extent, really come true."<ref>{{cite journal|author=Winkler|first=C.|year=1887|title=Mittheilungen über das Germanium|url=https://zenodo.org/record/1427916|journal=Journal für Praktische Chemie|language=de|volume=36|issue=1|pages=182–183|doi=10.1002/prac.18870360119}}</ref>}} Some contemporary chemists were not convinced by these discoveries, noting the dissimilarities between the new elements and the predictions or claiming those similarities that did exist were coincidental.{{Sfn|Scerri|2019|pp=170–172}} However, success of Mendeleev's predictions helped spread the word about his periodic table.{{Sfn|Scerri|2019|p=156}} Later, chemists used the successes of these Mendeleev's predictions to justify his table.<ref name=":0" />

By 1890, Mendeleev's periodic table had been universally recognized as a piece of basic chemical knowledge.{{Sfn|Scerri|2019|p=157}} Apart from his own correct predictions, a number of aspects may have contributed to this, such as the correct accommodation of many elements whose atomic weights were thought to have wrong values but were later corrected.{{Sfn|Scerri|2019|p=156}} The debate on the position of the rare-earth metals helped spur the discussion about the table as well.{{Sfn|Scerri|2019|p=156}}{{Efn|Meyer's tables, in contrast, did not at all attempt to incorporate those elements.{{citation needed|date=April 2020}}}} In 1889, Mendeleev noted at the Faraday Lecture to the [[Royal Institution]] in London that he had not expected to live long enough "to mention their discovery to the Chemical Society of Great Britain as a confirmation of the exactitude and generality of the periodic law".<ref>{{Cite web|last=Rouvray|first=R.|url=https://www.newscientist.com/people/dmitri-mendeleev/|title=Dmitri Mendeleev|website=New Scientist|language=en-US|access-date=2020-04-19}}</ref>

== Inert gases and ether ==
{{blockquote|The great value of Newland's, Mendeleef's, and Lothar Meyer's generalisation, known as the periodic arrangement of the elements, is universally acknowledged. But a study of this arrangement, it must be allowed, is a somewhat tantalising pleasure; for, although the properties of elements do undoubtedly vary qualitatively, and, indeed, show approximate quantitative relations to their position in the periodic table, yet there are inexplicable deviations from regularity, which hold forth hopes of the discovery of a still more far-reaching generalisation. What that generalisation may be is not yet to be divined; but that it must underlie what is known, and must furnish a clue to the explanation of irregularities, cannot be disputed. | source = British chemists [[William Ramsay]] and [[Morris Travers]] in 1900 discussion of their research of new inert gases<ref>{{cite journal|date=1901|last1=Ramsay|first1=W.|author-link=William Ramsay|last2=Travers|first2=M.|author-link2=Morris Travers|title=Argon and its companions|journal=Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character|language=en|volume=197|issue=287–299|pages=47–89|doi=10.1098/rsta.1901.0014|bibcode=1901RSPTA.197...47R|issn=0264-3952|doi-access=free}}</ref>}}

=== Inert gases ===
British chemist [[Henry Cavendish]], the discoverer of [[hydrogen]] in 1766, discovered that air is composed of more gases than [[nitrogen]] and [[oxygen]].<ref name="ardisc">{{cite journal |last=Wisniak |first=J. |title=The composition of air: Discovery of argon |date=2007 |doi=10.22201/fq.18708404e.2007.1.65979 |journal=Educación Química |volume=18 |issue=1 |pages=69–84 |s2cid=128942433 |url=https://www.researchgate.net/publication/236233522|doi-access=free }}</ref> He recorded these findings in 1784 and 1785; among them, he found a then-unidentified gas less reactive than nitrogen. [[Helium]] was first reported in 1868; the report was based on the new technique of [[spectroscopy]]; some spectral lines in light emitted by the Sun did not match those of any of the known elements. Mendeleev was not convinced by this finding since variance of temperature led to change of intensity of spectral lines and their location on the spectrum.<ref>{{Cite book|last=Assovskaya|first=A. S.|url=https://collectedpapers.com.ua/ru/helium-on-earth-and-in-the-universe/pershe-stolittya-geliyu|title=Гелий на Земле и во Вселенной|publisher=Nedra|year=1984|location=Leningrad|language=ru|trans-title=Helium on Earth and in the Universe|chapter=Первый век гелия|work=CollectedPapers |trans-chapter=The first century of helium}}</ref> This opinion was held by some other scientists of the day, some of whom believed the spectral lines were due to a particular state of hydrogen existing in the Sun's atmosphere. Others believed the spectral lines could belong to an element that occurred on the Sun but not on Earth; some believed it was yet to be found on Earth.<ref name=He-Kragh>{{cite journal |last=Kragh |first=Helge |date=2008 |title=The Solar Element: A Reconsideration of Helium's Early History |journal=Annals of Science |volume=69 |issue=2 |pages=157–182 |doi=10.1080/00033790902741633 <!--via TWL-->}}</ref>

In 1894, British chemist [[William Ramsay]] and British physicist [[Lord Rayleigh]] isolated [[argon]] from air and determined that it was a new element. Argon, however, did not engage in any chemical reactions and was—highly unusually for a gas—monatomic;{{Efn|The only other monatomic gas known at the time was vaporized mercury.{{sfn|Scerri|2019|p=151}}}} it did not fit into the periodic law and thus challenged the very notion of it. Not all scientists immediately accepted this report; Mendeleev's original response was that argon was a triatomic form of nitrogen rather than an element of its own.<ref>{{Cite journal|last=Lente|first=Gábor|date=2019|title=Where Mendeleev was wrong: predicted elements that have never been found|journal=ChemTexts|language=en|volume=5|issue=3|article-number=17|doi=10.1007/s40828-019-0092-5|bibcode=2019ChTxt...5...17L |s2cid=201644634|issn=2199-3793|doi-access=free}}</ref> While the notion of a possibility of a group between that of halogens and that of alkali metals had existed (some scientists believed that several atomic weight values between halogens and alkali metals were missing, especially since places in this half of group VIII remained vacant),{{Sfn|Petrov|1981|pp=38-44}} argon did not easily match the position between chlorine and potassium because its atomic weight exceeded those of both chlorine and potassium.{{Sfn|Petrov|1981|pp=58-59}} Other explanations were proposed; for example, Ramsay supposed argon could be a mixture of different gases.{{Sfn|Petrov|1981|pp=58-59}} For a while, Ramsay believed argon could be a mixture of three gases of similar atomic weights; this triad would resemble the triad of iron, cobalt, and nickel, and be similarly placed in group VIII.{{Sfn|Petrov|1981|pp=59-61}} Certain that shorter periods contain triads of gases at their ends, Ramsay suggested in 1898 the existence of a gas between helium and argon with an atomic weight of 20; after its discovery later that year (it was named [[neon]]), Ramsay continued to interpret it as a member of a horizontal triad at the end of that period.{{Sfn|Petrov|1981|pp=54-55}}

