View source for Period 4 element

{{Short description|Fourth row in the periodic table of chemical elements}}
{{Periodic table (micro)| title=Period 4 in the [[periodic table]] | mark=K,Ca,Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn,Ga,Ge,As,Se,Br,Kr}}
{{Sidebar periodic table|expanded=structure }}
A '''period 4 element''' is one of the [[chemical element]]s in the fourth row (or [[Period (periodic table)|period]]) of the [[periodic table|periodic table of the chemical elements]]. 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 fourth period contains 18 elements [[#s-block elements|beginning with potassium]] and ending with [[#Krypton|krypton]]&nbsp;– one element for each of the [[group (periodic table)|eighteen groups]]. It sees the first appearance of [[d-block]] (which includes [[transition metal]]s) in the table.

==Properties==

All 4th-period elements are [[Radioactive element|stable]],<ref>{{cite web|url=https://www.science.co.il/elements/?s=Earth |title=List of Elements of the Periodic Table – Sorted by Abundance in Earth's crust |publisher=Science.co.il |access-date=2012-08-14}}</ref> and many are extremely common in the [[Earth's crust]] and/or [[Earth's core|core]]; it is the last period with no unstable elements. Many transition metals in the period are very [[Strength of materials|strong]], and therefore common in [[industry (manufacturing)|industry]], especially [[#Iron|iron]].<ref name=Gray /> Some are [[toxic]], with all known [[#Vanadium|vanadium]] compounds toxic,<ref>{{cite journal |last=Srivastava |first=A. K. |year=2000 |title=Anti-diabetic and toxic effects of vanadium compounds |journal=Molecular and Cellular Biochemistry |volume=206 |issue=206 |pages=177–182 |doi=10.1023/A:1007075204494 |pmid=10839208 |s2cid=8871862}}</ref> [[#Arsenic|arsenic]] one of the most well-known [[poison]]s, and [[#Bromine|bromine]] a toxic liquid. Conversely, many elements are essential to human survival, such as [[#Calcium|calcium]], the main component in [[bones]].<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 structure===
Progressing towards increase of [[atomic number]], the [[Aufbau principle]] causes elements of the period to put [[electron]]s onto 4s, 3d, and 4p subshells, in that order. However, there are exceptions, such as [[#Chromium|chromium]]. The first twelve elements—[[#Potassium|K]], [[#Calcium|Ca]], [[#d-block elements|and transition metals]]—have from 1 to 12 [[valence electron]]s respectively, which are placed on 4s and 3d.

Twelve electrons over the [[electron configuration]] of [[argon]] reach the configuration of [[#Zinc|zinc]], namely 3d<sup>10</sup> 4s<sup>2</sup>. After this element, the filled 3d subshell effectively withdraws from chemistry and the subsequent [[periodic trends|trend]] looks much like trends in the periods [[period 2 element|2]] and [[period 3 element|3]]. [[#p-block elements|The p-block elements]] of period&nbsp;4 have their [[valence shell]] composed of 4s and 4p subshells of the [[electron shell|fourth ({{math|1=''n'' = 4}}) shell]] and obey the [[octet rule]].

For [[quantum chemistry]] namely this period sees transition from the simplified [[electron shell]] paradigm to research of many [[azimuthal quantum number|differently-shaped subshells]]. The relative disposition of their [[energy level]]s is governed by the interplay of various physical effects. The period's [[#s-block elements|s-block metals]] put their differentiating electrons onto 4s despite having vacancies among nominally lower [[principal quantum number|{{math|1=''n'' = 3}} states]]&nbsp;– a phenomenon unseen in lighter elements. Contrariwise, the six elements from [[#Gallium|gallium]] to [[#Krypton|krypton]] are the heaviest where all electron shells below the valence shell are filled ''completely''. This is no longer possible in further periods due to the existence of f-subshells starting from {{math|1=''n'' = 4}}.

