Question -
Answer -
Cations are formed by expelling an electron from the outermost orbit of an atom, thus cation has less electrons compared to parent atom which results in increased effective nuclear charge but the total nuclear charge remains same which results in the increased attraction of electrons towards nucleus than that of the parent atom. Thus, cations are having smaller radii than that of their parent atom.
Anions are formed by gaining an electron in the outermost orbit of an atom. Thus anion has more electrons compared to parent atom, which results in decreased effective nuclear charge but the total nuclear charge remains the same which results in the increased distance the nucleus and the valence electrons as the attraction of electrons towards nucleus decreases than that of the parent atom. Thus, anions are having larger radii than that of their parent atom.