Editing Calcium (section)

==Characteristics==
===Classification===
[[Image:Calcium crystals.jpg|thumb|left|Calcium crystals stored in mineral oil]]
Calcium is a very ductile silvery metal (sometimes described as pale yellow) whose properties are very similar to the heavier elements in its group, [[strontium]], [[barium]], and [[radium]]. A calcium atom has twenty electrons, with [[electron configuration]] [Ar]4s{{sup|2}}. Like the other elements placed in group 2 of the periodic table, calcium has two [[valence electron]]s in the outermost s-orbital, which are very easily lost in chemical reactions to form a dipositive ion with the stable electron configuration of a [[noble gas]], in this case [[argon]].{{sfn|Greenwood|Earnshaw|1997|pp = 112-13}}

Hence, calcium is almost always [[divalent]] in its compounds, which are usually [[ionic compound|ionic]]. Hypothetical univalent salts of calcium would be stable with respect to their elements, but not to [[disproportionation]] to the divalent salts and calcium metal, because the [[enthalpy of formation]] of MX{{sub|2}} is much higher than those of the hypothetical MX. This occurs because of the much greater [[lattice energy]] afforded by the more highly charged Ca{{sup|2+}} cation compared to the hypothetical Ca{{sup|+}} cation.{{sfn|Greenwood|Earnshaw|1997|pp = 112-13}}

Calcium, strontium, barium, and radium are always considered to be [[alkaline earth metal]]s; the lighter [[beryllium]] and [[magnesium]], also in group 2 of the periodic table, are often included as well. Nevertheless, beryllium and magnesium differ significantly from the other members of the group in their physical and chemical behaviour: they behave more like [[aluminium]] and [[zinc]] respectively and have some of the weaker metallic character of the [[post-transition metal]]s, which is why the traditional definition of the term "alkaline earth metal" excludes them.<ref>{{cite book |last=Parish |first=R. V. |date=1977 |title=The Metallic Elements |location=London |publisher=Longman |page=[https://archive.org/details/metallicelements0000pari/page/34 34] |isbn=978-0-582-44278-8 |url=https://archive.org/details/metallicelements0000pari/page/34 }}</ref>

===Physical properties===
Calcium metal melts at 842&nbsp;°C and boils at 1494&nbsp;°C; these values are higher than those for magnesium and strontium, the neighbouring group 2 metals. It crystallises in the [[face-centered cubic]] arrangement like strontium and barium; above {{convert|443|C|K}}, it changes to a [[body-centered cubic]].<ref name="Arblaster 2018" /><ref name="Smith 1956">{{cite journal |last1=Smith |first1=J. F. |last2=Carlson |first2=O. N. |last3=Vest |first3=R. W. |title=Allotropic Modifications of Calcium |journal=Journal of the Electrochemical Society |volume=103 |date=1956 |issue=7 |doi=10.1149/1.2430364 |page=409}}</ref> Its density of 1.526&nbsp;g/cm<sup>3</sup> (at 20&nbsp;°C)<ref name="Arblaster 2018" /><!-- ref name="Arblaster 2018" is defined in infobox: {{cite book |last=Arblaster |first= John W. |title=Selected Values of the Crystallographic Properties of Elements |publisher=ASM International |publication-place=Materials Park, Ohio |date=2018 |isbn=978-1-62708-155-9}}--> is the lowest in its group.{{sfn|Greenwood|Earnshaw|1997|pp = 112-13}}

Calcium is harder than [[lead]] but can be cut with a knife with effort. While calcium is a poorer conductor of electricity than [[copper]] or [[aluminium]] by volume, it is a better conductor by mass than both due to its very low density.{{Sfn|Hluchan|Pomerantz|2005|p=484}} While calcium is infeasible as a conductor for most terrestrial applications as it reacts quickly with atmospheric oxygen, its use as such in space has been considered.{{Sfn|Hluchan|Pomerantz|2005|p=484}}

===Chemical properties===
[[File:Ca(aq)6 improved image.tif|thumb|left|Structure of the polymeric [Ca(H{{sub|2}}O){{sub|6}}]{{sup|2+}} center in hydrated calcium chloride, illustrating the high coordination number typical for calcium complexes.]]

