Question

Incorrect order of atomic radius is

• A. \( \mathrm{B<Al} \)
• B. \( \mathrm{In<Tl} \)
• C. \( \mathrm{Al<Ga} \)
• D. \( \mathrm{Ga<In} \)

Hint:

While atomic radius usually increases down a group, exceptions like \( \mathrm{Ga} \) and \( \mathrm{Tl} \) occur because of poor shielding by \( d \)- and \( f \)-electrons. Always remember d-block contraction and lanthanoid contraction for such comparisons.

Answer 1

The Correct Option is C

To solve this question, we need to understand the trend of atomic radii across groups and periods of the periodic table. The atomic radius generally increases down a group as more electron shells are added, and it typically decreases across a period due to increased nuclear charge. Exceptions occur due to d-block and f-block contractions and peculiar arrangements of electron shells and charges.

Group 13 elements, which include B, Al, Ga, In, and Tl, exhibit some deviations from the general trend due to these factors:

1. \(\mathrm{B < Al}\): Correct. As we move from B to Al, the atomic radius increases down the group.
2. \(\mathrm{In < Tl}\): Correct. Although atomic radius generally increases down a group, in some cases like In and Tl, relativistic effects and filled f-orbitals influence the size, making Tl not significantly larger than In.
3. \(\mathrm{Al < Ga}\): Incorrect. The atomic radius of Ga is slightly less than that of Al due to the d-block contraction where 3d electrons are poor at shielding. Thus, enhanced nuclear charge affects the outer electrons, reducing Ga's atomic radius.
4. \(\mathrm{Ga < In}\): Correct. As expected, In is larger than Ga due to the additional electron shell.

Thus, the incorrect atomic radius order is indeed \(\mathrm{Al < Ga}\) because Al actually has a slightly larger atomic radius than Ga due to the d-block contraction effect.