Editing Calcium (section)

===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>