The correct order of atomic radii in group 13 elements is

Options

A. B `lt` Al `lt` In`lt` Ga `lt` Tl
B. B `lt` Al `lt` Ga `lt` In `lt` Tl
C. B `lt` Ga `lt` Al `lt` Tl `lt` In
D. B `lt` Ga `lt` Al `lt` In `lt` Tl

Answer

To determine the correct order of atomic radii in group 13 elements, we need to analyze the elements in this group: boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl).

### Step-by-Step Solution:

1. **Identify the Group 13 Elements**:
The elements in group 13 of the periodic table are boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl).

2. **Understand General Trends**:
In general, atomic radii increase as you move down a group in the periodic table. This is due to the addition of electron shells, which increases the distance between the nucleus and the outermost electrons.

3. **Proposed Order Based on General Trend**:
Following the general trend, one might propose the order of atomic radii as:
- Boron (B) < Aluminum (Al) < Gallium (Ga) < Indium (In) < Thallium (Tl)

4. **Consider Exceptions**:
However, there is an exception between aluminum and gallium. When moving from aluminum to gallium, a 3d subshell is added. The 3d electrons have a poor shielding effect, which means they do not effectively shield the outermost electrons from the nucleus's positive charge.

5. **Effect of Poor Shielding**:
Due to the poor shielding effect of the 3d electrons, the outermost electrons in gallium experience a stronger attraction to the nucleus compared to aluminum. This results in a smaller atomic radius for gallium than one might expect based on the general trend.

6. **Correct Order of Atomic Radii**:
Taking into account the exception, the correct order of atomic radii in group 13 elements is:
- Boron (B) < Gallium (Ga) < Aluminum (Al) < Indium (In) < Thallium (Tl)

### Final Answer:
The correct order of atomic radii in group 13 elements is:
**B < Ga < Al < In < Tl**