Q.Explain, with reference to atomic size and ionisation enthalpy, why gallium's first ionisation enthalpy is anomalously close to that of aluminium instead of being clearly lower.

[!TLDR] Ten intervening 3d electrons between Al and Ga shield poorly, so Ga's effective nuclear charge stays high despite the added shell, keeping its ionisation enthalpy close to aluminium's rather than lower. [!ANSWER] Gallium's valence electron experiences poor shielding from the 10 filled $3d$ electrons that lie between it and the nucleus (added between Al and Ga in the periodic table), so its effective nuclear ch …

Under a simple 'increasing shell number decreases ionisation enthalpy' expectation, gallium (period 4) should have a noticeably lower first ionisation enthalpy than aluminium (period 3), since gallium's valence electron sits in a shell farther from the nucleus. In practice, gallium's ionisation enthalpy is measured to be very close to — not clearly lower than — aluminium's, and the reason lies in what happens electronically between aluminium and gallium in the periodic table: the entire first transition series (scandium through zinc) is filled in between, adding ten $3d$ electrons to the atom's core. $d$ electrons are notoriously poor at shielding the nuclear charge felt by electrons farther out (they are more penetrating/diffuse in a way that leaves outer electrons more exposed to the nucleus than simple shell-counting would suggest), so gallium's $4p$ valence electron experiences a considerably higher effective nuclear charge than a naive extrapolation from aluminium would predict. This unusually high effective nuclear charge almost exactly offsets the ionisatio …

Identify the ten intervening 3d electrons between Al and Ga, explain their poor shielding effect on effective nuclear charge, and connect this directly to the near-c …

Attributing the anomaly vaguely to 'd-block contraction' without specifying the shielding mechanism; forgetting that it is specifically the poor shielding by d-electrons (not …