==List of elements==
:{| class="wikitable sortable"
! colspan="3" | [[Chemical element]]
! [[Block (periodic table)|Block]]
! [[Electron configuration]]
|-
! &nbsp;
!
!
!
!
|- bgcolor="{{element color|s-block}}"
|| 19  || '''K''' || [[Potassium]] || [[s-block]] || [Ar] 4s<sup>1</sup>
|- bgcolor="{{element color|s-block}}"
|| 20  || '''Ca''' || [[Calcium]] || [[s-block]] || [Ar] 4s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 21  || '''Sc''' || [[Scandium]] || [[d-block]] || [Ar] 3d<sup>1</sup> 4s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 22  || '''Ti''' || [[Titanium]] || [[d-block]] || [Ar] 3d<sup>2</sup> 4s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 23  || '''V''' || [[Vanadium]] || [[d-block]] || [Ar] 3d<sup>3</sup> 4s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 24  || '''Cr''' || [[Chromium]] || [[d-block]] || [Ar] 3d<sup>5</sup> 4s<sup>1</sup> (*)
|- bgcolor="{{element color|d-block}}"
|| 25  || '''Mn''' || [[Manganese]] || [[d-block]] || [Ar] 3d<sup>5</sup> 4s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 26 || '''Fe''' || [[Iron]] || [[d-block]] || [Ar] 3d<sup>6</sup> 4s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 27 || '''Co''' || [[Cobalt]] || [[d-block]] || [Ar] 3d<sup>7</sup> 4s<sup>2</sup>
|- bgcolor="{{element color|d-block}}"
|| 28 || '''Ni''' || [[Nickel]] || [[d-block]] || [Ar] 3d<sup>8</sup> 4s<sup>2</sup> 
|- bgcolor="{{element color|d-block}}"
|| 29 || '''Cu''' || [[Copper]] || [[d-block]] || [Ar] 3d<sup>10</sup> 4s<sup>1</sup> (*)
|- bgcolor="{{element color|d-block}}"
|| 30 || '''Zn''' || [[Zinc]] || [[d-block]] || [Ar] 3d<sup>10</sup> 4s<sup>2</sup>
|- bgcolor="{{element color|p-block}}"
|| 31 || '''Ga''' || [[Gallium]] || [[p-block]] || [Ar] 3d<sup>10</sup> 4s<sup>2</sup> 4p<sup>1</sup>
|- bgcolor="{{element color|p-block}}"
|| 32 || '''Ge''' || [[Germanium]] || [[p-block]] || [Ar] 3d<sup>10</sup> 4s<sup>2</sup> 4p<sup>2</sup>
|- bgcolor="{{element color|p-block}}"
|| 33 || '''As''' || [[Arsenic]] || [[p-block]] || [Ar] 3d<sup>10</sup> 4s<sup>2</sup> 4p<sup>3</sup>
|- bgcolor="{{element color|p-block}}"
|| 34 || '''Se''' || [[Selenium]] || [[p-block]] || [Ar] 3d<sup>10</sup> 4s<sup>2</sup> 4p<sup>4</sup>
|- bgcolor="{{element color|p-block}}"
|| 35 || '''Br''' || [[Bromine]] || [[p-block]] || [Ar] 3d<sup>10</sup> 4s<sup>2</sup> 4p<sup>5</sup>
|- bgcolor="{{element color|p-block}}"
|| 36 || '''Kr''' || [[Krypton]] || [[p-block]] || [Ar] 3d<sup>10</sup> 4s<sup>2</sup> 4p<sup>6</sup>
|}

(*) Exception to the [[Madelung rule]]

==s-block elements==
===Potassium===
[[File:Potassium-2.jpg|frameless|left|upright=0.35]]
[[Potassium]] (K) is an [[alkali metal]], underneath [[sodium]] and above [[rubidium]],<ref>{{cite web|url=http://www.tutorvista.com/content/science/science-ii/periodic-classification-elements/position-elements.php |title=Elements in the Modern Periodic Table, Periodic Classification of Elements |publisher=Tutorvista.com |access-date=2012-08-14 |archive-url=https://web.archive.org/web/20180126054026/http://www.tutorvista.com/content/science/science-ii/periodic-classification-elements/position-elements.php |archive-date=26 January 2018}}</ref> and the first element of period 4.  One of the most [[reactivity (chemistry)|reactive]] chemical elements, it is usually found only in [[Chemical compound|compound]]s.  It is a silvery metal<ref name=ChemExplained /> that tarnishes rapidly when exposed to the [[oxygen]] in air, which [[redox|oxidizes]] it. It is [[softness|soft]] enough to be cut with a knife<ref>{{cite web |url=http://education.jlab.org/itselemental/ele019.html |title=The Element Potassium |website=It's Elemental |author=Office of Science Education |publisher=[[Thomas Jefferson National Accelerator|Jefferson Science Associates]] |location=[[Newport News]] |access-date=2012-08-14 |archive-date=2023-04-15 |archive-url=https://web.archive.org/web/20230415101943/https://education.jlab.org/itselemental/ele019.html |url-status=dead }}</ref> and the second least-[[density|dense]] element.{{citation needed|date=September 2023}} [[Potassium]] has a relatively low [[melting point]]; it will melt under a small open flame.<ref name=ChemExplained>{{cite web|url=http://www.chemistryexplained.com/elements/L-P/Potassium.html |title=Potassium |website=Chemistry Explained: Foundations and Applications |publisher=[[Advameg]] |location=[[Hinsdale, IL]] |access-date=2012-08-14 |year=2023}}</ref> It also is less dense than water, and can, in principle, float<ref>{{cite web|url=http://www.lenntech.com/periodic/elements/k.htm |title=Potassium - K  |publisher=Lenntech |access-date=2012-08-14 |year=1998}}</ref> (although it will react with any water it is exposed to).<ref name=ChemExplained />