In 1896, Ramsay tested a report of American chemist [[William Francis Hillebrand]], who found a stream of an unreactive gas from a sample of [[uraninite]]. Wishing to prove it was nitrogen, Ramsay analyzed a different uranium mineral, [[cleveite]], and found a new element, which he named krypton. This finding was corrected by British chemist [[William Crookes]], who matched its spectrum to that of the Sun's helium.<ref>{{Cite book|last=Sears|first=W. M. Jr. |url=https://books.google.com/books?id=5SvABgAAQBAJ&q=william+crookes+%22krypton%22&pg=PA52|title=Helium: The Disappearing Element|date=2015|publisher=Springer|isbn=978-3-319-15123-6|pages=50–52|language=en}}</ref> Following this discovery, Ramsay, using [[fractional distillation]] to separate the components air, discovered several more such gases in 1898:<ref>{{Cite journal |last=Davies |first=Alwyn G. |date=March 2012 |title=Sir William Ramsay and the Noble Gases |journal=Science Progress |language=en |volume=95 |issue=1 |pages=23–49 |doi=10.3184/003685012X13307058213813 |pmid=22574384 |s2cid=12592582 |issn=0036-8504|pmc=10365523 }}</ref> metargon, [[krypton]], neon, and [[xenon]]; detailed spectroscopic analysis of the first of these demonstrated it was argon contaminated by a carbon-based impurity.<ref>{{Cite journal |last1=Ramsay |first1=William |last2=Travers |first2=Morris William |date=1901 |title=II. Argon and its companions |journal=Philosophical Transactions of the Royal Society A |language=en |volume=197 |issue=287–299 |page=48 |doi=10.1098/rsta.1901.0014 |doi-access=free |bibcode=1901RSPTA.197...47R |issn=0264-3952}}</ref> Ramsay was initially skeptical about the existence of gases heavier than argon, and the discovery of krypton and xenon came as a surprise to him; however, Ramsay accepted his own discovery, and the five newly discovered inert gases (now [[noble gases]]) were placed in a single column in the periodic table.{{Sfn|Petrov|1981|pp=63-64}} Although Mendeleev's table predicted several undiscovered elements, it did not predict the existence of such inert gases, and Mendeleev originally rejected those findings as well.<ref name="Stewart07">{{cite journal |last=Stewart |first=P. J. |title=A century on from Dmitrii Mendeleev: Tables and spirals, noble gases, and Nobel prizes |date=2007 |journal=[[Foundations of Chemistry]] |volume=9 |issue=3 |pages=235–245 |doi=10.1007/s10698-007-9038-x|s2cid=97131841 }}</ref>{{Efn|Mendeleev did consider that some atomic weight values could be missing from the set of known values. However, Mendeleev could not have made a prediction of a group of unreactive gases in a fashion similar to the one in which he made his predictions on reactive elements and their chemical properties.{{sfn|Petrov|1981|p=40}}|name=|group=}}

=== Changes to the periodic table ===
Although the sequence of atomic weights suggested that inert gases should be located between halogens and alkali metals, and there were suggestions to put them into group VIII coming from as early as 1895,{{Sfn|Petrov|1981|pp=64-65}} such placement contradicted one of Mendeleev's basic considerations, that of the highest oxides. Inert gases did not form any oxides, and no other compounds at all, and as such, their placement in a group where elements should form tetroxides was seen as merely auxiliary and not natural; Mendeleev doubted inclusion of those elements in group VIII.{{Sfn|Petrov|1981|pp=64-65}} Later developments, particularly by British scientists, focused on correspondence of inert gases with halogens to their left and alkali metals to their right. In 1898, when only helium, argon, and krypton were definitively known, Crookes suggested these elements be placed in a single column between the hydrogen group and the fluorine group.<ref>{{Cite journal|last=Crookes|first=W.|date=1898|title=On the position of helium, argon, and krypton in the scheme of elements|journal=Proceedings of the Royal Society of London|language=en|volume=63|issue=389–400|pages=408–411|doi=10.1098/rspl.1898.0052|s2cid=94778359|issn=0370-1662}}</ref> In 1900, at the [[Prussian Academy of Sciences]], Ramsay and Mendeleev discussed the new inert gases and their location in the periodic table; Ramsay proposed that these elements be put in a new group between halogens and alkali metals, to which Mendeleev agreed.{{sfn|Scerri|2019|p=156}} Ramsay published an article after his discussions with Mendeleev; the tables in it featured halogens to the left of inert gases and alkali metals to the right.{{Sfn|Petrov|1981|pp=64-66}} Two weeks before that discussion, Belgian botanist [[Léo Errera]] had proposed to the [[Royal Academy of Science, Letters and Fine Arts of Belgium]] to put those elements in a new group 0. In 1902, Mendeleev wrote that those elements should be put in a new group 0; he said this idea was consistent with what Ramsay suggested to him and referred to Errera as to the first person to suggest the idea.<ref name="Trifonov">{{Cite news|last=Trifonov|first=D. N.|url=http://www.chem.msu.su/rus/elibrary/trifonov/noble-gases.html|title=Сорок лет химии благородных газов|access-date=2020-04-12|publisher=[[Moscow State University]]|language=ru|trans-title=Forty years of noble gas chemistry}}</ref> Mendeleev himself added these elements to the table as group 0 in 1902, without disturbing the basic concept of the periodic table.<ref name="Trifonov" /><ref name="mendeleevether">{{cite book|last=Mendeleev|first=D.|title=Popytka khimicheskogo ponimaniia mirovogo efira|year=1903|location=St. Petersburg|language=ru|author-link=Dmitri Mendeleev}}<br />An English translation appeared as<br />{{cite book|last=Mendeléeff|first=D.|title=An Attempt Towards A Chemical Conception Of The Ether|publisher=Longmans, Green & Co.|year=1904|translator=G. Kamensky |author-link=Dmitri Mendeleev}}</ref>

