| Cue | Main Notes |
|---|---|
| Atomic radius — across period | Decreases — Zeff increases at constant n; electrons pulled closer |
| Atomic radius — down group | Increases — new principal shell added |
| Ionic radius — cation vs atom | Cation smaller than parent atom; loss of electrons raises Zeff per remaining electron |
| Ionic radius — anion vs atom | Anion larger than parent atom; extra electrons increase repulsion |
| IE — across period | Increases (generally) — Zeff rises |
| IE — down group | Decreases — larger atomic radius, greater shielding |
| IE — key exceptions | Be > B (2s vs 2p); N > O (half-filled 2 stability) |
| EGE — across period | More negative (generally) |
| EGE — key exception | Cl (−349 kJ/mol) > F (−328 kJ/mol) due to F's small 2p orbital repulsion |
| Electronegativity — across period | Increases |
| Electronegativity — down group | Decreases |
| Most electronegative element | Fluorine (4.0) on Pauling scale |
Summary: All major periodic properties are driven by two competing factors — effective nuclear charge (Zeff) and principal quantum number (n). Exceptions arise when subshell stability (filled or half-filled) overrides the general trend.