View source for Period 6 element

{{Short description|Row 6 of the periodic table}}
{{Periodic table (micro)| title=Period 6 in the [[periodic table]] | mark=Cs,Ba,La,Ce,Pr,Nd,Pm,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu,Hf,Ta,W,Re,Os,Ir,Pt,Au,Hg,Tl,Pb,Bi,Po,At,Rn}}
{{Sidebar periodic table|expanded=structure }}
A '''period 6 element''' is one of the [[chemical element]]s in the sixth row (or [[Periodic table period|period]]) of the [[periodic table|periodic table of the chemical elements]], including the [[lanthanide]]s. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The sixth period contains 32 elements, tied for the most with [[Period 7 element|period 7]], beginning with [[caesium]] and ending with [[radon]]. [[Lead]] is currently the last stable element; all subsequent elements are [[radioactive]]. For [[bismuth]], however, its only [[primordial isotope]], <sup>209</sup>Bi, has a half-life of more than 10<sup>19</sup> years, over a billion times longer than the current [[age of the universe]]. As a rule, period 6 elements fill their 6s [[electron shell|shells]] first, then their 4f, 5d, and 6p shells, in that order; however, there are exceptions, such as [[gold]].

==Properties==
This period contains the [[lanthanides]], also known as the ''rare earths''. Many lanthanides are known for their magnetic properties, such as [[neodymium]]. Many period 6 [[transition metals]] are very valuable, such as [[gold]], however many period 6 [[other metals]] are incredibly toxic, such as [[thallium]]. Period 6 contains the last stable element, [[lead]]. All subsequent elements in the periodic table are [[radioactive]]. After [[bismuth]], which has a half-life or more than 10<sup>19</sup> years, [[polonium]], [[astatine]], and [[radon]] are some of the [[Half-life|shortest-lived]] and rarest elements known; less than a gram of astatine is estimated to exist on earth at any given time.<ref name="Gray" >{{cite book|last=Gray|first=Theodore|title=The Elements: A Visual Exploration of Every Known Atom in the Universe|year=2009|publisher=Black Dog & Leventhal Publishers|location=New York|isbn=978-1-57912-814-2|url-access=registration|url=https://archive.org/details/elementsvisualex0000gray}}</ref>

==Atomic characteristics==
:{| class="wikitable sortable"
! colspan="3" | [[Chemical element]]
! [[Block (periodic table)|Block]]
! [[Electron configuration]]
|- bgcolor="{{element color|s-block}}"
|| 55  || '''Cs''' || [[Caesium]] || [[s-block]] || [Xe] 6s<sup>1</sup>
|- bgcolor="{{element color|s-block}}"
|| 56  || '''Ba''' || [[Barium]] || [[s-block]] || [Xe] 6s<sup>2</sup>
|- bgcolor="{{element color|f-block}}"
|| 57 || '''La''' || [[Lanthanum]] || [[f-block]] {{ref label|Note1|a|a}} || [Xe] 5d<sup>1</sup> 6s<sup>2</sup> {{ref label|Note2|b|b}}
|- bgcolor="{{element color|f-block}}"
|| 58 || '''Ce''' || [[Cerium]] || [[f-block]] || [Xe] 4f<sup>1</sup> 5d<sup>1</sup> 6s<sup>2</sup> {{ref label|Note2|b|b}}
|- bgcolor="{{element color|f-block}}"
|| 59 || '''Pr''' || [[Praseodymium]] || [[f-block]] || [Xe] 4f<sup>3</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|f-block}}"
|| 60 || '''Nd''' || [[Neodymium]] || [[f-block]] || [Xe] 4f<sup>4</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|f-block}}"
|| 61 || '''Pm''' || [[Promethium]] || [[f-block]] || [Xe] 4f<sup>5</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|f-block}}"
|| 62 || '''Sm''' || [[Samarium]] || [[f-block]] || [Xe] 4f<sup>6</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|f-block}}"
|| 63 || '''Eu''' || [[Europium]] || [[f-block]] || [Xe] 4f<sup>7</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|f-block}}"
|| 64 || '''Gd''' || [[Gadolinium]] || [[f-block]] || [Xe] 4f<sup>7</sup> 5d<sup>1</sup> 6s<sup>2</sup> {{ref label|Note2|b|b}}
|- bgcolor="{{element color|f-block}}"
|| 65 || '''Tb''' || [[Terbium]] || [[f-block]] || [Xe] 4f<sup>9</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|f-block}}"
|| 66 || '''Dy''' || [[Dysprosium]] || [[f-block]] || [Xe] 4f<sup>10</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|f-block}}"
|| 67 || '''Ho''' || [[Holmium]] || [[f-block]] || [Xe] 4f<sup>11</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|f-block}}"
|| 68 || '''Er''' || [[Erbium]] || [[f-block]] || [Xe] 4f<sup>12</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|f-block}}"
|| 69 || '''Tm''' || [[Thulium]] || [[f-block]] || [Xe] 4f<sup>13</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|f-block}}"
|| 70 || '''Yb''' || [[Ytterbium]] || [[f-block]] || [Xe] 4f<sup>14</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 71  || '''Lu''' || [[Lutetium]] || [[d-block]] {{ref label|Note1|a|a}} || [Xe] 4f<sup>14</sup> 5d<sup>1</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 72  || '''Hf''' || [[Hafnium]] || [[d-block]] || [Xe] 4f<sup>14</sup> 5d<sup>2</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 73  || '''Ta''' || [[Tantalum]] || [[d-block]] || [Xe] 4f<sup>14</sup> 5d<sup>3</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 74  || '''W''' || [[Tungsten]] || [[d-block]] || [Xe] 4f<sup>14</sup> 5d<sup>4</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 75  || '''Re''' || [[Rhenium]] || [[d-block]] || [Xe] 4f<sup>14</sup> 5d<sup>5</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 76 || '''Os''' || [[Osmium]] || [[d-block]] || [Xe] 4f<sup>14</sup> 5d<sup>6</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 77 || '''Ir''' || [[Iridium]] || [[d-block]] || [Xe] 4f<sup>14</sup> 5d<sup>7</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 78 || '''Pt''' || [[Platinum]] || [[d-block]] || [Xe] 4f<sup>14</sup> 5d<sup>9</sup> 6s<sup>1</sup> {{ref label|Note2|b|b}}
|- bgcolor="{{element color|d-block}}"
|| 79 || '''Au''' || [[Gold]] || [[d-block]] || [Xe] 4f<sup>14</sup> 5d<sup>10</sup> 6s<sup>1</sup> {{ref label|Note2|b|b}}
|- bgcolor="{{element color|d-block}}"
|| 80 || '''Hg''' || [[Mercury (element)|Mercury]] || [[d-block]] || [Xe] 4f<sup>14</sup> 5d<sup>10</sup> 6s<sup>2</sup>
|- bgcolor="{{element color|p-block}}"
|| 81 || '''Tl''' || [[Thallium]] || [[p-block]] || [Xe] 4f<sup>14</sup> 5d<sup>10</sup> 6s<sup>2</sup> 6p<sup>1</sup>
|- bgcolor="{{element color|p-block}}"
|| 82 || '''Pb''' || [[Lead]] || [[p-block]] || [Xe] 4f<sup>14</sup> 5d<sup>10</sup> 6s<sup>2</sup> 6p<sup>2</sup>
|- bgcolor="{{element color|p-block}}"
|| 83 || '''Bi''' || [[Bismuth]] || [[p-block]] || [Xe] 4f<sup>14</sup> 5d<sup>10</sup> 6s<sup>2</sup> 6p<sup>3</sup>
|- bgcolor="{{element color|p-block}}"
|| 84 || '''Po''' || [[Polonium]] || [[p-block]] || [Xe] 4f<sup>14</sup> 5d<sup>10</sup> 6s<sup>2</sup> 6p<sup>4</sup>
|- bgcolor="{{element color|p-block}}"
|| 85 || '''At''' || [[Astatine]] || [[p-block]] || [Xe] 4f<sup>14</sup> 5d<sup>10</sup> 6s<sup>2</sup> 6p<sup>5</sup>
|- bgcolor="{{element color|p-block}}"
|| 86 || '''Rn''' || [[Radon]] || [[p-block]] || [Xe] 4f<sup>14</sup> 5d<sup>10</sup> 6s<sup>2</sup> 6p<sup>6</sup>
|}

