Calcium has the same electron configuration as magnesium, but with three full shells within that have the identical configuration to argon. Being one shell larger than magnesium, calcium’s valence electrons are less well bound, causing it to be more reactive than magnesium, but much less so than potassium.
Calcium will give up its valence electrons in an ionic interaction in order to reach the stability of the 3s23p6 noble gas configuration of argon, which is a multi-di-electron state with three concentric full shells. That is why calcium forms a 2+ ionic state.
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OTHER GROUP II ELEMENTS: Beryllium, Magnesium, Calcium, Strontium, Barium