===Calcium===
[[File:Calcium unter Argon Schutzgasatmosphäre.jpg|frameless|left|upright=0.45]]
[[Calcium]] (Ca) is the second element in the period. An [[alkali earth metal]], [[native element|native]] calcium is almost never found in nature,{{citation needed|date=September 2023}} because it reacts with water.<ref>{{cite web|url=http://www.chemguide.co.uk/inorganic/group2/reacth2o.html |title=Reactions of the Group 2 elements with water |last1=Clark |first1=Jim |date=December 2021 |access-date=2012-08-14}}</ref> It has one of the most widely-known biological roles in all animals and some plants, making up structural elements such as bones and teeth.<ref>{{cite report |title=Human Vitamin and Mineral Requirements |publisher=FAO Information Division Publishing and Multimedia Service |location=Rome, Italy |year=2002 |chapter=Calcium |chapter-url=http://www.fao.org/DOCREP/004/Y2809E/y2809e0h.htm |access-date=2012-08-14}}</ref>  It also has applications in [[Cell (biology)|cells]], such as signals for [[cellular processes]]es. It is regarded as the most abundant [[mineral]] in the human body.{{citation needed|date=September 2023}}

==d-block elements==
===Scandium===
[[File:Scandium sublimed dendritic and 1cm3 cube.jpg|frameless|left|upright=0.3]]
[[Scandium]] (Sc) is the third element in the period, and is the first [[transition metal]] in the periodic table. Scandium is quite common in nature, but difficult to isolate because its chemistry mirrors that of the other [[rare earth element|rare earth]] compounds quite closely. Scandium has very few commercial applications, the major exception being [[aluminium]] [[alloy]]s.

===Titanium===
[[File:Titan-crystal bar.JPG|frameless|left|upright=0.3]]
[[Titanium]] (Ti) is an element in [[group 4 element|group&nbsp;4]]. Titanium is both one of the least dense metals and one of the strongest and most corrosion-resistant.  As such, it has many applications, especially in alloys with other elements, such as iron. It is commonly used in [[airplane]]s, [[golf club]]s, and other objects that must be strong, but lightweight.

===Vanadium===
[[File:Vanadium etched.jpg|frameless|left|upright=0.24]]
[[Vanadium]] (V) is an element in [[group 5 element|group&nbsp;5]]. Vanadium is never found in pure form in nature, but is commonly found in compounds. Vanadium is similar to titanium in many ways, such as being very corrosion-resistant, however, unlike titanium, it oxidizes in air even at room temperature. All vanadium compounds have at least some level of toxicity, with some of them being extremely toxic.

===Chromium===
[[File:Chromium crystals and 1cm3 cube.jpg|frameless|left|upright=0.3]]
[[Chromium]] (Cr) is an element in [[group 6 element|group&nbsp;6]]. Chromium is, like titanium and vanadium before it, extremely resistant to corrosion, and is indeed one of the main components of [[stainless steel]]. Chromium also has many colorful compounds, and as such is very commonly used in pigments, such as [[chrome green]].

===Manganese===
[[File:Manganese electrolytic and 1cm3 cube.jpg|frameless|left|upright=0.3]]
[[Manganese]] (Mn) is an element in [[group 7 element|group&nbsp;7]]. Manganese is often found in combination with iron. Manganese, like chromium before it, is an important component in [[stainless steel]], preventing the iron from rusting. Manganese is also often used in pigments, again like chromium. Manganese is also poisonous; if enough is inhaled, it can cause irreversible neurological damage.