The chemistry of calcium is that of a typical heavy alkaline earth metal. For example, calcium spontaneously reacts with water more quickly than magnesium and less quickly than strontium to produce [[calcium hydroxide]] and hydrogen gas. It also reacts with the [[oxygen]] and [[nitrogen]] in air to form a mixture of [[calcium oxide]] and [[calcium nitride]].<ref name="CRC">C. R. Hammond ''The elements'' (pp. 4–35) in {{RubberBible86th}}</ref> When finely divided, it spontaneously burns in air to produce the nitride. Bulk calcium is less reactive: it quickly forms a hydration coating in moist air, but below 30% [[relative humidity]] it may be stored indefinitely at room temperature.{{sfn|Hluchan|Pomerantz|2005|p=483}}

Besides the simple oxide CaO, [[calcium peroxide]], CaO{{sub|2}}, can be made by direct oxidation of calcium metal under a high pressure of oxygen, and there is some evidence for a yellow [[superoxide]] Ca(O{{sub|2}}){{sub|2}}.{{sfn|Greenwood |Earnshaw|1997|p=119}}Calcium hydroxide, Ca(OH){{sub|2}}, is a strong base, though not as strong as the hydroxides of strontium, barium or the alkali metals.{{sfn|Greenwood |Earnshaw|1997|p=121}} All four dihalides of calcium are known.{{sfn|Greenwood|Earnshaw|1997|p = 117}} [[Calcium carbonate]] (CaCO{{sub|3}}) and [[calcium sulfate]] (CaSO{{sub|4}}) are particularly abundant minerals.{{sfn|Greenwood|Earnshaw|1997|pp = 122-15}} Like strontium and barium, as well as the alkali metals and the divalent [[lanthanide]]s [[europium]] and [[ytterbium]], calcium metal dissolves directly in liquid [[ammonia]] to give a dark blue solution.{{sfn|Greenwood|Earnshaw|1997|p=112}}

Due to the large size of the calcium ion (Ca{{sup|2+}}), high coordination numbers are common, up to 24 in some [[intermetallic compound]]s such as CaZn{{sub|13}}.{{sfn|Greenwood|Earnshaw|1997|p = 115}} Calcium is readily complexed by oxygen [[chelate]]s such as [[ethylenediaminetetraacetic acid|EDTA]] and [[polyphosphate]]s, which are useful in [[analytic chemistry]] and removing calcium ions from [[hard water]]. In the absence of [[steric hindrance]], smaller group 2 cations tend to form stronger complexes, but when large [[polydentate]] [[macrocycle]]s are involved the trend is reversed.{{sfn|Greenwood|Earnshaw|1997|pp = 122-15}}

Though calcium is in the same group as magnesium and [[organomagnesium compound]]s are very widely used throughout chemistry, organocalcium compounds are not similarly widespread because they are more difficult to make and more reactive, though they have recently been investigated as possible [[catalyst]]s.<ref>{{cite journal|last1=Harder|first1=S.|last2=Feil|first2=F.|last3=Knoll|first3=K.|year=2001|title=Novel Calcium Half-Sandwich Complexes for the Living and Stereoselective Polymerization of Styrene|journal=Angew. Chem. Int. Ed.|volume=40|issue=22|pages=4261–64|doi=10.1002/1521-3773(20011119)40:22<4261::AID-ANIE4261>3.0.CO;2-J|pmid=29712082}}</ref><ref>{{cite journal|last1=Crimmin|first1=Mark R.|last2=Casely|first2=Ian J.|last3=Hill|first3=Michael S.|title=Calcium-Mediated Intramolecular Hydroamination Catalysis|journal=[[Journal of the American Chemical Society]]|year=2005|volume=127|issue=7|pages=2042–43|doi=10.1021/ja043576n|pmid=15713071|bibcode=2005JAChS.127.2042C }}</ref><ref>{{cite journal|last1=Jenter|first1=Jelena|last2=Köppe|first2=Ralf|last3=Roesky|first3=Peter W.|title=2,5-Bis{''N''-(2,6-diisopropylphenyl)iminomethyl}pyrrolyl Complexes of the Heavy Alkaline Earth Metals: Synthesis, Structures, and Hydroamination Catalysis|journal=Organometallics|year=2011|volume=30|issue=6|pages=1404–13|doi=10.1021/om100937c}}</ref><ref>{{cite journal|last1=Arrowsmith|first1=Merle|last2=Crimmin|first2=Mark R.|last3=Barrett|first3=Anthony G. M.|last4=Hill|first4=Michael S.|last5=Kociok-Köhn|first5=Gabriele|last6=Procopiou|first6=Panayiotis A.|title=Cation Charge Density and Precatalyst Selection in Group 2-Catalyzed Aminoalkene Hydroamination|journal=Organometallics|year=2011|volume=30|issue=6|pages=1493–1506|doi=10.1021/om101063m}}</ref><ref>{{cite journal|last1=Penafiel|first1=J.|last2=Maron|first2=L.|last3=Harder|first3=S.|year=2014|title=Early Main Group Metal Catalysis: How Important is the Metal?|journal=Angew. Chem. Int. Ed.|volume=54|issue=1|pages=201–06|doi=10.1002/anie.201408814|pmid=25376952|url=https://pure.rug.nl/ws/files/83571601/Early_Main_Group_Metal_Catalysis_How_Important_is_the_Metal.pdf }}</ref> Organocalcium compounds tend to be more similar to organoytterbium compounds due to the similar [[ionic radius|ionic radii]] of Yb{{sup|2+}} (102&nbsp;pm) and Ca{{sup|2+}} (100&nbsp;pm).{{sfn|Greenwood|Earnshaw|1997|pp = 136-37}}

