First ionisation potential of Mg is .... than that of Al.

Options

A. less
B. more
C. equal
D. None of these

Explanation

To determine the first ionization potential of magnesium (Mg) compared to aluminum (Al), we need to analyze their electronic configurations and the stability of their outermost electrons.

### Step-by-Step Solution:

1. **Identify Atomic Numbers and Electronic Configurations**:
- Magnesium (Mg) has an atomic number of 12. Its electronic configuration is:
\[ \text{Mg: } 1s^2, 2s^2, 2p^6, 3s^2 \]
- Aluminum (Al) has an atomic number of 13. Its electronic configuration is:
\[ \text{Al: } 1s^2, 2s^2, 2p^6, 3s^2, 3p^1 \]

2. **Determine the Valence Electrons**:
- For Mg, the outermost shell (3rd shell) has 2 electrons in the 3s orbital.
- For Al, the outermost shell (3rd shell) has 3 electrons: 2 in the 3s orbital and 1 in the 3p orbital.

3. **Analyze the Stability of the Outer Electrons**:
- The 3s orbital in Mg is fully filled with 2 electrons, which provides extra stability.
- The 3p orbital in Al has only 1 electron, which is less stable compared to a fully filled orbital.

4. **Compare Ionization Energies**:
- The first ionization potential is the energy required to remove the outermost electron.
- Since Mg has a fully filled 3s orbital, it requires more energy to remove one of its 2 electrons compared to Al, which has a single electron in the 3p orbital that is easier to remove.

5. **Conclusion**:
- Therefore, the first ionization potential of magnesium is **more** than that of aluminum.

### Final Answer:
The first ionization potential of Mg is **more** than that of Al.