Editing Calcium (section)

===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>