In 1905, Swiss chemist [[Alfred Werner]] resolved the dead zone of Mendeleev's table. He determined that the [[rare-earth element]]s ([[lanthanide]]s), 13 of which were known, lay within that gap. Although Mendeleev knew of [[lanthanum]], cerium, and [[erbium]], they were previously unaccounted for in the table because their total number and exact order were not known; Mendeleev still could not fit them in his table by 1901.<ref name=Stewart07/> This was in part a consequence of their similar chemistry and the imprecise determination of their atomic masses. Combined with the lack of a known group of similar elements, this rendered the placement of the lanthanides in the periodic table difficult.<ref name="LnChem">{{cite book|last=Cotton|first=S.|title=Lanthanide and Actinide Chemistry|date=2006|publisher=John Wiley & Sons, Ltd.|isbn=978-0-470-01005-1|pages=1–7|chapter=Introduction to the lanthanides}}</ref> This discovery led to a restructuring of the table and the first appearance of the [[Periodic table#The long- or 32-column table|32-column form]].<ref name="1871table">{{cite journal|last1=Stewart|first1=P.J.|date=2019|title=Mendeleev's predictions: success and failure|journal=Foundations of Chemistry|volume=21|issue=1|pages=3–9|doi=10.1007/s10698-018-9312-0|s2cid=104132201|doi-access=free}}</ref>

=== Ether ===
By 1904, Mendeleev's table rearranged several elements, and included the noble gases along with most other newly discovered elements. It still had the dead zone, and a row zero was added above hydrogen and helium to include [[coronium]] and [[the ether]], which were widely believed to be elements at the time.<ref name="1871table" /> Although the [[Michelson–Morley experiment]] in 1887 cast doubt on the possibility of a [[luminiferous ether]] as a space-filling medium, physicists set constraints for its properties.<ref name="michel2">{{Cite journal |last1=Michelson |first1=Albert A. |last2=Morley |first2=Edward W.|title=On the Relative Motion of the Earth and the Luminiferous Ether |journal=American Journal of Science |volume=34 |issue=203 |year=1887 |pages=333–345 |doi=10.2475/ajs.s3-34.203.333|title-link=s:On the Relative Motion of the Earth and the Luminiferous Ether |bibcode=1887AmJS...34..333M |s2cid=124333204 }}</ref> Mendeleev believed it to be a very light gas, with an atomic weight several orders of magnitude smaller than that of hydrogen. He also postulated that it would rarely interact with other elements, similar to the noble gases of his group zero, and instead permeate substances at a velocity of {{convert|2250|km|mi|sp=us}} per second.{{Efn|The notion of ether was disproven by German physicist [[Albert Einstein]] in 1905 with his [[special theory of relativity]]; the idea that ether did not exist was accepted in the scientific community rather quickly.|name=|group=}}

Mendeleev was not satisfied with the lack of understanding of the nature of this periodicity; this would only be possible through the understanding of the composition of the atom. However, Mendeleev firmly believed that future would only develop the notion rather than challenge it and reaffirmed his belief in writing in 1902.<ref name=unconventional>{{Cite news|last=Trifonov|first=D. N.|url=http://www.chem.msu.su/rus/journals/xr/istor6-9.html|title=Д.И. Менделеев. Нетрадиционный взгляд (II)|access-date=2020-04-12|publisher=[[Moscow State University]]|language=ru|trans-title=D.I. Mendeleev. An unconventional view (II)}}</ref>

<gallery class="center" widths="200" caption="Early developments of Mendeleev's table">
File:Masson main table 1895.png|Main table of the periodic table published by Australian chemist David Orme Masson in 1895
File:Ramsay 1896 fragment.png|Fragment of a periodic table published by Ramsay in 1896
File:Ramsay 1900 fragment.png|Fragment of a periodic table published by Ramsay in 1900
File:Errera 1900.png|Periodic table as published by Errera in 1900
File:Mendeleev 1904 Periodic Table.png|Mendeleev's 1904 table. It includes the noble gases in group 0, and it has two reserved gaps for [[coronium]] and [[Luminiferous aether|ether]].
File:Taula periòdica de Werner (1905).gif|Werner's 32-column 1905 table. This table left spaces for many then-unknown elements, and several elements had their positions revised following advances in atomic theory.
</gallery>

==Atomic theory and isotopes==
<!--[[Image:Henry Moseley.jpg|thumb|upright|[[Henry Moseley]]]]-->=== Radioactivity and isotopes ===
[[File:Moseley step ladder.jpg|upright=0.9|thumb|right|Photographic recording of the characteristic [[X-ray]] emission lines of elements with atomic number between 20 and 29 (From works of Henry Moseley)]]{{Main|Radioactive decay|Isotope}}
In 1907 it was discovered that thorium and radiothorium, products of radioactive decay, were physically different but chemically identical; this led [[Frederick Soddy]] to propose in 1910 that they were the same element but with different atomic weights.<ref>[[Muriel Howorth| Howorth, Muriel]] (1958), The Life of Frederick Soddy (London: New World)</ref> Soddy later proposed to call these elements with complete chemical identity "isotopes".<ref>Soddy, Frederick (1913), 'Intra-Atomic Charge', Nature, 92, 399–400</ref>

The problem of placing isotopes in the periodic table had arisen beginning in 1900 when four radioactive elements were known: [[radium]], [[actinium]], [[thorium]], and [[uranium]]. These radioactive elements (termed "radioelements") were accordingly placed at the bottom of the periodic table, as they were known to have greater atomic weights than stable elements, although their exact order was not known. Researchers believed there were still more radioactive elements yet to be discovered, and during the next decade, the [[decay chain#Actinide alpha decay chains|decay chain]]s of thorium and uranium were extensively studied. Many new radioactive substances were found, including the noble gas [[radon]], and their chemical properties were investigated.<ref name=ley196610/> By 1912, almost 50 different radioactive substances had been found in the decay chains of thorium and uranium. American chemist [[Bertram Boltwood]] proposed several decay chains linking these radioelements between uranium and lead. These were thought at the time to be new chemical elements, substantially increasing the number of known "elements" and leading to speculations that their discoveries would undermine the concept of the periodic table which had long been established to obey the [[octet rule]].<ref name=pastfuture>{{cite journal |last=Scerri |first=E.R. |title=The past and future of the periodic table |journal=American Scientist |volume=96 |issue=1 |pages=52–58 |doi=10.1511/2008.69.52  |url=https://www.researchgate.net/publication/263442539|year=2008 }}</ref> For example, there was not enough room between lead and uranium to accommodate these discoveries, even assuming that some discoveries were duplicates or incorrect identifications. It was also believed that radioactive decay violated one of the central principles of the periodic table, namely that chemical elements could not undergo [[nuclear transmutation|transmutations]] and always had unique identities.<ref name=ley196610/>

