Here's why:

* Calcium's electron configuration: Calcium has an atomic number of 20, meaning it has 20 protons and 20 electrons. Its electron configuration is [Ar] 4s².

* Tendency to lose electrons: Calcium is in group 2 of the periodic table (alkaline earth metals). These elements tend to lose their two valence electrons (the electrons in the outermost shell) to achieve a stable, noble gas configuration like Argon.

* Formation of cation: By losing these two electrons, calcium becomes a positively charged ion called a cation, denoted as Ca²⁺.

Therefore, calcium always forms a +2 charge in its compounds.