===Iron===
[[File:Iron electrolytic and 1cm3 cube.jpg|frameless|left|upright=0.3]]
[[Iron]] (Fe) is an element in [[group 8 element|group&nbsp;8]]. Iron is [[Abundance of elements on Earth|the most common on Earth]] among elements of the period, and probably the most well-known of them. It is the principal component of [[steel]]. [[Iron-56]] has the lowest energy density of any isotope of any element, meaning that it is the most massive element that can be produced in [[supergiant]] [[star]]s. Iron also has some applications in the human body; [[hemoglobin]] is partly iron.

===Cobalt===
[[File:Kobalt electrolytic and 1cm3 cube.jpg|frameless|left|upright=0.3]]
[[Cobalt]] (Co) is an element in [[group 9 element|group&nbsp;9]]. Cobalt is commonly used in pigments, as many compounds of cobalt are blue in color. Cobalt is also a core component of many magnetic and high-strength alloys. The only stable isotope, [[cobalt-59]], is an important component of [[vitamin B-12]], while [[cobalt-60]] is a component of nuclear fallout and can be dangerous in large enough quantities due to its radioactivity.

===Nickel===
[[File:Nickel chunk.jpg|frameless|left|upright=0.2]]
[[Nickel]] (Ni) is an element in [[group 10 element|group&nbsp;10]]. Nickel is rare in the Earth's crust, mainly due to the fact that it reacts with oxygen in the air, with most of the nickel on Earth coming from [[nickel iron meteorite]]s. However, nickel is very abundant in the [[Earth's core]]; along with iron it is one of the two main components. Nickel is an important component of stainless steel, and in many [[superalloys]].

===Copper===
[[File:NatCopper.jpg|frameless|left|upright=0.24]]
[[Copper]] (Cu) is an element in [[group 11 element|group&nbsp;11]]. Copper is one of the few metals that is not white or gray in color, the only{{citation needed|date=August 2019}} others being [[gold]], [[osmium]] and [[caesium]]. Copper has been used by humans for thousands of years to provide a reddish tint{{clarification needed|date=August 2019|reason=In elemental form? As a powder, sheets, or how in the hell at all? Compounds of copper are indeed turquoise or cyan.}} to many objects, and is even an essential nutrient to humans, although too much is poisonous. Copper is also commonly used as a wood preservative or [[fungicide]]s.

===Zinc===
[[File:Zinc fragment sublimed and 1cm3 cube.jpg|frameless|left|upright=0.3]]
[[Zinc]] (Zn) is an element in [[group 12 element|group&nbsp;12]]. Zinc is one of the main components of [[brass]], being used since the 10th century BCE. Zinc is also incredibly important to humans; almost 2 billion people in the world suffer from zinc deficiency.  However, too much zinc can cause copper deficiency. Zinc is often used in batteries, aptly named [[carbon-zinc batteries]], and is important in many platings, as zinc is very corrosion resistant.

==p-block elements==

===Gallium===
[[File:Gallium crystals.jpg|frameless|left|upright=0.3]]
[[Gallium]] (Ga) is an element in [[boron group|group&nbsp;13]], under [[aluminium]]. Gallium is noteworthy because it has a melting point at about 303 [[kelvin]]s, right around room temperature. For example, it will be solid on a typical spring day, but will be liquid on a hot summer day. Gallium is an important component in the alloy [[galinstan]], along with tin. Gallium can also be found in semiconductors.

===Germanium===
[[File:Polycrystalline-germanium.jpg|frameless|left|upright=0.3]]
[[Germanium]] (Ge) is an element in [[carbon group|group&nbsp;14]]. Germanium, like [[silicon]] above it, is an important [[semiconductor]] and is commonly used in diodes and transistors, often in combination with arsenic. Germanium is fairly rare on Earth, leading to its comparatively late discovery. Germanium, in compounds, can sometimes irritate the eyes, skin, or lungs.

===Arsenic===
[[File:Arsen 1a.jpg|frameless|left|upright=0.3]]
[[Arsenic]] (As) is an element in [[pnictogen|group&nbsp;15]], the pnictogens. Arsenic, as mentioned above, is often used in semiconductors in alloys with germanium. Arsenic, in pure form and some alloys, is incredibly poisonous to all multicellular life, and as such is a common component in pesticides. Arsenic was also used in some pigments before its toxicity was discovered.

===Selenium===
[[File:SeBlackRed.jpg|frameless|left|upright=0.3]]
[[Selenium]] (Se) is an element in [[chalcogen|group&nbsp;16]], the chalcogens. Selenium is the first nonmetal in period&nbsp;4, with properties similar to [[sulfur]]. Selenium is quite rare in pure form in nature, mostly being found in minerals such as [[pyrite]], and even then it is quite rare. Selenium is necessary for humans in trace amounts, but is toxic in larger quantities. Selenium is red in monomolar structure but metallic gray in its crystalline structure.

