Question

Explain why cation are smaller and anions are larger in radii than their parent atoms?

Answer 1

A gaseous atom looses one or more electrons to form cation. The nuclear charge remains same but the number of electrons decreases. Same nuclear charge acts on lesser number of electrons. The effective nuclear charge per electron increases. The electrons are more strongly attracted and pulled towards the nucleus. Thus, the size decreases.
A gaseous atom gains one or more electrons to form anion. The nuclear charge remains same but the number of electrons increases. Same nuclear charge acts on greater number of electrons. The effective nuclear charge per electron increases. The electrons are less strongly attracted and pulled away from the nucleus. Thus, the size decreases.