Q.How would you explain the lower atomic radius of Ga as compared to Al?

Gallium's electronic configuration is $[\text{Ar}]3d^{10}4s^24p^1$, so between the noble-gas core and the valence electrons there sits a full 3d subshell. d-electrons shield the nucleus very poorly (the "d-block contraction"), so gallium's valence electrons feel an unusually high effective nuclear charge, pulling them closer to the nucleus than the simple increase in shell number would suggest — resulting in a radius close to (even slightly less than) that of aluminium. …

1. Naive expectation. Since gallium (period 4) lies directly below aluminium (period 3) in Group 13, one might expect Ga's atomic radius to be clearly larger, following the usual "radius increases down a group" trend.
2. Electronic configurations. Al: $[\text{Ne}]3s^23p^1$. Ga: $[\text{Ar}]3d^{10}4s^24p^1$ — note that gallium's core, unlike aluminium's, includes a complete 3d$^{10}$ subshell just beneath the valence electrons.
3. Poor shielding by d-electrons. d-orbitals are diffuse and shield the nuclear charge much less effectively than s- or p-orbitals of the same or lower shell. So the 10 additional 3d electrons in Ga do not shield the valence $4s/4p$ electrons nearly as well as an equivalent number of s/p electrons would.
4. Effect on effective nuclear charge. As a result, gallium's valence electrons experience a considerably higher effective nuclear charge than would be expected purely from its position in the periodic table. …

Compare the electronic configurations of Al and Ga, note that Ga has a filled 3d subshell absent in Al, and explain that d-electrons shield poorly, raising the effective nuclear charge felt by Ga's valence electrons — which contracts its size relative to the naive …

• Simply stating "Ga is smaller due to more electrons" without identifying that it is specifically the poorly-shielding filled 3d subshell responsible.
• Confusing this effect with the lanthanide contraction (which affects period 6 elements) rather than the analogous but distinct d-block contraction re …