*{{note label|Note1|a|a}} In many periodic tables, the f-block is erroneously shifted one element to the right, so that lanthanum and actinium become d-block elements, and Ce–Lu and Th–Lr form the f-block, tearing the d-block into two very uneven portions. This is a holdover from early erroneous measurements of electron configurations.<ref name="Jensen1982">{{cite journal |title=The Positions of Lanthanum (Actinium) and Lutetium (Lawrencium) in the Periodic Table |author=William B. Jensen |journal=J. Chem. Educ. |year=1982 |volume=59 |issue = 8|pages=634–636 |doi=10.1021/ed059p634|bibcode=1982JChEd..59..634J }}</ref> [[Lev Landau]] and [[Evgeny Lifshitz]] pointed out in 1948 that lutetium is not an f-block element,<ref name=Landau>{{cite book
 |author=[[Lev Landau|L. D. Landau]], [[Evgeny Lifshitz|E. M. Lifshitz]]
 |year=1958
 |title=Quantum Mechanics: Non-Relativistic Theory
 |edition=1st |volume=3
 |publisher=[[Pergamon Press]]
 |pages=256–7
}}</ref> and since then physical, chemical, and electronic evidence has overwhelmingly supported that the f-block contains the elements La–Yb and Ac–No,<ref name=Jensen1982/><ref name=Fluck/> as shown here and as supported by [[International Union of Pure and Applied Chemistry]] reports dating from 1988<ref name="Fluck">{{cite journal |last1=Fluck |first1=E. |year=1988 |title=New Notations in the Periodic Table |journal=[[Pure and Applied Chemistry|Pure Appl. Chem.]] |volume=60 |pages=431–436|doi=10.1351/pac198860030431 |url=https://www.iupac.org/publications/pac/1988/pdf/6003x0431.pdf |access-date=24 March 2012 |issue=3 |s2cid=96704008 |url-status=live |archive-url=https://web.archive.org/web/20120325152951/https://www.iupac.org/publications/pac/1988/pdf/6003x0431.pdf |archive-date=25 March 2012}}</ref> and 2021.<ref name="2021IUPAC">{{cite journal |last1=Scerri |first1=Eric |date=18 January 2021 |title=Provisional Report on Discussions on Group 3 of the Periodic Table |url=https://iupac.org/wp-content/uploads/2021/04/ChemInt_Jan2021_PP.pdf |journal=Chemistry International |volume=43 |issue=1 |pages=31–34 |doi=10.1515/ci-2021-0115 |s2cid=231694898 |access-date=9 April 2021 |archive-date=13 April 2021 |archive-url=https://web.archive.org/web/20210413150110/https://iupac.org/wp-content/uploads/2021/04/ChemInt_Jan2021_PP.pdf |url-status=live }}</ref>
*{{note label|Note2|b|b}} An exception to the [[Madelung rule]].

==s-block elements==

===Caesium===
{{main|Caesium}}
'''Caesium''' or '''cesium'''{{#tag:ref|''Caesium'' is the spelling recommended by the [[International Union of Pure and Applied Chemistry]] (IUPAC).<ref>{{RedBook2005|pages=248–49}}.</ref> The [[American Chemical Society]] (ACS) has used the spelling ''cesium'' since 1921,<ref>{{Cite book|editor1-first = Anne M.|editor1-last = Coghill|editor2-first = Lorrin R.|editor2-last = Garson|year = 2006|title = The ACS Style Guide: Effective Communication of Scientific Information|edition = 3rd|publisher = American Chemical Society|location = Washington, D.C.|isbn = 978-0-8412-3999-9|page = [https://archive.org/details/acsstyleguideeff0000unse/page/127 127]|url = https://archive.org/details/acsstyleguideeff0000unse/page/127}}</ref><ref>{{Cite journal|journal=Pure Appl. Chem.|volume=70|issue=1|last1=Coplen|pages = 237–257|year = 1998|first1=T. B.|url =http://old.iupac.org/reports/1998/7001coplen/history.pdf|last2=Peiser|first2=H. S.|title = History of the recommended atomic-weight values from 1882 to 1997: a comparison of differences from current values to the estimated uncertainties of earlier values|doi = 10.1351/pac199870010237|s2cid=96729044}}</ref> following ''Webster's New International Dictionary''. The element was named after the Latin word ''caesius'', meaning "bluish gray". More spelling explanation at [[American and British English spelling differences#ae and oe|ae/oe vs e]].|group=note}} is the [[chemical element]] with the symbol '''Cs''' and [[atomic number]] 55. It is a soft, silvery-gold [[alkali metal]] with a melting point of 28&nbsp;°C (82&nbsp;°F), which makes it one of only five elemental metals that are liquid at (or near) [[room temperature]].{{#tag:ref|Along with [[rubidium]] (39&nbsp;°C [102&nbsp;°F]), [[francium]] (estimated at 27&nbsp;°C [81&nbsp;°F]), [[mercury (element)|mercury]] (−39&nbsp;°C [−38&nbsp;°F]), and [[gallium]] (30&nbsp;°C [86&nbsp;°F]); bromine is also liquid at room temperature (melting at −7.2&nbsp;°C, 19&nbsp;°F) but it is a [[halogen]], not a metal.<ref>{{Cite web|url=http://www.webelements.com/|publisher=University of Sheffield|access-date=2010-12-01|title=WebElements Periodic Table of the Elements}}</ref>|group=note}} Caesium is an [[alkali metal]] and has physical and chemical properties similar to those of [[rubidium]] and [[potassium]]. The metal is extremely reactive and [[pyrophoricity|pyrophoric]], reacting with water even at−116&nbsp;°C (−177&nbsp;°F). It is the least [[electronegativity|electronegative]] element having a stable isotope, caesium-133. Caesium is mined mostly from [[pollucite]], while the [[Radionuclide|radioisotopes]], especially [[caesium-137]], a [[fission product]], are extracted from waste produced by [[nuclear reactor technology|nuclear reactors]].

Two German chemists, [[Robert Bunsen]] and [[Gustav Kirchhoff]], discovered caesium in 1860 by the newly developed method of [[Atomic emission spectroscopy#Flame emission spectroscopy|flame spectroscopy]]. The first small-scale applications for caesium have been as a "[[getter]]" in [[vacuum tube]]s and in [[Solar cell|photoelectric cells]]. In 1967, a specific frequency from the [[emission spectrum]] of caesium-133 was chosen to be used in the definition of the [[second]] by the [[International System of Units]]. Since then, caesium has been widely used in [[atomic clock]]s.

Since the 1990s, the largest [[#Applications|application of the element]] has been as caesium formate for [[drilling fluid]]s. It has a range of applications in the production of electricity, in electronics, and in chemistry. The radioactive isotope caesium-137 has a [[half-life]] of about 30 years and is used in medical applications, industrial gauges, and hydrology. Although the element is only mildly toxic, it is a hazardous material as a metal and its radioisotopes present a high health risk in case of radioactivity releases.

===Barium===
{{main|Barium}}
'''Barium''' is a [[chemical element]] with the symbol '''Ba''' and [[atomic number]] 56. It is the fifth element in Group 2, a soft silvery [[metal]]lic [[alkaline earth metal]]. Barium is never found in nature in its pure form due to its [[Reactivity (chemistry)|reactivity]] with [[Earth's atmosphere|air]]. Its oxide is historically known as [[Barium hydroxide|baryta]] but it reacts with water and carbon dioxide and is not found as a mineral. The most common naturally occurring minerals are the very insoluble barium sulfate, BaSO<sub>4</sub> ([[barite]]), and [[barium carbonate]], BaCO<sub>3</sub>([[witherite]]). Barium's name originates from [[Greek language|Greek]] ''barys'' (βαρύς), meaning "heavy", describing the high density of some common barium-containing ores.

