The atomic radius of Gallium () is observed to be slightly less than or almost equal to that of Alum — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

Gallium comes immediately after the first row of d-block (transition) elements. The ten $3d$ -electrons offer poor shielding of the increased nuclear charge. This increases the effective nuclear charge ($Z_{eff}$) on the valence electrons, pulling them closer to the nucleus and causing a reduction in size known as transition contraction, where $r_{Ga} \le r_{Al}$.