Question

The reason(s) for small radius of $Ga$ compared to $Al$ is/are :

• A. poor screening effect of d orbitals.
• B. decrease in nuclear charge
• C. presence of higher orbital
• D. higher atomic number

Answer 1

The correct option is A poor screening effect of d orbitals.

$Ga\left(31\right)=\left[Ar\right]4s^{2}3d^{10}4p^{1}Al\left(13\right)=1s^{2}2s^{2}2p^{6}3s^{2}3p^1$

On moving down the group from $AltoGa$, atomic radius decrease (exception) due to poor shielding by d-electrons. On moving form $AltoGa$, shielding effect of d-electrons is unable to compensate increased nuclear charge.
Hence, successive increase of atomic radius as expected is not observed.