Barium has few industrial applications, but the metal has been historically used to [[getter|scavenge air]] in [[vacuum tube]]s. Barium compounds impart a green color to flames and have been used in fireworks. [[Barium sulfate]] is used for its density, insolubility, and X-ray opacity. It is used as an insoluble heavy additive to oil well drilling mud, and in purer form, as an X-ray [[radiocontrast agent]] for imaging the human gastrointestinal tract. Soluble barium compounds are poisonous due to release of the soluble barium ion, and have been used as rodenticides. New uses for barium continue to be sought. It is a component of some "high temperature" [[YBCO]][[superconductors]], and electroceramics.

==f-block elements (lanthanides)==
{{main|Lanthanides}}
The '''lanthanide''' or '''lanthanoid''' ([[Chemical nomenclature|IUPAC nomenclature]])<ref>The current [[IUPAC]] recommendation is that the name ''lanthanoid'' be used rather than ''lanthanide'', as the suffix "-ide" is preferred for negative [[ion]]s whereas the suffix "-oid" indicates similarity to one of the members of the containing family of elements. However, ''lanthanide'' is still favored in most (~90%) scientific articles and is currently adopted on Wikipedia. In the older literature, the name "lanthanon" was often used.</ref> series comprises the fifteen [[metal]]lic [[chemical element]]s with [[atomic number]]s 57 through 71, from [[lanthanum]] through [[lutetium]].<ref name="Gray"/>{{rp|240}}<ref>[http://www.britannica.com.ph/chemistry/lanthanide-369725.html Lanthanide] {{webarchive|url=https://web.archive.org/web/20110911062241/http://www.britannica.com.ph/chemistry/lanthanide-369725.html |date=2011-09-11 }}, Encyclopædia Britannica on-line</ref><ref name="Holden2004">{{cite journal|author1=Holden, Norman E.|author2=Coplen, Tyler|name-list-style=amp|date=January–February 2004|journal=Chemistry International|title=The Periodic Table of the Elements|publisher=IUPAC|volume=26|issue=1|page=8|url=http://www.iupac.org/publications/ci/2004/2601/2_holden.html|access-date=March 23, 2010|archive-url=https://web.archive.org/web/20040217100116/http://www.iupac.org/publications/ci/2004/2601/2_holden.html|archive-date=February 17, 2004|url-status=dead}}</ref> These fifteen elements, along with the chemically similar elements [[scandium]] and [[yttrium]], are often collectively known as the '''[[rare-earth elements]].'''

The informal chemical symbol '''Ln''' is used in general discussions of lanthanide chemistry. All but one of the lanthanides are [[f-block]] elements, corresponding to the filling of the 4f [[electron shell]]; [[lanthanum]], a [[d-block]] element, is also generally considered to be a lanthanide due to its chemical similarities with the other fourteen. All lanthanide elements form trivalent cations, Ln<sup>3+</sup>, whose chemistry is largely determined by the [[ionic radius]], which decreases steadily from lanthanum to lutetium.

{| class="wikitable" style="background:#ffe; font-size:95%;"
![[Chemical element]]!![[Lanthanum|La]]!![[Cerium|Ce]]!![[praseodymium|Pr]]!![[neodymium|Nd]]!![[promethium|Pm]]!![[samarium|Sm]]!![[europium|Eu]]!![[gadolinium|Gd]]!![[terbium|Tb]]!![[dysprosium|Dy]]!![[holmium|Ho]]!![[erbium|Er]]!![[thulium|Tm]]!![[ytterbium|Yb]]!![[lutetium|Lu]]
|-
| [[Atomic number]]
|57||58||59||60||61||62||63||64||65||66||67||68||69||70||71
|-
|Image|| [[File:Lanthanum-2.jpg|50px]]||[[File:Cerium2.jpg|50px]]||[[File:Praseodymium.jpg|50px]]||[[File:Neodymium2.jpg|50px]]||||[[File:Samarium-2.jpg|50px]]||[[File:Europium.jpg|50px]]||[[File:Gadolinium-4.jpg|50px]]||[[File:Terbium-2.jpg|50px]]||[[File:Dy chips.jpg|50px]]||[[File:Holmium2.jpg|50px]]||[[File:Erbium-crop.jpg|50px]]||[[File:Thulium sublimed dendritic and 1cm3 cube.jpg|50px]]||[[File:Ytterbium-3.jpg|50px]]||[[File:Lutetium sublimed dendritic and 1cm3 cube.jpg|50px]]
|-
|Density (g/cm<sup>3</sup>)
|6.162 ||6.770 ||6.77 ||7.01 ||7.26 ||7.52 ||5.244 ||7.90 ||8.23 ||8.540 ||8.79 ||9.066 ||9.32 ||6.90 ||9.841
|-
|Melting point (°C)
|920 ||795 ||935 ||1024 ||1042 ||1072 ||826 ||1312 ||1356 ||1407 ||1461 ||1529 ||1545 ||824 ||1652
|-
| Atomic [[electron configuration]]*||'''5d<sup>1</sup>'''||4f<sup>1</sup>'''5d<sup>1</sup>'''||4f<sup>3</sup>|| 4f<sup>4</sup>|| 4f<sup>5</sup>||4f<sup>6</sup>||4f<sup>7</sup>|| 4f<sup>7</sup>'''5d<sup>1</sup>'''||4f<sup>9</sup>||4f<sup>10</sup>||4f<sup>11</sup>||4f<sup>12</sup>||4f<sup>13</sup>||4f<sup>14</sup>||4f<sup>14</sup>'''5d<sup>1</sup>'''
|-
| Ln<sup>3+</sup> electron configuration*<ref>{{cite book|author=Walter Koechner |title=Solid-state laser engineering |url=https://books.google.com/books?id=RK3jK0XWjdMC&pg=PA47 |access-date=15 January 2012 |year=2006 |publisher=Springer |isbn=978-0-387-29094-2|pages=47–}}</ref>||4f<sup>0</sup><ref>[http://www.chemistryexplained.com/Kr-Ma/Lanthanum.html Lanthanum – Chemistry Encyclopedia – reaction, water, elements, metal, gas, name, atom]. Chemistryexplained.com. Retrieved on 2012-01-15.</ref> || 4f<sup>1</sup>||4f<sup>2</sup>|| 4f<sup>3</sup>|| 4f<sup>4</sup>||4f<sup>5</sup>||4f<sup>6</sup>|| 4f<sup>7</sup>||4f<sup>8</sup>|| 4f<sup>9</sup>|| 4f<sup>10</sup>||4f<sup>11</sup>||4f<sup>12</sup>|| 4f<sup>13</sup>||
4f<sup>14</sup>
|-
| Ln<sup>3+</sup> radius ([[picometer|pm]])<ref name=Greenwood>{{Greenwood&Earnshaw|page=1233}}</ref> || 103 || 102 || 99|| 98.3|| 97|| 95.8|| 94.7|| 93.8|| 92.3|| 91.2|| 90.1|| 89||88|| 86.8|| 86.1
|}
*Between initial [Xe] and final 6s<sup>2</sup> electronic shells

The lanthanide elements are the group of elements with [[atomic number]] increasing from 57 (lanthanum) to 71 (lutetium). They are termed lanthanide because the lighter elements in the series are chemically similar to [[lanthanum]]. Strictly speaking, both lanthanum and lutetium have been labeled as [[group 3 element]]s, because they both have a single valence electron in the d shell. However, both elements are often included in any general discussion of the chemistry of the lanthanide elements.

