Usually the atomic and ionic radii should increase with increasing atomic number down the group. But there is not much increase at Ga and TI in group-13 and is due to

Options

A. high ionisation energies of these elements
B. the high nuclear charge of Ga and Tl
C. the intervening of and f-electrons
D. None of the above

Answer

To understand why there is not much increase in atomic and ionic radii at Gallium (Ga) and Thallium (Tl) in Group 13, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding Atomic and Ionic Radii**:
- Atomic radius is the distance from the nucleus to the outermost shell of an atom. Ionic radius is similar but refers to the size of an ion.
- Generally, as we move down a group in the periodic table, the atomic and ionic radii increase due to the addition of electron shells.

**Hint**: Remember that more electron shells typically lead to larger atomic and ionic sizes.

2. **Identifying Group 13 Elements**:
- Group 13 elements include Boron (B), Aluminum (Al), Gallium (Ga), Indium (In), and Thallium (Tl).
- We focus on Ga and Tl, where the expected increase in size is not observed.

**Hint**: List the elements in Group 13 to visualize the trend.

3. **Examining the Transition Series**:
- After Gallium, we enter the transition metals, which have d orbitals.
- The presence of d electrons starts affecting the atomic size due to their unique properties.

**Hint**: Consider how transition metals differ from main group elements in terms of electron configuration.

4. **Understanding Shielding Effect**:
- The d electrons have a poor shielding effect. This means they do not effectively shield the outer electrons from the nucleus's positive charge.
- As a result, the effective nuclear charge (Z_eff) felt by the outer electrons increases.

**Hint**: Recall that effective nuclear charge is the net positive charge experienced by electrons in the outer shell.

5. **Impact of Increased Z_eff**:
- With increased Z_eff, the outer electrons are pulled closer to the nucleus, resulting in a decrease in atomic and ionic radii.
- This is contrary to the expected trend of increasing size down the group.

**Hint**: Think about how a stronger pull from the nucleus affects the size of the electron cloud.

6. **Conclusion**:
- The lack of significant increase in atomic and ionic radii at Ga and Tl is primarily due to the poor shielding effect of d electrons, leading to an increase in effective nuclear charge and a resultant decrease in size.

**Hint**: Summarize the key points: poor shielding by d electrons and increased effective nuclear charge.

### Final Answer:
The limited increase in atomic and ionic radii at Gallium (Ga) and Thallium (Tl) in Group 13 is due to the poor shielding effect of d electrons, which results in an increased effective nuclear charge (Z_eff) that pulls the outer electrons closer to the nucleus, causing a decrease in size.