Question

The correct order of metallic radius of Al, Ga, In, Tl is

• A. \(Tl>In>Al>Ga\)
• B. \(Tl>In>Ga>Al\)
• C. \(In>Ga>Al>Tl\)
• D. \(Ga>In>Tl>Al\)

Hint:

Remember anomaly: Gallium is slightly smaller than aluminium because of d-block contraction.

Answer 1

The Correct Option is A

Step 1: List the Group 13 elements.
The metals in question are Al, Ga, In, and Tl, and we want the order of their metallic radii.
Step 2: State the naive expectation.
Normally radius increases down a group, which would suggest Al < Ga < In < Tl.
Step 3: Bring in the d-block contraction.
Gallium comes right after the first transition series, so its added 3d electrons shield the nucleus poorly. The increased effective nuclear charge pulls the electrons in.
Step 4: Correct gallium's place.
Because of this, gallium is actually slightly smaller than aluminium, so $\text{Ga} < \text{Al}$.
Step 5: Order the larger atoms.
Indium and thallium remain larger, with thallium the biggest, giving Tl > In at the top.
Step 6: Assemble the full order.
Combining, the order is Tl > In > Al > Ga, which is option 1.
\[ \boxed{\text{Tl} > \text{In} > \text{Al} > \text{Ga}} \]