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.