Explanation:
Atomic number of calcium is 20 and its electronic configuration is 2, 8, 8, 2. So, to acquire stability it loses 2 valence electrons and hence forms [tex]Ca^{2+}[/tex] ion.
Whereas atomic number of oxygen is 8 and its electronic distribution is 2, 6. And, to attain stability its needs to gain two more electrons and hence then it forms [tex]O^{2-}[/tex] ion.
Therefore, when both calcium and oxygen chemically combine together then calcium atom will donate its 2 valence electrons to the oxygen atom.
Hence, they will form calcium oxide, that is, CaO.
