Step 1: Understanding the Concept:
General trend: Atomic radius increases down a group.
Exception: Group 13 shows a deviation between Aluminum (Al) and Gallium (Ga).
Step 2: Key Formula or Approach:
Effect of d-block contraction: The filling of 3d orbitals before Gallium results in poor shielding of the nuclear charge. This pulls the valence electrons closer, making Ga smaller than or nearly equal to Al.
Step 3: Detailed Explanation:
The elements are B, Al, Ga, In, Tl.
Normal trend expectation:
B < Al < Ga < In < Tl
Due to the poor shielding effect of 3d electrons in Ga, its effective nuclear charge becomes higher, causing contraction in atomic size.
Approximate atomic radii:
Al ≈ 143 pm
Ga ≈ 135 pm
Therefore:
Ga < Al
The remaining elements follow the general increasing trend down the group:
Al < In < Tl
So, the combined increasing order is:
Ga < Al < In < Tl
Hence, the decreasing order is:
Tl > In > Al > Ga
Step 4: Final Answer:
The correct order is:
Tl > In > Al > Ga.