Soddy and [[Kazimierz Fajans]], who had been following these developments, published in 1913 that although these substances emitted different radiation,<ref name=isodisc>{{Thoennessen2016|pages=5}}</ref> many of these substances were identical in their chemical characteristics, so shared the same place in the periodic table.<ref>{{cite journal|last1=Soddy|first1=Frederick|title=Radioactivity|journal=Annual Reports on the Progress of Chemistry|date=1913|volume=10|pages=262–288|doi=10.1039/ar9131000262|url=https://zenodo.org/record/2268323}}</ref><ref>{{cite journal|last1=Soddy|first1=Frederick|title=The radio-elements and the periodic law|journal=The Chemical News|date=28 February 1913|volume=107|issue=2779|pages=97–99|url=https://babel.hathitrust.org/cgi/pt?id=njp.32101075379725;view=1up;seq=105}}</ref> They became known as [[isotope]]s, from the Greek ''{{lang|el|isos topos}}'' ("same place").<ref name=ley196610/><ref>Soddy first used the word "isotope" in: {{cite journal|last1=Soddy|first1=Frederick|title=Intra-atomic charge|journal=Nature|date=4 December 1913|volume=92|issue=2301|pages=399–400|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015038751833;view=1up;seq=441|bibcode=1913Natur..92..399S|doi=10.1038/092399c0|s2cid=3965303|url-access=subscription}} See p. 400.</ref> Austrian chemist [[Friedrich Paneth]] cited a difference between "real elements" (elements) and "simple substances" (isotopes), also determining that the existence of different isotopes was mostly irrelevant in determining chemical properties.<ref name=pastfuture/>

Following British physicist [[Charles Glover Barkla]]'s discovery of characteristic [[X-ray]]s emitted from metals in 1906, British physicist [[Henry Moseley]] considered a possible correlation between X-ray emissions and physical properties of elements. Moseley, along with [[Charles Galton Darwin]], [[Niels Bohr]], and [[George de Hevesy]], proposed that the [[nuclear charge]] (''Z'') might be mathematically related to physical properties.<ref name=moseley2010>{{cite magazine |last1=Marshall |first1=J.L. |last2=Marshall |first2=V.R. |date=2010 |title=Rediscovery of the Elements: Moseley and Atomic Numbers |pages=42–47 |magazine=The Hexagon |volume=101 |issue=3 |publisher=[[Alpha Chi Sigma]] |s2cid=94398490 }}</ref> The significance of these atomic properties was determined in the [[Geiger–Marsden experiments]], in which the atomic nucleus and its charge were discovered, conducted between 1908 and 1913.<ref name=rfnucleus>{{cite journal |last1=Rutherford |first1=Ernest |last2=Nuttal |first2=John Mitchell |year=1913 |title=Scattering of α-Particles by Gases |url=https://archive.org/details/ClassicalScientificPapersPhysics |journal=[[Philosophical Magazine]] |series=Series 6 |volume=26 |issue=154  |pages=702–712 |doi=10.1080/14786441308635014}}</ref>

=== Rutherford model and atomic number ===
{{Main|Rutherford model|Atomic number}}
<!-- This section should discuss how the model of an atom in which the electron shell was separate from the nucleus came about. In particular, the section should mention obsolete models like the [[Plum pudding model]] -->

In 1913, amateur Dutch physicist [[Antonius van den Broek]] was the first to propose that the [[atomic number]] (nuclear charge) determined the placement of elements in the periodic table. He correctly determined the atomic number of all elements up to atomic number 50 ([[tin]]), though he made several errors with heavier elements. However, Van den Broek did not have any method to experimentally verify the atomic numbers of elements; thus, they were still believed to be a consequence of atomic weight, which remained in use in ordering elements.<ref name="moseley2010" />

Moseley was determined to test Van den Broek's hypothesis.<ref name=moseley2010/> After a year of investigation of the [[characteristic x-ray]]s of various elements, he found a relationship between the X-ray [[wavelength]] of an element and its atomic number.<ref>{{cite journal|last1=Moseley|first1=H.G.J.|title=The high-frequency spectra of the elements|journal=Philosophical Magazine|date=1914|volume=27|pages=703–713|url=https://babel.hathitrust.org/cgi/pt?id=uva.x002266051;view=1up;seq=731|series=6th series|doi=10.1080/14786440408635141}}</ref> With this, Moseley obtained the first accurate measurements of atomic numbers and determined an absolute sequence to the elements, allowing him to restructure the periodic table. Moseley's research immediately resolved discrepancies between atomic weight and chemical properties, where sequencing strictly by atomic weight would result in groups with inconsistent chemical properties. For example, his measurements of X-ray wavelengths enabled him to correctly place [[argon]] (''Z''&nbsp;=&nbsp;18) before [[potassium]] (''Z''&nbsp;=&nbsp;19), [[cobalt]] (''Z''&nbsp;=&nbsp;27) before [[nickel]] (''Z''&nbsp;=&nbsp;28), as well as [[tellurium]] (''Z''&nbsp;=&nbsp;52) before [[iodine]] (''Z''&nbsp;=&nbsp;53), in line with [[periodic trends]]. The determination of atomic numbers also clarified the order of chemically similar rare-earth elements; it was also used to confirm that [[Georges Urbain]]'s claimed discovery of a new rare-earth element ([[celtium]]) was invalid, earning Moseley acclamation for this technique.<ref name=moseley2010/>

Swedish physicist [[Karl Siegbahn]] continued Moseley's work for elements heavier than gold (''Z''&nbsp;=&nbsp;79), and found that the heaviest known element at the time, [[uranium]], had atomic number 92. In determining the largest identified atomic number, gaps in the atomic number sequence were conclusively determined where an atomic number had no known corresponding element; the gaps occurred at atomic numbers 43 ([[technetium]]), 61 ([[promethium]]), 72 ([[hafnium]]), 75 ([[rhenium]]), 85 ([[astatine]]), and 87 ([[francium]]).<ref name=moseley2010/>

