why is it highly unlikely for calcium to form a Ca+ cation and for sodium to form a Na^2+ cation?

The goal of any element is to form a complete octet. In other words, to attain a total of 8 valence electrons. To do this, elements either gain or lose electrons.

Ca is an Alkaline earth metal. Because it is in group II, it has exactly 2 valence electrons. So, it loses 2 electrons and gains a cation with a positive charge of 2.
**Ca should be a Ca2+ cation, not a Ca+ cation!

Na is an Alkali metal. Because it is in group I, it has exactly 1 valence electron. So, like Ca, it loses an electron to form a complete octet. It should have a charge of Na+.
**Na should be a Na+ cation, not a Na2+ cation!