JEE Main · 2025 · 04 Apr 2025 · Shift 1 · p-Block Elements

Going down group 13 the atomic radius rises from boron to aluminium, but gallium follows immediately after the first transition series and the poorly shielding intervening $3d$ electrons leave it with an atomic radius slightly smaller than aluminium; the relation written as $\mathrm{Al}<\mathrm{Ga}$ is therefore the incorrect pair. For the tripositive ions this anomaly is reversed, the additional $d$ electrons making ${\mathrm{Ga}}^{3+}$ larger than ${\mathrm{Al}}^{3+}$, so within that pair the ion of higher radius is that of gallium, whose atomic number is $31$.