Write the formula of calcium oxide  in terms of the configurations of calcium and oxygen

In electronic configuration the number of elements in each period is twice the number of the atomic orbitals available in the energy level that is being filled, the period number corresponds to the shell number in which the distant electrons from the nucleus of the atom are present 

Given, Caliciumhasconfiguration. It loses two electrons to form ion. Oxygen atom gains both of these electrons to form oxide ion

Hence, the formula of calcium oxide is