Most of these compounds can only be prepared at low temperatures; bulky ligands tend to favour stability. For example, calcium di[[cyclopentadienyl]], Ca(C{{sub|5}}H{{sub|5}}){{sub|2}}, must be made by directly reacting calcium metal with [[mercurocene]] or [[cyclopentadiene]] itself; replacing the C{{sub|5}}H{{sub|5}} ligand with the bulkier C{{sub|5}}(CH{{sub|3}}){{sub|5}} ligand on the other hand increases the compound's solubility, volatility, and kinetic stability.{{sfn|Greenwood|Earnshaw|1997|pp = 122-15}}

===Isotopes===
{{main|Isotopes of calcium}}

Natural calcium is a mixture of five stable [[isotope]]s ({{sup|40}}Ca, {{sup|42}}Ca, {{sup|43}}Ca, {{sup|44}}Ca, and {{sup|46}}Ca) and one isotope with a half-life so long that it is for all practical purposes stable ([[calcium-48|{{sup|48}}Ca]], with a half-life of about 4.3&nbsp;×&nbsp;10{{sup|19}}&nbsp;years). Calcium is the first (lightest) element to have six naturally occurring isotopes.<ref name="CRC" />

By far the most common isotope of calcium in nature is {{sup|40}}Ca, which makes up 96.941% of all natural calcium. It is produced in the [[silicon-burning process]] from fusion of [[alpha particle]]s and is the heaviest stable nuclide with equal proton and neutron numbers; its occurrence is also supplemented slowly by the decay of [[primordial nuclide|primordial]] [[potassium-40|{{sup|40}}K]].  Adding another alpha particle leads to unstable {{sup|44}}Ti, which decays via two successive [[electron capture]]s to stable {{sup|44}}Ca; this makes up 2.806% of all natural calcium and is the second-most common isotope.<ref name="Cameron"/><ref name="Clayton"/>

The other four natural isotopes, {{sup|42}}Ca, {{sup|43}}Ca, {{sup|46}}Ca, and {{sup|48}}Ca, are significantly rarer, each comprising less than 1% of all natural calcium.  The four lighter isotopes are mainly products of the [[oxygen-burning process|oxygen-burning]] and silicon-burning processes, leaving the two heavier ones to be produced via [[neutron capture]] processes. {{sup|46}}Ca is mostly produced in a "hot" [[s-process]], as its formation requires a rather high neutron flux to allow short-lived {{sup|45}}Ca to capture a neutron. {{sup|48}}Ca is produced by electron capture in the [[r-process]] in [[type Ia supernova]]e, where high neutron excess and low enough entropy ensures its survival.<ref name="Cameron">{{cite journal | last1 = Cameron |first1 = A. G. W. | year = 1973 | title = Abundance of the Elements in the Solar System | url = https://pubs.giss.nasa.gov/docs/1973/1973_Cameron_ca06310p.pdf | journal = Space Science Reviews | volume = 15 |issue = 1 | pages = 121–46 | doi = 10.1007/BF00172440 | bibcode = 1973SSRv...15..121C |s2cid = 120201972 }}</ref><ref name="Clayton">{{cite book |last=Clayton |first=Donald |date=2003 |title=Handbook of Isotopes in the Cosmos: Hydrogen to Gallium |publisher=Cambridge University Press |pages=184–98 |isbn=9780521530835}}</ref>

