Quick Recap — Shielding, Contraction & Reasoning
- Effective nuclear charge rises across a period (poor shielding by same-shell electrons); shielding order .
- Lanthanoid contraction makes the second- and third-row transition metals similar in size (e.g. Zr Hf).
- Anomalous 2nd-period behaviour: small size, no d-orbitals, high electronegativity.
- Successive ionisation enthalpies jump sharply when a noble-gas core is broken into.