=== Electron shell and quantum mechanics ===
<!-- Janet and his table should definitely be mentioned here. Another thing to be mentioned (probably) here is the emergence of the 18=column table -->In 1914, Swedish physicist [[Johannes Rydberg]] noticed that the atomic numbers of the noble gases was equal to doubled sums of squares of simple numbers: 2 = 2·1<sup>2</sup>, 10 = 2(1<sup>2</sup> + 2<sup>2</sup>), 18 = 2(1<sup>2</sup> + 2<sup>2</sup> + 2<sup>2</sup>), 36 = 2(1<sup>2</sup> + 2<sup>2</sup> + 2<sup>2</sup> + 3<sup>2</sup>), 54 = 2(1<sup>2</sup> + 2<sup>2</sup> + 2<sup>2</sup> + 3<sup>2</sup> + 3<sup>2</sup>), 86 = 2(1<sup>2</sup> + 2<sup>2</sup> + 2<sup>2</sup> + 3<sup>2</sup> + 3<sup>2</sup> + 4<sup>2</sup>). This finding was accepted as an explanation of the fixed lengths of periods and eventually paved the way to repositioning of the noble gases from the left edge of the table, in group 0, to the right, in group VIII.<ref name="Trifonov" /> Unwillingness of the noble gases to engage in chemical reaction was explained in the alluded stability of closed noble gas electron configurations; from this notion emerged the [[octet rule]] originally referred to as Abegg's Rule of 1904.<ref>See [[Octet Rule]]</ref> Among the notable works that established the importance of the periodicity of eight were the [[valence bond theory]], published in 1916 by American chemist [[Gilbert N. Lewis]]<ref>{{cite journal|last1=Lewis|first1=Gilbert N.|date=1916|title=The atom and the molecule|url=https://babel.hathitrust.org/cgi/pt?id=hvd.hs1t2w;view=1up;seq=772|journal=Journal of the American Chemical Society|volume=38|issue=4|pages=762–785|doi=10.1021/ja02261a002|bibcode=1916JAChS..38..762L |s2cid=95865413 |url-access=subscription}}</ref> and the octet theory of chemical bonding, published in 1919 by American chemist [[Irving Langmuir]].<ref>{{cite journal|last1=Langmuir|first1=Irving|date=1919|title=The structure of atoms and the octet theory of valence|url=https://www.biodiversitylibrary.org/item/178935#page/298/mode/1up|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=5|issue=7|pages=252–259|bibcode=1919PNAS....5..252L|doi=10.1073/pnas.5.7.252|pmc=1091587|pmid=16576386|doi-access=free}}</ref><ref>{{cite journal|last1=Langmuir|first1=Irving|date=1919|title=The arrangement of electrons in atoms and molecules|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015011975565;view=1up;seq=884|journal=Journal of the American Chemical Society|volume=41|issue=6|pages=868–934|doi=10.1021/ja02227a002|bibcode=1919JAChS..41..868L |url-access=subscription}}</ref> The chemists' approach during the period of the Old Quantum Theory (1913 to 1925) was incorporated into the understanding of the electron shells and orbitals under current [[quantum mechanics]]. In his 1919 paper, Langmuir postulated the existence of "cells", which we now call [[Atomic orbital|atomic orbitals]], and these were arranged in "equidistant layers" which we now call [[Electron shell|electron shells]]. These postulates were introduced on the basis of Rydberg's rule which [[Niels Bohr]] had used not in chemistry, but in physics, to apply to the orbits of electrons around the nucleus. The Langmuir paper introduced the rule as 2N<sup>2</sup> where N was a positive integer.<ref>I. Langmuir, "The Arrangement of Electrons in Atoms and Molecules", J. Am. Chem. Soc., 41 (1919), p.868-934.</ref>

British chemist [[Charles Rugeley Bury]] made the next major step toward the modern theory in 1921, by suggesting that eight and eighteen electrons in a shell form stable configurations. Bury's scheme was built upon that of earlier chemists and was a chemical model. Bury proposed that the electron configurations in transitional elements depended upon the valency electrons in their outer shell.<ref>C.R. Bury, “Langmuir’s Theory on the Arrangement of Electrons in Atoms and Molecules, J. Am. Chem. Soc., 43 (1921), 1602-1609.</ref> In some early papers, the model was called the "Bohr-Bury Atom". He introduced the word ''transition'' to describe the elements now known as [[transition metal]]s or transition elements.<ref>{{cite journal |journal = Journal of Chemical Education |volume = 80 |issue = 8 |year = 2003 |url = http://www.uv.es/~borrasj/ingenieria_web/temas/tema_1/lecturas_comp/p952.pdf |title = The Place of Zinc, Cadmium, and Mercury in the Periodic Table|author1-link=William B. Jensen |first = William B. |last = Jensen |pages=952–961 |quote=The first use of the term “transition” in its modern electronic sense appears to be due to the British chemist C. R.Bury, who first used the term in his 1921 paper on the electronic structure of atoms and the periodic table |doi = 10.1021/ed080p952 |bibcode = 2003JChEd..80..952J }}</ref>

In the 1910s and 1920s, pioneering research into quantum mechanics led to new developments in atomic theory and small changes to the periodic table. In the 19th century, Mendeleev had already asserted that there was a fixed periodicity of eight, and expected a mathematical correlation between atomic number and chemical properties.<ref name=ptat>{{cite journal |url=https://www.researchgate.net/publication/226356457 |last1=Hettema |first1=H. |last2=Kuipers |first2=T.A.F. |title=The periodic table — its formalization, status, and relation to atomic theory |journal=Erkenntnis |date=1998 |volume=28 |issue=3 |pages=387–408 |doi=10.1007/BF00184902|s2cid=118775532 }}</ref> The [[Bohr model]] was developed beginning 1913, and championed the idea of [[electron configuration]]s that determine chemical properties. Bohr proposed that elements in the same group behaved similarly because they have similar electron configurations, and that noble gases had filled [[valence (chemistry)|valence]] shells;<ref name=scerri98>{{cite journal |last1=Scerri |first1=E.R. |title=The Evolution of the Periodic System |journal=Scientific American |date=1998 |volume=279 |issue=3 |pages=78–83 |doi=10.1038/scientificamerican0998-78 |bibcode=1998SciAm.279c..78S |url=https://mipt.ru/dasr/upload/62f/f_brx2-arphh81ii9w.pdf}}</ref> this forms the basis of the modern [[octet rule]]. Bohr's study of spectroscopy and chemistry was not usual among theoretical atomic physicists. Even Rutherford told Bohr that he was struggling "to form an idea of how you arrive at your conclusions".<ref>Pais, Abraham (1991), Niels Bohr’s Times, in Physics, Philosophy, and Polity (Oxford: Clarendon Press), p. 205.</ref> This is because none of the quantum mechanical equations describe the number of electrons per shell and orbital. Bohr acknowledged that he was influenced by the work of [[Walther Kossel]], who in 1916 was the first to establish an important connection between the quantum atom and the periodic table. He noticed that the difference between the atomic numbers 2, 10, 18 of the first three noble gases, helium, neon, argon, was 8, and argued that the electrons in such atoms orbited in "closed shells". The first contained only 2 electrons, the second and third, 8 each.<ref>Walther Kossel Bemerkung zum Seriencharakter der Röntgenstrahlen, Verh. D. Deutsch. Phys Ges (2) 18 339-359 (1916). Received 31 August 1916, published in issue No. 15-18 of 30 September 1916. As cited in Mehra, Volume 5, Part 2, 2001, p.&nbsp;919.</ref><ref>Walther Kossel, "''Uber Molkulbildung als Frage der Atombau''", Ann. Phys., 1916, 49:229-362.</ref> Bohr's research then led Austrian physicist [[Wolfgang Pauli]] to investigate the length of periods in the periodic table in 1924. Pauli demonstrated that this was not the case. Instead, the [[Pauli exclusion principle]] was developed, not upon a mathematical basis, but upon the previous developments in alignment with chemistry.<ref>Pauli admitted in his paper, "On the Connection between the Closing of Electron Groups in Atoms and the Complex Structure of Spectra", published on 21 March 1925 in ''Zeitschrift für Physik'': "We cannot give a more precise reason for this rule." Pais, Abraham (2000), The Genius of Science: A portrait gallery of twentieth-century physicists (New York: Oxford University Press), p. 223.</ref> This rule states that no electrons can coexist in the same quantum state, and showed, in conjunction with empirical observations, the existence of four [[quantum number]]s and the consequence on the order of shell filling.<ref name=scerri98/> This determines the order in which [[electron shell]]s are filled and explains the periodicity of the periodic table.

