(a) When an atom gains one or more electrons an anion is formed. During the change of an atom into an anion the one or more electrons are added to the valence shell of the atom. The nuclear charge remains the same. An increase in the number of electrons in the valence shell reduces the effective nuclear charge due to greater mutual shielding by the electrons. As a result, electron cloud expands, i.e., the ionic radius increases.
(b) A cation is formed when one or more electrons are lost by an atom. The cation formed is always smaller than the size of the parent atom. This may be due to the following reasons.
(i) Disappearance of the valence shell:- In certain cases, the loss of electrons leads to a complete disappearance of the valence shell. Removal of the outermost shell decreases the size.
(ii) Increase of effective nuclear charge:- In a cation, the number of electrons is less than in the corresponding atom. The total nuclear charge remains the same. This increases the effective nuclear charge. As a result, the electrons are attracted towards the nucleus more strongly, thereby leading to a decrease in its size.