How would you explain the lower atomic radii of `Ga` as compared to `Al`?

Options

A. poor screening effect of d-electrons for the outer electrons from increased nuclear charge
B. increased force of attraction of increased nuclear charge on electrons
C. increased ionisation enthalpy of Ga as compared to Al
D. anomalous behaviour of Ga.

Explanation

To explain the lower atomic radii of Gallium (Ga) compared to Aluminum (Al), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Atomic Radius**:
- The atomic radius is defined as the distance from the nucleus to the outermost shell of electrons. Generally, as we move down a group in the periodic table, atomic radii increase due to the addition of electron shells.

2. **Position in the Periodic Table**:
- Aluminum (Al) is in the third period, while Gallium (Ga) is in the fourth period of the periodic table. You would expect Ga to have a larger atomic radius than Al due to the additional electron shell.

3. **Electron Configuration**:
- The electron configuration of Aluminum is [Ne] 3s² 3p¹, while Gallium has the configuration [Ar] 3d¹⁰ 4s² 4p¹. The presence of the 3d electrons in Gallium plays a significant role in its atomic size.

4. **Poor Shielding Effect**:
- The 3d electrons in Gallium do not shield the outer 4p electrons effectively from the increased nuclear charge (more protons in the nucleus). This is known as the "poor shielding effect" of d electrons.

5. **Increased Nuclear Charge**:
- Gallium has more protons (31) compared to Aluminum (13). The increased nuclear charge pulls the outer electrons closer to the nucleus, resulting in a smaller atomic radius.

6. **Transition Contraction**:
- The phenomenon where the atomic size decreases due to the poor shielding of d electrons is referred to as "transition contraction." This contraction is significant in transition metals and their adjacent elements in the periodic table.

7. **Conclusion**:
- Due to the poor shielding effect of the 3d electrons and the increased nuclear charge in Gallium, its atomic radius is smaller than that of Aluminum.

### Final Answer:
The lower atomic radius of Gallium compared to Aluminum can be explained by the poor shielding effect of the 3d electrons in Gallium, which leads to a greater effective nuclear charge acting on the outer electrons, resulting in a smaller atomic radius.

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