British chemist Charles Bury is credited with the first use of the term ''transition metal'' in 1921 to refer to elements between the [[main-group element]]s of groups II and III. He explained the chemical properties of transition elements as a consequence of the filling of an inner subshell rather than the valence shell. This proposition, based upon the work of American chemist [[Gilbert N. Lewis]], suggested the appearance of the ''d'' subshell in period&nbsp;4 and the ''f'' subshell in period&nbsp;6, lengthening the periods from 8 to 18 and then 18 to 32 elements, thus explaining the position of the lanthanides in the periodic table.<ref name=Jensen>{{cite journal |journal = Journal of Chemical Education |volume = 80 |issue = 8 |year = 2003 |url = http://www.uv.es/~borrasj/ingenieria_web/temas/tema_1/lecturas_comp/p952.pdf |title = The Place of Zinc, Cadmium, and Mercury in the Periodic Table |first = William B. |last = Jensen |pages=952–961 |doi = 10.1021/ed080p952 |bibcode = 2003JChEd..80..952J }}</ref>

=== Proton and neutron ===
{{Main|Proton|Neutron}}

The discovery of proton and neutron demonstrated that an atom was divisible; this rendered Lavoisier's definition of a chemical element obsolete. A chemical element is defined today as a species of atoms with a consistent number of protons<ref>{{GoldBookRef |title=chemical element |file=C01022 }}</ref> and that number is now known to be precisely the atomic number of an element. The discovery also explained the mechanism of several types of radioactive decay, such as [[alpha decay]].

Eventually, it was proposed that protons and neutrons were made of even smaller particles called [[quark]]s; their discovery explained the transmutation of neutrons into protons in [[beta decay]].

==From short form into long form (into -A and -B groups)==
[[File:TUM Chemistry 1909.jpg|thumb|The periodic table on the wall of a chemistry lecture hall in Munich, Germany (1909)]]
Circa 1925, the periodic table changed by shifting some ''Reihen'' (series) to the right, into an extra set of columns (groups). The original groups I–VII were repeated, distinguished by adding "A" and "B". Group VIII (with three columns) remained sole.

Thus, ''Reihen'' 4 and 5 were shifted, and together formed new ''period'' 4 with groups IA–VIIA, VIII, IB–VIIB.

{{Periodic table (Reihen and periods)}}

==Later expansions and the end of the periodic table==

{{blockquote|We already feel that we have neared the moment when this [periodic] law begins to change, and change fast.| source = Russian physicist [[Yuri Oganessian]], co-discoverer of several [[superheavy element]]s, in 2019<ref>{{Cite interview|last=Oganessian|first=Yu.|interviewer-last=Sidorova|interviewer-first=Ye.|date=2019|url=https://elementy.ru/nauchno-populyarnaya_biblioteka/434705/My_priblizilis_k_granitsam_primenimosti_periodicheskogo_zakona|title=Мы приблизились к границам применимости периодического закона|trans-title=We have neared the limits of applicability of the periodic law|website=Elementy|language=ru|access-date=2020-04-23}}</ref> }}

=== Actinides ===
As early as 1913, Bohr's research on [[electronic structure]] led physicists such as [[Johannes Rydberg]] to extrapolate the properties of undiscovered elements heavier than uranium. Many agreed that the next noble gas after radon would most likely have the atomic number 118, from which it followed that the transition series in the [[period 7 element|seventh period]] should resemble those in the [[period 6 element|sixth]]. Although it was thought that these transition series would include a series analogous to the rare-earth elements, characterized by filling of the 5f shell, it was unknown where this series began. Predictions ranged from atomic number 90 (thorium) to 99, many of which proposed a beginning beyond the known elements (at or beyond atomic number 93). The elements from [[actinium]] to uranium were instead believed to form part of a fourth series of transition metals because of their high [[oxidation state]]s; accordingly, they were placed in groups 3 through 6.<ref name=seaborg2>{{cite book |last=Seaborg |first=G. |title=Lanthanides/Actinides: Chemistry |chapter=Origin of the Actinide Concept |series=Handbook on the Physics and Chemistry of Rare Earths |volume=18 |edition=1 |date=1994 |isbn=9780444536648 |id=LBL-31179 |chapter-url=https://cloudfront.escholarship.org/dist/prd/content/qt7cz371hx/qt7cz371hx.pdf}}</ref>

In 1940, [[neptunium]] and [[plutonium]] were the first [[transuranic element]]s to be discovered; they were placed in sequence beneath [[rhenium]] and [[osmium]], respectively. However, preliminary investigations of their chemistry suggested a greater similarity to uranium than to lighter transition metals, challenging their placement in the periodic table.<ref name=clark/> During his [[Manhattan Project]] research in 1943, American chemist [[Glenn T. Seaborg]] experienced unexpected difficulties in isolating the elements [[americium]] and [[curium]], as they were believed to be part of a fourth series of transition metals. Seaborg wondered if these elements belonged to a different series, which would explain why their chemical properties, in particular the instability of higher [[oxidation state]]s, were different from predictions.<ref name=clark>{{cite report |title=The Discovery of Plutonium Reorganized the Periodic Table and Aided the Discovery of New Elements |last=Clark |first=D.L. |date=2009 |publisher=Los Alamos National Laboratory |url=http://large.stanford.edu/courses/2017/ph241/kennedy-c1/docs/la-ur-09-04783.pdf}}</ref> In 1945, against the advice of colleagues, he proposed a significant change to Mendeleev's table: the [[Actinide|actinide series]].<ref name=seaborg2 /><ref name=legacy>{{cite journal |last1=Clark |first1=D.L. |last2=Hobart |first2=D.E. |title=Reflections on the Legacy of a Legend: Glenn T. Seaborg, 1912–1999 |journal=Los Alamos Science |volume=26 |date=2000 |pages=56–61 |url=https://fas.org/sgp/othergov/doe/lanl/pubs/00818011.pdf}}</ref>

