Question

I want to ask why Aluminium is larger than Gallium. If it's because of poor shielding effect of d orbitals, then why doesn't it follow in other cases like Silicon-Germanium, Phosphorus-Arsenic, etc.?"

Answer 1

The explanation for Al being larger than Ga is due to the poor shielding effect of 3d electrons in Ga, leading to d-block contraction. This effect is also present in Ge and As, but it only reduces the expected increase in atomic radius down the group, rather than reversing the trend as seen in Al-Ga.

Answer 2

The atomic radius of Aluminium (Al) is larger than that of Gallium (Ga) due to the d-block contraction. This phenomenon arises because Gallium (Ga) has 10 d-electrons in its penultimate shell (3d¹⁰). These d-electrons have a poor shielding effect on the outermost 4s and 4p valence electrons. Consequently, the effective nuclear charge (Z_eff) experienced by the valence electrons in Ga is significantly higher than expected. This increased Z_eff pulls the outer electrons closer to the nucleus, causing a contraction in atomic size that counteracts the effect of adding a new electron shell, leading to Ga being smaller than Al.

This poor shielding effect of d-orbitals does occur in Germanium (Ge) and Arsenic (As) as well. However, in these cases (Group 14 and Group 15), the d-block contraction is not strong enough to completely reverse the general trend of increasing atomic radius down a group. Instead, it reduces the expected increase in atomic radius. Therefore, while Ge is smaller than it would have been without the d-electrons, it is still larger than Si. Similarly, As is larger than P, but the increase in size is less pronounced than it would be without the d-block contraction. The net effect in Groups 14 and 15 is still an increase in atomic radius down the group, albeit a smaller increase.