Compare the radii of two species X and Y. Give reasons for your answer.
(a) X has 12 protons and 12 electrons
(b) Y has 12 protons and 10 electrons.

Explanation

Here X has same number of electrons and protons, therefore it is a neutral atom whereas Y contains 12 protons and 10 electrons so there are 2 protons extra giving Y a charge of +2. The electronic configurations of the two species can be expressed as :

The atomic size of a cation is always smaller than a neutral atom containing same number of protons this is because cation has less number of electrons due to which the nuclear attractions are more on electrons in cation causing its size to be smaller than that of the neutral atom.
Hence, atomic radius of Y is smaller than that of X.