The number of electron would have been equal to number of proton if the atom were a neutral atom or represented as Z without Z\(^{2+}\). But because it is Z\(^{2+}\), carrying +2, meaning 2electrons were given out, therefore number of protons will be more than number electron by 2. i.e number of electron + 2 = 18 + 2 = 20. Therefore, the number of protons will be 20.