Cations are always smaller than their parent atoms: fewer electrons, same nuclear charge → higher Zeff per electron → contraction. Na (186 pm) → Na^+ (102 pm). Anions are always larger: more electrons, same nuclear charge → lower Zeff per electron → expansion. Cl (99 pm) → Cl^- (181 pm). Isoelectronic species (same number of electrons): radius decreases with increasing nuclear charge. For 10-electron species: N^3- (146) > O^2- (140) > F^- (133) > Ne (112) > Na^+ (102) > Mg^2+ (72) > Al^3+ (54 pm). The more protons pulling the same number of electrons, the smaller the ion.