In terms of electronegativity and electrons why does CaO contain an ionic bond?

Calcium oxide is ionic in nature. The Ca-O bond in CaO is formed by the transfer of electron from highly electropositive Ca to the highly electronegative oxygen atom.  The electronegativity difference should be high for the formation of ionic bond.

Explanation:

The ionic compound contains one highly electronegative atom and one highly electropositive atom. The ionic bond is formed by the transfer of electron from the more electropositive atom to electronegative atom.

In calcium oxide , Calcium is alkaline earth metal and it is highly electropositive atom while oxygen is highly electronegative atom. Calcium will transfer its 2 valence electrons to the oxygen atom. The electronegativity difference between calcium and oxygen is greater than 2.0. Hence, the bond in CaO is found to be ionic in nature.