In presentations of the [[periodic table]], the [[lanthanides]] and the [[actinides]] are customarily shown as two additional rows below the main body of the table,<ref name="Gray" /> with placeholders or else a selected single element of each series (either [[lanthanum]] or [[lutetium]], and either [[actinium]] or [[lawrencium]], respectively) shown in a single cell of the main table, between [[barium]] and [[hafnium]], and [[radium]] and [[rutherfordium]], respectively. This convention is entirely a matter of [[aesthetics]] and formatting practicality; a rarely used [[Periodic table (detailed cells)#32-column layout|wide-formatted periodic table]] inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods).

==d-block elements==

===Lutetium===
{{main|Lutetium}}
'''Lutetium''' ({{IPAc-en|l|juː|ˈ|t|iː|ʃ|i|ə|m}} {{respell|lew|TEE|shee-əm}}) is a [[chemical element]] with the symbol '''Lu''' and [[atomic number]] 71. It is the last element in the [[lanthanide]] series, which, along with the [[lanthanide contraction]], explains several important properties of lutetium, such as it having the highest hardness or density among lanthanides. Unlike other lanthanides, which lie in the [[f-block]] of the [[periodic table]], this element lies in the [[d-block]]; however, [[lanthanum]] is sometimes placed on the d-block lanthanide position. Chemically, lutetium is a typical lanthanide: its only common oxidation state is +3, seen in its oxide, halides and other compounds. In an aqueous solution, like compounds of other late lanthanides, soluble lutetium compounds form a complex with nine water molecules.

Lutetium was independently discovered in 1907 by French scientist [[Georges Urbain]], Austrian mineralogist Baron [[Carl Auer von Welsbach]], and American chemist [[Charles James (chemist)|Charles James]]. All of these men found lutetium as an impurity in the mineral [[ytterbia]], which was previously thought to consist entirely of ytterbium. The dispute on the priority of the discovery occurred shortly after, with Urbain and von Welsbach accusing each other of publishing results influenced by the published research of the other; the naming honor went to Urbain as he published his results earlier. He chose the name lutecium for the new element but in 1949 the spelling of element 71 was changed to lutetium. In 1909, the priority was finally granted to Urbain and his names were adopted as official ones; however, the name cassiopeium (or later cassiopium) for element 71 proposed by von Welsbach was used by many German scientists until the 1950s. Like other lanthanides, lutetium is one of the elements that traditionally were included in the classification "[[rare-earth element|rare earth]]s."

Lutetium is rare and expensive; consequently, it has few specific uses. For example, a [[radioactive isotope]] lutetium-176 is used in [[nuclear technology]] to determine the age of [[meteorite]]s. Lutetium usually occurs in association with the element [[yttrium]] and is sometimes used in metal [[alloy]]s and as a [[catalyst]] in various chemical reactions. <sup>177</sup>Lu-[[DOTA-TATE]] is used for radionuclide therapy (see [[Nuclear medicine]]) on neuroendocrine tumours.
<!------------<ref>{{cite news| url=http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html|title =IUPAC Provisional Recommendations for the Nomenclature of Inorganic Chemistry (online draft of an updated version of the "''Red Book''" IR 3-6)| date =2004| accessdate = 2009-06-06}}</ref>----------------->

===Hafnium===
{{main|Hafnium}}
'''Hafnium''' is a [[chemical element]] with the [[element symbol|symbol]] '''Hf''' and [[atomic number]] 72. A [[lustre (mineralogy)|lustrous]], silvery gray, [[tetravalence|tetravalent]] [[transition metal]], hafnium chemically resembles [[zirconium]] and is found in zirconium [[mineral]]s. Its existence was [[Mendeleev's predicted elements|predicted by Dmitri Mendeleev]] in 1869. Hafnium was the penultimate [[stable isotope]] element to be discovered ([[rhenium]] was identified two years later). Hafnium is named for ''Hafnia'', the [[Latin]] name for "[[Copenhagen]]", where it was discovered.

Hafnium is used in filaments and electrodes.  Some [[semiconductor]] fabrication processes use its oxide for [[integrated circuits]] at 45&nbsp;nm and smaller feature lengths. Some [[superalloy]]s used for special applications contain hafnium in combination with [[niobium]], [[titanium]], or [[tungsten]].

Hafnium's large [[neutron capture]] cross-section makes it a good material for [[neutron]] absorption in [[control rod]]s in [[nuclear power plant]]s, but at the same time requires that it be removed from the neutron-transparent corrosion-resistant zirconium alloys used in nuclear reactors.

===Tantalum===
{{main|Tantalum}}
'''Tantalum''' is a [[chemical element]] with the symbol '''Ta''' and [[atomic number]] 73. Previously known as ''tantalium'', the name comes from ''[[Tantalus]]'', a character from Greek mythology.<ref>[[Euripides]], ''[[Orestes (play)|Orestes]]''</ref> Tantalum is a rare, hard, blue-gray, [[lustre (mineralogy)|lustrous]] [[transition metal]] that is highly corrosion resistant. It is part of the [[refractory metals]] group, which are widely used as minor component in alloys. The chemical inertness of tantalum makes it a valuable substance for laboratory equipment and a substitute for [[platinum]], but its main use today is in [[tantalum capacitor]]s in [[electronics|electronic]] equipment such as [[mobile phone]]s, [[DVD player]]s, [[video game systems]] and [[Personal computer|computers]].
Tantalum, always together with the chemically similar [[niobium]], occurs in the [[mineral]]s [[tantalite]], [[columbite]] and [[coltan]] (a mix of columbite and tantalite).

===Tungsten===
{{main|Tungsten}}
'''Tungsten''', also known as '''wolfram''', is a [[chemical element]] with the chemical symbol '''W''' and [[atomic number]] 74. The word ''tungsten'' comes from the Swedish language ''tung sten'' directly translatable to ''heavy stone'',<ref>{{OED|Tungsten}}</ref> though the name is ''volfram'' in Swedish to distinguish it from [[Scheelite]], in Swedish alternatively named ''tungsten''.

A hard, rare [[metal]] under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as a metal in 1783. Its important [[ore]]s include [[wolframite]] and [[scheelite]]. The [[free element]] is remarkable for its robustness, especially the fact that it has the highest [[melting point]] of all the non-[[alloy]]ed metals and the second highest of all the elements after [[carbon]]. Also remarkable is its high density of 19.3 times that of water, comparable to that of [[uranium]] and [[gold]], and much higher (about 1.7 times) than that of [[lead]].<ref name="daintith">{{cite book |last=Daintith |first=John |title=Facts on File Dictionary of Chemistry |edition=4th |location=New York |publisher=Checkmark Books |year=2005 |isbn=978-0-8160-5649-1 }}</ref> Tungsten with minor amounts of impurities is often [[brittle]]<ref>{{cite book |title=Tungsten: properties, chemistry, technology of the element, alloys, and chemical compounds|first = Erik|last = Lassner|author2=Schubert, Wolf-Dieter | publisher = Springer|year = 1999|isbn = 978-0-306-45053-2|chapter-url = https://books.google.com/books?id=foLRISkt9gcC&pg=PA20|chapter = low temperature brittleness|pages = 20–21}}</ref> and [[hardness|hard]], making it difficult to [[metalworking|work]]. However, very pure tungsten, though still hard, is more [[ductility|ductile]], and can be cut with a hard-steel [[hacksaw]].<ref name="albert">{{cite book |last=Stwertka |first=Albert |title=A Guide to the elements |edition=2nd |location=New York |publisher=Oxford University Press |year=2002 |isbn=978-0-19-515026-1 }}</ref>

The unalloyed elemental form is used mainly in electrical applications. Tungsten's many alloys have numerous applications, most notably in incandescent [[light bulb]] filaments, [[X-ray tube]]s (as both the filament and target), electrodes in [[TIG welding]], and [[superalloys]]. Tungsten's hardness and high [[density]] give it military applications in penetrating [[projectile]]s. Tungsten compounds are most often used industrially as [[catalyst]]s.

