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

Answer

The ionic radius of a cation is always smaller than the parent atom because the loss of one or more electrons increases the effective nuclear charge `(Z_(eff))`. As a result, the force of attraction of nucleus for the remaining electrons increases and hence the electron cloud contracts and ionic radii decreases.
In contrast, the ionic radius of an anion is always larger than its parent atom because the adition of one or more electrons decreases the effective nuclear charge `(Z_(eff))`. As a result, the force of attraction of the nucleus for the remaining electrons decreases and hence electron cloud expands and the ionic radii increases.