Seaborg's [[actinide concept]] of heavy element electronic structure proposed that the actinides form an [[inner transition]] series analogous to the [[Rare-earth element|rare-earth]] series of [[lanthanide]] elements—they would comprise the second row of the f-block (the 5f series), in which the lanthanides formed the 4f series. This facilitated chemical identification of americium and curium,<ref name=legacy/> and further experiments corroborated Seaborg's hypothesis; a spectroscopic study at the [[Los Alamos National Laboratory]] by a group led by American physicist [[Edwin McMillan]] indicated that 5f orbitals, rather than 6d [[atomic orbital|orbital]]s, were indeed being filled. However, these studies could not unambiguously determine the first element with 5f electrons and therefore the first element in the actinide series;<ref name=clark /> it was thus also referred to as the "thoride" or "uranide" series until it was later found that the series began with actinium.<ref name=seaborg2 /><ref name=hoffman96>{{cite conference |last=Hoffman |first=D. C. |date=1996 |title=The Transuranium Elements: From Neptunium and Plutonium to Element 112 |conference=NATO Advanced Study Institute on "Actinides and the Environment" |publisher=Lawrence Livermore National Laboratory |url=https://inis.iaea.org/collection/NCLCollectionStore/_Public/28/017/28017156.pdf}}</ref>

In light of these observations and an apparent explanation for the chemistry of transuranic elements, and despite fear among his colleagues that it was a radical idea that would ruin his reputation, Seaborg nevertheless submitted it to ''[[Chemical & Engineering News]]'' and it gained widespread acceptance; new periodic tables thus placed the actinides below the lanthanides.<ref name=legacy /> Following its acceptance, the actinide concept proved pivotal in the groundwork for discoveries of heavier elements, such as [[berkelium]] in 1949.<ref name=bk>{{cite journal |last=Trabesinger |first=A. |title=Peaceful berkelium |date=2017 |journal=[[Nature Chemistry]] |volume=9 |issue=9 |page=924 |doi=10.1038/nchem.2845 |pmid=28837169 |bibcode=2017NatCh...9..924T |doi-access=free }}</ref> It also supported experimental results for a trend towards +3 oxidation states in the elements beyond americium—a trend observed in the analogous 4f series.<ref name=seaborg2 />

=== Relativistic effects and breakdown of the periodic law in period 7 ===
{{Main|Relativistic quantum chemistry}}
{{Empty section|date=August 2025}}

=== Expansions beyond period 7 ===
Seaborg's subsequent elaborations of the actinide concept theorized a series of [[Transuranium element|superheavy elements]] in a [[transactinide]] series comprising elements from [[Rutherfordium|104]] to [[Unbiunium|121]] and a [[superactinide]] series of elements from [[Unbibium|122]] to 153.<ref name=clark /> He proposed an [[extended periodic table]] with an additional period of 50&nbsp;elements (thus reaching element&nbsp;168); this eighth period was derived from an extrapolation of the [[Aufbau principle]] and placed elements&nbsp;121 to&nbsp;138 in a g-block, in which a new g subshell would be filled.<ref name=transu1>{{cite book |last1=Hoffman |first1=D.C |last2=Ghiorso |first2=A. |last3=Seaborg |first3=G.T. |title=The Transuranium People: The Inside Story |publisher=Imperial College Press |date=2000 |isbn=978-1-86094-087-3 |pages=435–436}}</ref> Seaborg's model, however, did not take into account [[relativistic quantum chemistry|relativistic effects]] resulting from high atomic number and electron orbital speed. [[Burkhard Fricke]] in 1971<ref name="Fricke">{{cite journal |last1=Fricke |first1=B. |last2=Greiner |first2=W. |last3=Waber |first3=J. T. |year=1971 |title=The continuation of the periodic table up to Z = 172. The chemistry of superheavy elements |journal=Theoretica Chimica Acta |volume=21 |issue=3 |pages=235–260 |doi=10.1007/BF01172015|s2cid=117157377 }}</ref> and [[Pekka Pyykkö]] in 2010<ref name=PT172>{{Cite journal|last1=Pyykkö|first1=Pekka|author-link=Pekka Pyykkö|title=A suggested periodic table up to Z≤ 172, based on Dirac–Fock calculations on atoms and ions|journal=Physical Chemistry Chemical Physics|volume=13|issue=1|pages=161–8|year=2011|pmid=20967377|doi=10.1039/c0cp01575j|bibcode = 2011PCCP...13..161P }}</ref> used computer modeling to calculate the positions of elements up to ''Z''&nbsp;=&nbsp;172, and found that the positions of several elements were different from those predicted by Seaborg. Although models from Pyykkö and Fricke generally place element&nbsp;172 as the next noble gas, there is no clear consensus on the electron configurations of elements beyond [[unbinilium|120]] and thus their placement in an extended periodic table. It is now thought that because of relativistic effects, such an extension will feature elements that break the periodicity in known elements, thus posing another hurdle to future periodic table constructs.<ref name=PT172/>

The discovery of [[tennessine]] in 2010 filled the last remaining gap in the seventh period. Any newly discovered elements will thus be placed in an eighth period.

Despite the completion of the seventh period, experimental chemistry of some transactinides has been shown to be inconsistent with the periodic law. In the 1990s, Ken Czerwinski at [[University of California, Berkeley]] observed similarities between rutherfordium and plutonium and between dubnium and protactinium, rather than a clear continuation of periodicity in groups&nbsp;4 and&nbsp;5. More recent experiments on [[copernicium]] and [[flerovium]] have yielded inconsistent results, some of which suggest that these elements behave more like the noble gas [[radon]] rather than mercury and lead, their respective [[congener (chemistry)|congener]]s. As such, the chemistry of many superheavy elements has yet to be well characterized, and it remains unclear whether the periodic law can still be used to extrapolate the properties of undiscovered elements.<ref name="NYT-20190827">{{cite news |last=Roberts |first=Siobhan |title=Is It Time to Upend the Periodic Table? - The iconic chart of elements has served chemistry well for 150 years. But it's not the only option out there, and scientists are pushing its limits. |url=https://www.nytimes.com/2019/08/27/science/periodic-table-elements-chemistry.html |date=27 August 2019 |work=[[The New York Times]] |access-date=27 August 2019 }}</ref><ref name=EScerri>{{cite magazine |last=Scerri |first=E. |title=Cracks in the periodic table |magazine=Scientific American |date=2013 |volume=308 |issue=6 |pages=68–73 |issn=0036-8733}}</ref>