Tungsten is the only metal from the third [[Transition metal|transition]] series that is known to occur in [[biomolecule]]s, where it is used in a few species of bacteria. It is the heaviest element known to be used by any living organism. Tungsten interferes with [[molybdenum]] and [[copper]]  metabolism, and is somewhat toxic to animal life.<ref>{{cite journal
|title = The active sites of molybdenum- and tungsten-containing enzymes
|author1=McMaster, J.  |author2=Enemark, John H
 |name-list-style=amp |journal = Current Opinion in Chemical Biology
|volume = 2
|issue = 2
|pages = 201–207
|year = 1998
|doi = 10.1016/S1367-5931(98)80061-6
|pmid = 9667924}}</ref><ref>{{cite journal
|title = Molybdenum and tungsten in biology
|author = Hille, Russ
|journal = Trends in Biochemical Sciences
|volume = 27
|issue = 7
|pages = 360–367
|year = 2002
|doi = 10.1016/S0968-0004(02)02107-2
|pmid = 12114025}}</ref>

===Rhenium===
{{main|Rhenium}}
'''Rhenium''' is a [[chemical element]] with the symbol '''Re''' and [[atomic number]] 75. It is a silvery-white, heavy, third-row [[transition metal]] in [[group 7 element|group 7]] of the [[periodic table]]. With an ''estimated'' average concentration of 1 [[Parts-per notation|part per billion]] (ppb), rhenium is one of the rarest elements in the [[Earth's crust]]. The free element has the [[List of elements by melting point|third-highest]] [[melting point]] and highest boiling point of any element. Rhenium resembles [[manganese]] chemically and is obtained as a [[by-product]] of [[molybdenum]] and [[copper]] ore's extraction and refinement. Rhenium shows in its compounds a wide variety of [[oxidation state]]s ranging from −1 to +7.

Discovered in 1925, rhenium was the last [[stable element]] to be discovered. It was named after the river [[Rhine]] in Europe.

[[Nickel]]-based [[superalloys]] of rhenium are used in the combustion chambers, turbine blades, and exhaust nozzles of [[jet engine]]s, these alloys  contain up to 6% rhenium, making jet engine construction the largest single use for the element, with the chemical industry's catalytic uses being next-most important. Because of the low availability relative to demand, rhenium is among the most expensive of metals, with an average price of approximately US$4,575 per [[kilogram]] (US$142.30 per troy ounce) as of August 2011; it is also of critical strategic military importance, for its use in high performance military jet and rocket engines.<ref>{{cite web |url=http://www.metalprices.com/FreeSite/metals/re/re.asp |title=Rhenium |work=MetalPrices.com |access-date=February 2, 2012 |archive-date=January 15, 2012 |archive-url=https://web.archive.org/web/20120115004912/http://www.metalprices.com/FreeSite/metals/re/re.asp |url-status=dead }}</ref>

===Osmium===
{{main|Osmium}}
'''Osmium''' is a [[chemical element]] with the symbol '''Os''' and [[atomic number]] 76. It is a hard, brittle, blue-gray or blue-black [[transition metal]] in the [[platinum family]] and is the densest naturally occurring element, with a [[density]] of  {{val|22.59|ul=g/cm3}} (slightly greater than that of [[iridium]] and twice that of [[lead]]). It is found in nature as an alloy, mostly in platinum ores; its [[alloy]]s with [[platinum]], [[iridium]], and other platinum group metals are employed in [[fountain pen]] tips, electrical contacts, and other applications where extreme durability and hardness are needed.<ref>Hammond "Osmium", C. R., p. 4-25 in {{RubberBible86th}}</ref>

===Iridium===
{{main|Iridium}}
'''Iridium''' is the [[chemical element]] with [[atomic number]] 77, and is represented by the symbol '''Ir'''. A very hard, brittle, silvery-white [[transition metal]] of the [[platinum group|platinum family]], iridium is the second-[[density|densest]] element (after [[osmium]]) and is the most [[corrosion]]-resistant metal, even at temperatures as high as 2000&nbsp;°C. Although only certain molten salts and [[halogen]]s are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.

Iridium was discovered in 1803 among insoluble impurities in natural [[platinum]]. [[Smithson Tennant]], the primary discoverer, named the iridium for the goddess [[Iris (mythology)|Iris]], personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is [[Abundance of elements in Earth's crust|one of the rarest elements]] in the [[Crust (geology)#Earth's crust|Earth's crust]], with annual production and consumption of only three [[tonne]]s. {{chem|191|Ir}} and {{chem|193|Ir}} are the only two naturally occurring [[isotope]]s of iridium as well as the only [[stable isotope]]s; the latter is the more abundant of the two.

The most important iridium compounds in use are the salts and acids it forms with [[chlorine]], though iridium also forms a number of [[organometallic compound]]s used in industrial [[catalysis]], and in research. Iridium metal is employed when high corrosion resistance at high temperatures is needed, as in high-end [[spark plug]]s, [[crucible]]s for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the [[chloralkali process]]. Iridium radioisotopes are used in some [[radioisotope thermoelectric generator]]s.

Iridium is found in meteorites with an abundance much higher than its average abundance in the Earth's crust. For this reason the unusually high abundance of iridium in the clay layer at the [[Cretaceous–Paleogene boundary]] gave rise to the [[Alvarez hypothesis]] that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 66&nbsp;million years ago. It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum group metals, the high density and [[Goldschmidt classification#Siderophile elements|tendency]] of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.

===Platinum===
{{main|Platinum}}
'''Platinum''' is a [[chemical element]] with the [[chemical symbol]] '''Pt''' and an [[atomic number]] of 78.

Its name is derived from the Spanish term ''platina'', which is literally translated into "little silver".<ref>[http://www.britannica.com/EBchecked/topic/464081/platinum-Pt "platinum (Pt)."] Encyclopædia Britannica Online. Encyclopædia Britannica Inc., 2012. Web. 24 April 2012</ref><ref>{{OEtymD|platinum}}</ref><!--source for "platina del Pinto"<ref>{{cite book|last=Woods|first=Ian|title=The Elements: Platinum|publisher=Benchmark Books|year=2004|series=The Elements|isbn=978-0-7614-1550-3}}</ref>--> It is a [[density|dense]], [[malleability|malleable]], [[ductility|ductile]], [[precious metal|precious]], gray-white [[transition metal]].

Platinum has six naturally occurring [[isotopes of platinum|isotopes]]. It is one of the [[Abundance of elements in Earth's crust|rarest elements in the Earth's crust]] and has an average abundance of approximately 5&nbsp;μg/kg. It is the [[Reactivity series|least reactive metal]]. It occurs in some [[nickel]] and [[copper]] ores along with some native deposits, mostly in South Africa, which accounts for 80% of the world production.

As a member of the [[platinum group]] of elements, as well as of the [[group 10 element|group 10]] of the [[periodic table of elements]], platinum is generally non-reactive. It exhibits a remarkable resistance to corrosion, even at high temperatures, and as such is considered a [[noble metal]]. As a result, platinum is often found chemically uncombined as native platinum. Because it occurs naturally in the [[alluvium|alluvial sands]] of various rivers, it was first used by [[pre-Columbian]] South American natives to produce artifacts. It was referenced in European writings as early as 16th century, but it was not until [[Antonio de Ulloa]] published a report on a new metal of [[Colombia]]n origin in 1748 that it became investigated by scientists.

