Why is calcium oxide cao, and not cao2?

The Calcium ion is an Alkaline earth metal and wants to give up the 2 s orbital elections and become a +2 cation.

Oxygen has six valence electrons and is looking to gain two electrons to complete the octet (8) electron count in the valence shell making it a -2 anion.

When the charges of the Calcium +2 and the Oxygen -2 are equal and opposite, the ions for an electrical attraction. (Remember Paula Abdul told us "Opposites Attract")

This one to one ratio of charges makes the formula CaO

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.