===Bromine===
[[File:Bromine 25ml.jpg|frameless|left|upright=0.3]]
[[Bromine]] (Br) is an element in [[halogen|group&nbsp;17 (halogen)]]. It does not exist in elemental form in nature. Bromine is barely liquid at room temperature, boiling at about 330 kelvins. Bromine is also quite toxic and corrosive, but bromide ions, which are relatively inert, can be found in [[halite]], or table salt. Bromine is often used as a [[fire retardant]] because many compounds can be made to release free bromine atoms.

===Krypton===
[[File:Krypton discharge tube.jpg|frameless|left|upright=0.3]]
[[Krypton]] (Kr) is a [[noble gas]], placed under [[argon]] and over [[xenon]]. Being a noble gas, krypton rarely interacts with itself or other elements; although compounds have been detected, they are all unstable and decay rapidly, and as such, krypton is often used in fluorescent lights. Krypton, like most noble gases, is also used in lighting because of its many spectral lines and the aforementioned reasons.

==Biological role==
Many period 4 elements find roles in controlling protein function as [[secondary messengers]], structural components, or enzyme [[Cofactor (biochemistry)|cofactors]]. A gradient of potassium is used by cells to maintain a [[membrane potential]] which enables [[Action potential|neurotransmitter firing]] and [[facilitated diffusion]] among other processes. Calcium is a common signaling molecule for proteins such as [[calmodulin]] and plays a critical role in triggering [[Muscle contraction#Excitation-contraction coupling|skeletal muscle contraction]] in vertebrates. Selenium is a component of the [[Non-proteinogenic amino acids|noncanonical]] [[amino acid]], [[selenocysteine]]; proteins which contain selenocysteine are known as [[selenoprotein]]s. [[:Category:Manganese enzymes|Manganese enzymes]] are utilized by both [[eukaryote]]s and [[prokaryote]]s, and may play a role in the virulence of some pathogenic bacteria.<ref>{{Cite journal|last1=Makhlynets|first1=Olga|last2=Boal|first2=Amie K.|last3=Rhodes|first3=Delacy V.|last4=Kitten|first4=Todd|last5=Rosenzweig|first5=Amy C.|last6=Stubbe|first6=JoAnne|date=2014-02-28|title=Streptococcus sanguinis class Ib ribonucleotide reductase: high activity with both iron and manganese cofactors and structural insights|journal=The Journal of Biological Chemistry|volume=289|issue=9|pages=6259–6272|doi=10.1074/jbc.M113.533554|issn=1083-351X|pmc=3937692|pmid=24381172|doi-access=free}}</ref> [[Vanabins]], also known as vanadium-associated proteins, are found in the blood cells of some species of [[Ascidiacea|sea squirts]]. The role of these proteins is disputed, although there is some speculation that they function as oxygen carriers. Zinc ions are used to stabilize the [[zinc finger]] [[Structural motif|milieu]] of many [[DNA-binding protein]]s.

Period 4 elements can also be found [[Coordination complex|complexed]] with organic [[small molecule]]s to form cofactors. The most famous example of this is [[heme]]: an iron-containing [[porphyrin]] compound responsible for the oxygen-carrying function of [[myoglobin]] and [[hemoglobin]] as well as the catalytic activity of [[Cytochrome P450|cytochrome enzymes]].<ref>{{Cite journal|last1=Caputo|first1=Gregory A.|last2=Vaden|first2=Timothy D.|last3=Calabro|first3=Anthony|last4=Lee|first4=Joshua Y.|last5=Kohn|first5=Eric M.|date=December 2018|title=Heme Dissociation from Myoglobin in the Presence of the Zwitterionic Detergent N,N-Dimethyl-N-Dodecylglycine Betaine: Effects of Ionic Liquids|journal=Biomolecules|language=en|volume=8|issue=4|pages=126|doi=10.3390/biom8040126|pmid=30380655|pmc=6315634|doi-access=free}}</ref> [[Hemocyanin]] replaces hemoglobin as the oxygen carrier of choice in the blood of certain invertebrates, including [[horseshoe crab]]s, [[tarantula]]s, and [[octopus]]es. [[Vitamin B12|Vitamin B<sub>12</sub>]] represents one of the few biochemical applications for cobalt.

==References==
<references/>

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