Platinum is used in [[catalytic converter]]s, laboratory equipment, [[electrical contacts]] and [[electrode]]s, [[platinum-resistance thermometer]]s, [[dentistry]] equipment, and jewelry. Because only a few hundred tonnes are produced annually, it is a scarce material, and is highly valuable. Being a [[heavy metals|heavy metal]], it leads to health issues upon exposure to its salts, but due to its corrosion resistance, it is not as toxic as some metals.<ref>{{cite web|url= http://www.euro.who.int/__data/assets/pdf_file/0015/123081/AQG2ndEd_6_11Platinum.PDF|title= Platinum&#124;|access-date= 2012-08-28|archive-date= 2012-10-18|archive-url= https://web.archive.org/web/20121018173735/http://www.euro.who.int/__data/assets/pdf_file/0015/123081/AQG2ndEd_6_11Platinum.PDF|url-status= dead}}</ref> Its compounds, most notably [[cisplatin]], are applied in [[chemotherapy]] against certain types of cancer.<ref>{{cite journal | pmid = 20593091 | year = 2010 | last1 = Wheate | first1 = N. J. | last2 = Walker | first2 = S. | last3 = Craig | first3 = G. E. | last4 = Oun | first4 = R. | title = The status of platinum anticancer drugs in the clinic and in clinical trials | volume = 39 | issue = 35 | pages = 8113–27 | doi = 10.1039/C0DT00292E | journal = Dalton Transactions| url = https://ses.library.usyd.edu.au/bitstream/2123/9269/2/41%20Dalton%20perspective.pdf | hdl = 2123/14271 | hdl-access = free }}</ref>

===Gold===
{{main|Gold}}
'''Gold''' is a dense, soft, shiny, malleable and ductile metal.  It is a [[chemical element]] with the symbol '''Au''' and [[atomic number]] 79.

Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a [[transition metal]] and a [[group 11 element]]. It is one of the least reactive chemical elements solid under standard conditions. The metal therefore occurs often in free elemental (native) form, as [[gold nugget|nuggets]] or grains in rocks, in [[vein (geology)|veins]] and in [[alluvial deposit]]s. Less commonly, it occurs in minerals as gold compounds, usually with [[tellurium]].

Gold resists attacks by individual acids, but it can be dissolved by the [[aqua regia]] (nitro-[[hydrochloric acid]]), so named because it dissolves gold. Gold also dissolves in alkaline solutions of [[cyanide]], which have been used in mining. Gold dissolves in [[mercury (element)|mercury]], forming [[amalgam (chemistry)|amalgam]] alloys. Gold is insoluble in [[nitric acid]], which dissolves [[silver]] and [[base metal]]s, a property that has long been used to confirm the presence of gold in items, giving rise to the term ''the [[acid test (gold)|acid test]].''

Gold has been a valuable and highly sought-after [[precious metal]] for [[coin]]age, jewelry, and other arts since long before the beginning of [[recorded history]]. [[Gold standard]]s have been a common basis for [[monetary policy|monetary policies]] throughout human history,{{Citation needed|date=April 2012}} later being supplanted by [[fiat currency]] starting in the 1930s. The last [[gold certificate]] and [[gold coin]] currencies were issued in the U.S. in 1932. In Europe, most countries left the gold standard with the start of [[World War I]] in 1914 and, with huge war debts, failed to return to gold as a medium of exchange.

A total of 165,000 [[tonne]]s of gold have been mined in human history, as of 2009.<ref name="World Gold Council FAQ">[http://www.gold.org/faq/answer/76/how_much_gold_has_been_mined/ World Gold Council FAQ]. www.gold.org</ref> This is roughly equivalent to 5.3 billion [[troy ounce]]s or, in terms of volume, about 8500 m<sup>3</sup>, or a [[cube]] 20.4 m on a side. The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.<ref name='oil-price.com-worlds-gold-consumption 2011'>{{cite news|first = Andy |last = Soos|title = Gold Mining Boom Increasing Mercury Pollution Risk|date = 2011-01-06|publisher = Oilprice.com|url = http://oilprice.com/Metals/Gold/Gold-Mining-Boom-Increasing-Mercury-Pollution-Risk.html|work = Advanced Media Solutions, Inc.|access-date = 2011-03-26}}</ref>

Besides its widespread monetary and symbolic functions, gold has many practical uses in [[dentistry]], [[electronics]], and other fields. Its high [[malleability]], [[ductility]], resistance to corrosion and most other chemical reactions, and conductivity of electricity led to many uses of gold, including [[electric wiring]], colored-glass production and even [[gold leaf]] eating.

It has been claimed that most of the Earth's gold lies at its core, the metal's high density having made it sink there in the planet's youth.  Virtually all of the gold that mankind has discovered is considered to have been deposited later by [[meteorites]] which contained the element.  This supposedly explains why, in prehistory, gold appeared as nuggets on the earth's surface.<ref>{{cite news| url=https://www.bbc.co.uk/news/science-environment-14827624 | work=BBC News | title=Meteorites delivered gold to Earth | date=2011-09-08}}</ref><ref>{{Cite web | url=https://www.sciencedaily.com/releases/2011/09/110907132044.htm |title = Where does all Earth's gold come from? Precious metals the result of meteorite bombardment, rock analysis finds}}</ref><ref>{{Cite journal |last1=Kirk |first1=Jason |last2=Ruiz |first2=Joaquin |last3=Chesley |first3=John |last4=Titley |first4=Spencer |date=2003 |title=The Origin of Gold in South Africa |url=http://www.sas.rochester.edu/ees/ees119/reading2.pdf |journal=American Scientist |volume=91 |issue=6 |pages=534–541 |doi=10.1511/2003.38.534 |issn=0003-0996}}</ref><ref>{{cite news| url=http://www.huffingtonpost.com/2011/09/10/meteor-shower-gold_n_955448.html | work=Huffington Post | title=Meteor Shower Rained Gold On Ancient Earth | date=2011-09-10}}</ref><ref>{{Cite journal |doi = 10.1038/nature10399|pmid = 21901010|title = The tungsten isotopic composition of the Earth's mantle before the terminal bombardment|journal = Nature|volume = 477|issue = 7363|pages = 195–198|year = 2011|last1 = Willbold|first1 = Matthias|last2 = Elliott|first2 = Tim|last3 = Moorbath|first3 = Stephen|bibcode = 2011Natur.477..195W|s2cid = 4419046}}</ref>

===Mercury===
{{main|Mercury (element)}}
'''Mercury''' is a [[chemical element]] with the symbol '''Hg''' and [[atomic number]] 80. It is also known as '''quicksilver'''<!--ref>{{cite web|url=http://dictionary.reference.com/browse/quicksilver|title=quicksilver definition |access-date=13 October 2008|publisher=Dictionary.com Unabridged (v 1.1)}}</ref---> or '''hydrargyrum''' ( < Greek "[[Wiktionary:en:hydr-|hydr-]]" ''water'' and "[[Wiktionary:en:άργυρος|argyros]]" ''silver''). A heavy, silvery [[d-block]] element, mercury is the only metal that is liquid at [[standard conditions for temperature and pressure]]; the only other element that is liquid under these conditions is [[bromine]], though metals such as [[caesium]], [[francium]], [[gallium]], and [[rubidium]] melt just above room temperature. With a [[freezing point]] of −38.83&nbsp;°C and [[boiling point]] of 356.73&nbsp;°C, mercury has one of the narrowest ranges of its liquid state of any metal.<ref>{{cite web| url=http://antoine.frostburg.edu/chem/senese/101/inorganic/faq/why-is-mercury-liquid.shtml| title=Why is mercury a liquid at STP?| access-date=May 1, 2007| publisher=General Chemistry Online at Frostburg State University| author=Senese, F| archive-date=August 31, 2011| archive-url=https://web.archive.org/web/20110831053345/http://antoine.frostburg.edu/chem/senese/101/inorganic/faq/why-is-mercury-liquid.shtml| url-status=dead}}</ref><ref name="Norrby">{{cite journal|author=Norrby, L.J.|title=Why is mercury liquid? Or, why do relativistic effects not get into chemistry textbooks?| journal= Journal of Chemical Education|volume=68|issue=2|page=110 |year=1991|doi=10.1021/ed068p110|bibcode=1991JChEd..68..110N}}</ref><ref>{{RubberBible86th|pages=4.125–4.126}}</ref>

Mercury occurs in deposits throughout the world mostly as [[cinnabar]] ([[mercuric sulfide]]). The red pigment [[vermilion]] is mostly obtained by reduction from cinnabar. Cinnabar is highly toxic by ingestion or inhalation of the dust. [[Mercury poisoning]] can also result from exposure to water-soluble forms of mercury (such as [[mercuric chloride]] or [[methylmercury]]), inhalation of mercury vapor, or eating seafood contaminated with mercury.