{{sup|46}}Ca and {{sup|48}}Ca are the first "classically stable" nuclides with a 6-neutron or 8-neutron excess respectively. Although extremely neutron-rich for such a light element, {{sup|48}}Ca is very stable because it is a [[magic number (physics)|doubly magic nucleus]], having 20 protons and 28 neutrons arranged in closed shells. Its [[beta decay]] to {{sup|48}}[[scandium|Sc]] is very hindered because of the gross mismatch of [[nuclear spin]]: {{sup|48}}Ca has zero nuclear spin, being [[even and odd atomic nuclei|even–even]], while {{sup|48}}Sc has spin 6+, so the decay is [[forbidden mechanism|forbidden]] by the conservation of [[angular momentum]]. While two excited states of {{sup|48}}Sc are available for decay as well, they are also forbidden due to their high spins. As a result, when {{sup|48}}Ca does decay, it does so by [[double beta decay]] to {{sup|48}}[[titanium|Ti]] instead, being the lightest nuclide known to undergo double beta decay.{{NUBASE2016|ref}}<ref>{{Cite journal
 |last1=Arnold |first1=R.
 |display-authors=etal
 |year=2016
 |collaboration=[[NEMO-3 Collaboration]]
 |title= Measurement of the double-beta decay half-life and search for the neutrinoless double-beta decay of <sup>48</sup>Ca with the NEMO-3 detector
 |journal=[[Physical Review D]]
 |volume=93 |issue=11
 |page=112008
 |doi= 10.1103/PhysRevD.93.112008
|arxiv=1604.01710|bibcode=2016PhRvD..93k2008A|s2cid=55485404
 }}</ref>

{{sup|46}}Ca can also theoretically undergo double beta decay to {{sup|46}}Ti, but this has never been observed. The most common isotope {{sup|40}}Ca is also doubly magic and could undergo [[double electron capture]] to {{sup|40}}[[argon|Ar]], but this has likewise never been observed. Calcium is the only element with two primordial doubly magic isotopes. The experimental lower limits for the half-lives of {{sup|40}}Ca and {{sup|46}}Ca are 5.9&nbsp;×&nbsp;10{{sup|21}} years and 2.8&nbsp;×&nbsp;10{{sup|15}} years respectively.{{NUBASE2016|ref}}

Apart from the practically stable {{sup|48}}Ca, the longest lived [[radioisotope]] of calcium is {{sup|41}}Ca. It decays by electron capture to stable {{sup|41}}[[potassium|K]] with a half-life of about 10{{sup|5}} years. Its existence in the early Solar System as an [[extinct radionuclide]] has been inferred from excesses of {{sup|41}}K: traces of {{sup|41}}Ca also still exist today, as it is a [[cosmogenic nuclide]], continuously produced through [[neutron activation]] of natural {{sup|40}}Ca.<ref name="Clayton" />

Many other calcium radioisotopes are known, ranging from {{sup|35}}Ca to {{sup|60}}Ca. They are all much shorter-lived than {{sup|41}}Ca, the most stable being {{sup|45}}Ca (half-life 163 days) and {{sup|47}}Ca (half-life 4.54 days). Isotopes lighter than {{sup|42}}Ca usually undergo [[beta plus decay]] to isotopes of potassium, and those heavier than {{sup|44}}Ca usually undergo [[beta minus decay]] to isotopes of [[scandium]], though near the [[nuclear drip line]]s, [[proton emission]] and [[neutron emission]] begin to be significant decay modes as well.{{NUBASE2016|ref}}

Like other elements, a variety of processes alter the relative abundance of calcium isotopes.<ref>{{Cite journal|last1=Russell|first1=W. A.|last2=Papanastassiou|first2=D. A.|last3=Tombrello|first3=T. A.|title=Ca isotope fractionation on the earth and other solar system materials|journal=Geochim Cosmochim Acta|date=1978|volume=42|pages=1075–90|doi=10.1016/0016-7037(78)90105-9|issue=8|bibcode = 1978GeCoA..42.1075R }}</ref> The best studied of these processes is the mass-dependent [[Isotope fractionation|fractionation]] of calcium isotopes that accompanies the precipitation of calcium minerals such as [[calcite]], [[aragonite]] and [[apatite]] from solution. Lighter isotopes are preferentially incorporated into these minerals, leaving the surrounding solution enriched in heavier isotopes at a magnitude of roughly 0.025% per atomic mass unit (amu) at room temperature. Mass-dependent differences in calcium isotope composition are conventionally expressed by the ratio of two isotopes (usually {{sup|44}}Ca/{{sup|40}}Ca) in a sample compared to the same ratio in a standard reference material. {{sup|44}}Ca/{{sup|40}}Ca varies by about 1–2‰ among organisms on Earth.<ref>{{Cite journal|last1=Skulan|first1=J.|last2=Depaolo|first2=D. J.|title=Calcium isotope fractionation between soft and mineralized tissues as a monitor of calcium use in vertebrates|journal=Proc Natl Acad Sci USA|date=1999|volume=96|pages=13709–13|doi=10.1073/pnas.96.24.13709|pmid=10570137|issue=24|pmc=24129 |bibcode = 1999PNAS...9613709S |doi-access=free}}</ref>