The radius of cation is smaller than that of the parent atom. Cation is formed by the loss of one or more electron from the gaseous atom. Now, in the cation the nuclear charge remains the same that in the parent atom but the number of electrons becomes less. As a result of this, the nuclear hold on the remaining electrons increases because of the increase in the effective nuclear charge per electron. This causes a decrease in the size.
The radius of anion is larger than that of parent atom. Anion is formed by the gain of one or more electrons by the gaseous atom. In the anion the nuclear charge is the same as that in the parent atom but the number of electrons has increased. Since same nuclear charge now acts on increased number of electrons, the effective nuclear charge per electron decreases in the anion. The electron cloud is held less tightly by the nucleus. This causes increase in the size.