Mercury is used in [[thermometer]]s, [[barometer]]s, [[manometer]]s, [[sphygmomanometer]]s, [[float valve]]s, [[mercury switch]]es, and other devices though concerns about the element's toxicity have led to mercury thermometers and sphygmomanometers being largely phased out in clinical environments in favor of [[ethanol|alcohol]]-filled, [[galinstan]]-filled, digital, or [[thermistor]]-based instruments. It remains in use in scientific research applications and in [[amalgam (dentistry)|amalgam]] material for [[dental restoration]]. It is used in lighting: electricity passed through mercury vapor in a phosphor tube produces short-wave [[ultraviolet light]] which then causes the phosphor to [[fluoresce]], making visible light.

==p-block elements==

===Thallium===
{{main|Thallium}}
'''Thallium''' is a chemical element with the symbol '''Tl''' and atomic number 81. This soft gray [[other metal]] resembles [[tin]] but discolors when exposed to air. The two chemists [[William Crookes]] and [[Claude-Auguste Lamy]] discovered thallium independently in 1861 by the newly developed method of [[Atomic emission spectroscopy#Flame emission spectroscopy|flame spectroscopy]]. Both discovered the new element in residues of [[sulfuric acid]] production.

Approximately 60–70% of thallium production is used in the [[electronics industry]], and the remainder is used in the [[pharmaceutical industry]] and in [[glass|glass manufacturing]].<ref name="sl2001">{{cite web|title=Chemical fact sheet&nbsp;— Thallium|publisher=Spectrum Laboratories|date=April 2001|url=http://www.speclab.com/elements/thallium.htm|access-date=2008-02-02|archive-url=https://web.archive.org/web/20080221222321/http://www.speclab.com/elements/thallium.htm|archive-date=2008-02-21|url-status=dead}}</ref> It is also used in [[infrared detector]]s. Thallium is highly [[toxic]] and was used in [[rat poison]]s and [[insecticide]]s. Its use has been reduced or eliminated in many countries because of its nonselective toxicity. Because of its use for [[murder]], thallium has gained the nicknames "The Poisoner's Poison" and "Inheritance Powder" (alongside [[arsenic]]).<ref>{{cite book|title = The Boron Elements: Boron, Aluminum, Gallium, Indium, Thallium| page = 14|first =Heather|last = Hasan|year = 2009| isbn = 978-1-4358-5333-1|publisher = Rosen Publishing Group}}</ref>

===Lead===
{{main|Lead}}
'''Lead''' is a main-group [[Chemical element|element]] in the [[carbon group]] with the symbol '''Pb''' (from {{langx|la|plumbum}}) and [[atomic number]] 82. Lead is a soft, [[malleable]] [[other metal]]. It is also counted as one of the [[heavy metal (chemistry)|heavy metal]]s. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed to air. Lead has a shiny chrome-silver luster when it is melted into a liquid.

Lead is used in building construction, [[lead–acid battery|lead-acid batteries]], [[bullet]]s and [[lead shot|shot]]s, weights, as part of [[solder]]s, [[pewter]]s, [[fusible alloy]]s and as a [[radiation shield]]. Lead has the highest [[atomic number]] of all of the [[stable element]]s, although the next higher element, [[bismuth]], has a [[half-life]] that is so long (much longer than the age of the universe) that it can be considered stable. Its four stable isotopes have 82 [[proton]]s, a [[magic number (physics)|magic number]] in the [[nuclear shell model]] of [[atomic nuclei]].

Lead, at certain exposure levels, is a poisonous substance to animals as well as for human beings. It damages the [[nervous system]] and causes [[brain]] disorders. Excessive lead also causes blood disorders in mammals. Like the element [[mercury (element)|mercury]], another heavy metal, lead is a [[neurotoxin]] that accumulates both in soft tissues and the bones. [[Lead poisoning]] has been documented from [[ancient Rome]], [[ancient Greece]], and [[History of China#Ancient China|ancient China]].

===Bismuth===
{{main|Bismuth}}
'''Bismuth''' is a [[chemical element]] with symbol '''Bi''' and [[atomic number]] 83. Bismuth, a trivalent [[other metal]], chemically resembles [[arsenic]] and [[antimony]]. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The [[free element]] is 86% as dense as [[lead]]. It is a brittle metal with a silvery white color when newly made, but often seen in air with a pink tinge owing to the surface oxide. Bismuth metal has been known from ancient times, although until the 18th century it was often confused with lead and tin, which each have some of bismuth's bulk physical properties. The etymology is uncertain but possibly comes from Arabic {{Transliteration|ar|bi ismid}} meaning having the properties of antimony<ref>[http://webmineral.com/data/Bismuth.shtml Bismuth]. Web Mineral. Retrieved on 2011-12-17.</ref> or German words {{lang|de|weisse masse}} or {{lang|de|wismuth}}  meaning "white mass".<ref name="arizona1">{{cite book|editor1=Anthony, John W. |editor2=Bideaux, Richard A. |editor3=Bladh, Kenneth W. |editor4=Nichols, Monte C. |title=Handbook of Mineralogy|publisher=Mineralogical Society of America |place=Chantilly, VA, US |volume=I (Elements, Sulfides, Sulfosalts) |chapter-url=http://rruff.geo.arizona.edu/doclib/hom/bismuth.pdf|chapter=Bismuth |date=1990 |access-date=December 5, 2011 |isbn=978-0-9622097-0-3 }}</ref>

Bismuth is the most naturally [[Diamagnetism|diamagnetic]] of all metals, and only [[mercury (element)|mercury]] has a lower [[thermal conductivity]].

Bismuth has classically been considered to be the heaviest naturally occurring stable element, in terms of atomic mass. Recently, however, it has been found to be very slightly radioactive: its only primordial isotope [[bismuth-209]] decays via [[alpha decay]] into [[thallium-205]] with a [[half-life]] of more than a [[1000000000 (number)|billion]] times the estimated [[age of the universe]].<ref>{{cite news| url=http://physicsweb.org/articles/news/7/4/16| title=Bismuth breaks half-life record for alpha decay| date=2003-04-23| publisher=Physicsweb| first=Belle| last=Dumé| archive-date=2007-06-30| access-date=2012-02-13| archive-url=https://web.archive.org/web/20070630034928/http://physicsweb.org/articles/news/7/4/16| url-status=dead}}</ref>

Bismuth compounds (accounting for about half the production of bismuth) are used in [[cosmetics]], pigments, and a few pharmaceuticals. Bismuth has unusually low [[toxicity]] for a heavy metal. As the toxicity of [[lead]] has become more apparent in recent years, alloy uses for bismuth metal (presently about a third of bismuth production), as a replacement for lead, have become an increasing part of bismuth's commercial importance.

