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Unit III — Periodic Properties · Effective Nuclear Charge & Radii
Across a period Z* rises so radius shrinks; down a group new shells dominate so radius grows.
Measured as covalent or van der Waals radius. Across a period: Z* increases, the outer electrons are pulled closer → radius decreases (Li → F). Down a group: principal quantum number increases, new shells added → radius increases (Li → Cs).
Cations are smaller than the parent atom (loss of an electron shell, or increased Z* per electron); anions are larger (extra electrons, weaker hold). Na⁺ (102 pm) < Na (186 pm); Cl⁻ (181 pm) > Cl (99 pm).
Species with the same number of electrons: O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺ — the more positive the nuclear charge, the smaller the ion(same electron count, stronger pull).
| Ion | Z | Radius (pm) |
|---|---|---|
| O²⁻ | 8 | 140 |
| F⁻ | 9 | 133 |
| Na⁺ | 11 | 102 |
| Mg²⁺ | 12 | 72 |
| Al³⁺ | 13 | 54 |
Radius ↓ across period (Z*↑), ↑ down group (shells). Cation < atom, anion > atom. Isoelectronic: smaller radius with larger Z.