Question

The correct order of atomic radii of group 13 elements is

• A. Al > Tl > Ga > In
• B. Al > Ga > In > Tl
• C. Tl > In > Ga > Al
• D. Tl > In > Al > Ga

Hint:

Remember the "zig-zag" at Al-Ga. \( \text{Radius}(\text{Ga}) \textless \text{Radius}(\text{Al}) \).

Answer 1

The Correct Option is D

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: \( \text{B} \textless \text{Al} \textless \text{Ga} \textless \text{In} \textless \text{Tl} \). - Due to the screening effect of 3d electrons in Ga, its effective nuclear charge is higher, causing a contraction. - Radius of Al \(\approx\) 143 pm. - Radius of Ga \(\approx\) 135 pm. - Therefore, \( \text{Ga} \textless \text{Al} \). - The rest follow the general trend: \( \text{Al} \textless \text{In} \textless \text{Tl} \). - Combined Order (Increasing): \( \text{Ga} \textless \text{Al} \textless \text{In} \textless \text{Tl} \). - Decreasing Order: \( \text{Tl} \textgreater \text{In} \textgreater \text{Al} \textgreater \text{Ga} \).
Step 4: Final Answer:

The correct order is Tl > In > Al > Ga.