===Polonium===
{{main|Polonium}}
'''Polonium''' is a [[chemical element]] with the symbol '''Po''' and [[atomic number]] 84, discovered in 1898 by [[Marie Curie|Marie Skłodowska-Curie]] and [[Pierre Curie]]. A rare and highly [[radioactive]] element, polonium is chemically similar to [[bismuth]]<ref>{{cite web| url = http://hyperphysics.phy-astr.gsu.edu/hbase/pertab/Po.html |title = Polonium| access-date = 2009-05-05}}</ref> and [[tellurium]], and it occurs in [[uranium]] [[ore]]s. Polonium has been studied for possible use in heating [[spacecraft]]. As it is unstable, all [[isotopes of polonium]] are radioactive. There is disagreement as to whether polonium is a [[post-transition metal]] or [[metalloid]].<ref>{{cite journal| doi=10.1021/ed100308w |title = Polonium and Astatine Are Not Semimetals|journal= Journal of Chemical Education| year=2010| last1=Hawkes| first1=Stephen J.| volume=87| issue=8| pages=783 |bibcode = 2010JChEd..87..783H }}</ref><ref>{{cite web |url=http://periodic.lanl.gov/metal.shtml |title=Characterizing the Elements |author=<!--Staff writer(s); no by-line.--> |publisher=[[Los Alamos National Laboratory]] |access-date=4 March 2013}}</ref>

===Astatine===
{{main|Astatine}}
'''Astatine''' is a [[radioactive]] [[chemical element]] with the symbol '''At''' and [[atomic number]] 85. It occurs on the Earth only as the result of decay of heavier elements, and decays away rapidly, so much less is known about this element than its upper neighbors in the [[periodic table]]. Earlier studies have shown this element follows periodic trends, being the heaviest known [[halogen]], with [[melting point|melting]] and [[boiling point]]s being higher than those of lighter halogens.

Until recently most of the chemical characteristics of astatine were inferred from comparison with other elements; however, important studies have already been done. The main difference between astatine and [[iodine]] is that the HAt molecule is chemically a [[hydride]] rather than a [[halide]]; however, in a fashion similar to the lighter halogens, it is known to form ionic astatides with metals. Bonds to [[Nonmetal (chemistry)|nonmetal]]s result in positive [[oxidation state]]s, with +1 best portrayed by monohalides and their derivatives, while the higher are characterized by bond to oxygen and carbon. Attempts to synthesize astatine fluoride have been met with failure. The second longest-living astatine-211 is the only one to find a commercial use, being useful as an [[alpha decay|alpha emitter]] in medicine; however, only extremely small quantities are used, and in larger ones it is very hazardous, as it is intensely radioactive.

Astatine was first produced by [[Dale R. Corson]], [[Kenneth Ross MacKenzie]], and [[Emilio Segrè]] in the [[University of California, Berkeley]] in 1940. Three years later, it was found in nature; however, with an estimated amount of less than 28&nbsp;grams (1&nbsp;oz) at given time, astatine is the least abundant element in Earth's crust among non-[[transuranium element]]s. Among astatine isotopes, four (with [[mass number]]s 215, 217, 218 and 219) are present in nature as the result of decay of heavier elements; however, the most stable astatine-210 and the industrially used astatine-211 are not.

===Radon===
{{main|Radon}}
'''Radon''' is a [[chemical element]] with symbol '''Rn''' and [[atomic number]] 86. It is a [[radioactive decay|radioactive]], colorless, odorless, tasteless<ref>{{Cite book|title=Britannica Concise Encyclopedia|publisher=Encyclopaedia Britannica: Britannica Digital Learning|year=2017|via=Credo Reference}}</ref> [[noble gas]], occurring naturally as the decay product of [[uranium]] or [[thorium]]. Its most stable [[isotope]], [[Radon-222|<sup>222</sup>Rn]], has a [[half-life]] of 3.8 days. Radon is one of the densest substances that remains a [[gas]] under normal conditions. It is also the only gas that is radioactive under normal conditions, and is considered a health hazard due to its radioactivity. Intense radioactivity also hindered chemical studies of radon and only a few compounds are known.

Radon is formed as part of the normal radioactive [[decay chain]] of uranium and thorium. Uranium and thorium have been around since the earth was formed and their [[isotopes of thorium|most common isotope]] has a very long half-life (14.05 billion years). Uranium and thorium, [[radium]], and thus radon, will continue to occur for millions of years at about the same concentrations as they do now.<ref name=USPHS90>[http://www.bvsde.paho.org/bvstox/i/fulltext/toxprofiles/radon.pdf Toxological profile for radon] {{Webarchive|url=https://web.archive.org/web/20160415161629/http://www.bvsde.paho.org/bvstox/i/fulltext/toxprofiles/radon.pdf |date=2016-04-15 }}, [[Agency for Toxic Substances and Disease Registry]], U.S. Public Health Service, In collaboration with U.S. Environmental Protection Agency, December 1990.</ref> As the radioactive gas of radon decays, it produces new radioactive elements called radon daughters or decay products. Radon daughters are solids and stick to surfaces such as dust particles in the air. If contaminated dust is inhaled, these particles can stick to the airways of the lung and increase the risk of developing lung cancer.<ref>{{cite web|url=http://www.mass.gov/eohhs/consumer/community-health/environmental-health/exposure-topics/radiation/radon/public-health-fact-sheet-on-radon.html |title=Public Health Fact Sheet on Radon – Health and Human Services |publisher=Mass.Gov |access-date=2011-12-04}}</ref>

Radon is responsible for the majority of the public exposure to [[ionizing radiation]]. It is often the single largest contributor to an individual's [[background radiation]] dose, and is the most variable from location to location. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as attics and basements. It can also be found in some [[Spring (hydrosphere)|spring waters]] and hot springs.<ref>{{cite web|title=Facts about Radon|publisher=Facts about|url=http://www.facts-about.org.uk/science-element-radon.htm|access-date=2008-09-07|url-status=dead|archive-url=https://web.archive.org/web/20050222004131/http://www.facts-about.org.uk/science-element-radon.htm|archive-date=2005-02-22}}</ref>

[[Epidemiological]] studies have shown a clear link between breathing high concentrations of radon and incidence of [[lung cancer]]. Thus, radon is considered a significant contaminant that affects [[indoor air quality]] worldwide. According to the [[United States Environmental Protection Agency]], radon is the second most frequent cause of lung cancer, after cigarette smoking, causing 21,000 lung cancer deaths per year in the [[United States]]. About 2,900 of these deaths occur among people who have never smoked. While radon is the second most frequent cause of lung cancer, it is the number one cause among non-smokers, according to EPA estimates.<ref name="epa">{{cite web|url=http://www.epa.gov/radon/pubs/citguide.html|archive-url=https://archive.today/20120527011445/http://www.epa.gov/radon/pubs/citguide.html|url-status=dead|archive-date=May 27, 2012|title=A Citizen's Guide to Radon|date=October 12, 2010|work=www.epa.gov|publisher=[[United States Environmental Protection Agency]]|access-date=January 29, 2012}}</ref>

==Biological role==
Of the period 6 elements, only tungsten and the early lanthanides<ref>{{cite journal |last1=Daumann |first1=Lena J. |date=25 April 2019 |title=Essential and Ubiquitous: The Emergence of Lanthanide Metallobiochemistry |url=https://onlinelibrary.wiley.com/doi/epdf/10.1002/anie.201904090?referrer_access_token=2weSaqy0h1TxFG1Y91YOeU4keas67K9QMdWULTWMo8ObaCCIszBDH3oGZO5RKOo1C6bwCnOveT5jp0zxjuFs4kN_POnWtLvIF7-qVmB_SDcaXWHzvlHJdFxs4BV7VRxcDJuBOi-tPV-x4uED2cp893utgoNphFeKKKe7HUBBSUU%3D |journal=Angewandte Chemie International Edition |volume=58 |issue=37 |pages=12795–12802 |doi=10.1002/anie.201904090 |pmid=31021478 |access-date=15 June 2019|url-access=subscription }}</ref> are known to have any biological role in organisms, and even then only in lower organisms (not mammals). However, gold, platinum, mercury, and some lanthanides such as gadolinium have applications as drugs.

==Toxicity==
Most of the period 6 elements are toxic (for instance lead) and produce [[heavy-element poisoning]]. Promethium, polonium, astatine and radon are radioactive, and therefore present radioactive hazards.

==Notes==
{{Reflist|group=note}}

==References==
{{Reflist}}
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[[Category:Periods (periodic table)]]
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