<!--== Main discovery periods==
IUPAC<ref>{{cite web|url=http://old.iupac.org/reports/periodic_table/index.html|title=International Union of Pure and Applied Chemistry > Periodic Table of the Elements|work=iupac.org|access-date=6 March 2017}}</ref> has suggested five 'main discovery periods'.
Seven elements can have their discovery traced to a tiny [[Sweden|Swedish]] island near [[Stockholm]]. At a [[quarry]] and mine near the Swedish village of [[Ytterby]] on the island of [[Resarö]], the [[rare-earth mineral]] was discovered, which was named ''yttria'' after the village. The mineral proved to be the source of four new elements, that were all named after the village: [[yttrium]] (Y), [[erbium]] (Er), [[terbium]] (Tb) and [[ytterbium]] (Yb).<ref name="Emsley">{{cite book|last=Emsley|first=John|title=Nature's Building Blocks|year=2001|publisher=Oxford University Press|isbn=0-19-850341-5|page=496}}</ref> They were first described in 1794, 1842, 1842 and 1878, respectively. In 1989, [[ASM International (society)|ASM International]] installed a plaque at the former entrance to the mine, commemorating it as a historical landmark.<ref name="Blom">{{cite web | url = http://www.hembygd.se/index.asp?DocID=2785 | title = Ytterby gruva | author = Blom, Carl-Hugo | publisher = Stockholms läns hembygdsförbund | date = 18 June 2006 | access-date = 9 June 2007 | language = sv}}</ref> Three other [[lanthanides]] ([[holmium]] (Ho), [[thulium]] (Tm) and [[gadolinium]] (Gd)) can also trace their discovery to the same quarry.<ref>{{cite web|last=Kean|first=Sam|title=Ytterby: The Tiny Swedish Island That Gave the Periodic Table Four Different Elements|url=http://www.slate.com/id/2258112/entry/2260782|work=Slate|access-date=22 March 2011|date=16 July 2010}}</ref> -->

== See also ==
{{div col|colwidth=27em}}
* [[History of chemistry]]
* [[Periodic systems of small molecules]]
* [[The Mystery of Matter (film)|''The Mystery of Matter: Search for the Elements'' (PBS film)]]
* [[Discovery of chemical elements]]
* [[Types of periodic tables]]
{{div col end}}

== Notes ==
{{notelist}}

== References ==
{{reflist}}

== Sources==
*{{Cite book|last=Gordin|first=M. D.|chapter=The Textbook Case of a Priority Dispute: D. I. Mendeleev, Lothar Meyer, and the Periodic System|date=2012|title=Nature Engaged|pages=59–82|editor-last=Biagioli|editor-first=M. |publisher=Palgrave Macmillan |doi=10.1057/9780230338029_4|isbn=978-1-349-28717-8|editor2-last=Riskin|editor2-first=J.}}
*{{Cite book|last=Mendeleev|first=D. I.|author-link=Dmitri Mendeleev |title=Естественная система элементов и применение ее к указанию свойств неоткрытых элементов|trans-title=The natural system of the elements and its application to indication of properties of unknown elements|year=1870|pages=102–176}}. Republished from {{cite journal |ref=none |last=Mendeleev|first=D. I. |author-link=Dmitri Mendeleev |title=Естественная система элементовъ и примѣненіе её къ указанію свойствъ неоткрытыхъ элементовъ|trans-title=The natural system of the elements and its application to indication of properties of unknown elements|year=1871|language=ru|journal=Journal of the Russian Physico-Chemical Society|volume=3|issue=2|pages=25–56 |url=http://www.knigafund.ru/books/56718/read#page31|archive-url=https://web.archive.org/web/20140317224410/http://www.knigafund.ru/books/56718/read#page31|archive-date=17 March 2014}}
*{{Cite book|last=Mendeleev|first=D. I.|title=Периодическая законность химических элементов|trans-title=Periodic regularity of the chemical elements|year=1871|pages=102–176 }}. Republished from {{cite journal |last=Mendelejeff|first=D.|title=Die periodische Gesetzmässigkeit der Elemente|trans-title=Periodic regularity of the chemical elements|year=1871|language=de|journal=[[Liebigs Annalen|Annalen der Chemie und Pharmacie]]|pages=133–229|url=https://babel.hathitrust.org/cgi/pt?id=uva.x002457982&view=1up&seq=542 |ref=none}}
*{{Cite book|last=Petrov|first=L. P.|title=Прогнозирование и размещение инертных элементов в периодической системе|publisher=|year=1981|isbn=|location=|pages=|language=ru|trans-title=Forecasting and placing of inert elements in the periodic system}}
* {{cite book|last=Scerri|first=E. R.|author-link=Eric Scerri|title=The Periodic Table: Its Story and Its Significance|url=https://books.google.com/books?id=9x2yDwAAQBAJ&pg=PA456|year=2019|publisher=Oxford University Press|isbn=978-0-19-091436-3}}
* {{cite book |doi=10.1007/978-94-017-9364-3_11 |chapter=Mendeleev and the Rare-Earth Crisis |title=Philosophy of Chemistry |series=Boston Studies in the Philosophy and History of Science |year=2015 |last1=Thyssen |first1=Pieter |last2=Binnemans |first2=Koen |volume=306 |pages=155–182 |isbn=978-94-017-9363-6 |url=http://philsci-archive.pitt.edu/10718/1/Thyssen_%26_Binnemans_%282013b%29.pdf }}

==Further reading==
*{{Cite book|last=Mendeleev|first=D. I. |author-link=Dmitri Mendeleev |title=Попытка химического понимания мирового эфира|trans-title=Attempt of chemical understanding of the world ether|year=1902|pages=470–517 |ref=none}}. Republished from {{cite book |last=Mendeleev|first=D.|title=Попытка химическаго пониманія мірового эѳира|trans-title=Attempt of chemical understanding of the world ether|url=http://ether-wind.narod.ru/Mendeleev_1905/|language=ru|publisher=M. P. Frolova's typo-lithography|year=1905|pages=5–40 |ref=none}}
*{{Cite book|last=Mendeleev|first=D. I.|url=https://runivers.ru/bookreader/book144952/#page/1/mode/1up|title=Периодический закон|trans-title=The periodic law|publisher=[[Academy of Sciences of the USSR]]|year=1958|editor-last=Kedrov|editor-first=K. M.|language=ru |ref=none}}
*{{Cite book|title=Учение о периодичности: история и современность|publisher=[[Nauka (publisher)|Nauka]]|year=1981|editor-last=Trifonov|editor-first=D. I.|pages=|language=ru|trans-title=Teaching of periodicity: history and modernity |ref=none}}

== External links ==
*[http://www.rsc.org/periodic-table/history/about Development of the periodic table] (part of a collection of pages that explores the periodic table and the elements) by the [http://www.rsc.org/ Royal Society of Chemistry]
*Dr. Eric Scerri's [http://www.ericscerri.com/ web page], which contains interviews, lectures and articles on various aspects of the periodic system, including the history of the periodic table.
*[http://www.meta-synthesis.com/webbook/35_pt/pt_database.php The Internet Database of Periodic Tables] – a large collection of periodic tables and periodic system formulations.
*[https://stats.stackexchange.com/q/96907 History of Mendeleev periodic table of elements as a data visualization] at [[Stack Exchange]]

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