Among the following, the third ionisation energy is highest for

Options

A. magnesium
B. boron
C. beryllium
D. aluminium

Explanation

To determine which element has the highest third ionization energy among magnesium, boron, beryllium, and aluminium, we need to analyze the electronic configurations and the general trends of ionization energy in the periodic table.

### Step-by-Step Solution:

1. **Identify the Electronic Configurations**:
- **Beryllium (Be)**: 1s² 2s²
- **Boron (B)**: 1s² 2s² 2p¹
- **Magnesium (Mg)**: 1s² 2s² 2p⁶ 3s²
- **Aluminium (Al)**: 1s² 2s² 2p⁶ 3s² 3p¹

2. **Understand Ionization Energy**:
- Ionization energy is the energy required to remove an electron from an atom in the gaseous state.
- The third ionization energy refers to the energy needed to remove the third electron from a neutral atom.

3. **Trends in Ionization Energy**:
- Ionization energy generally increases across a period (left to right) and decreases down a group (top to bottom).
- Elements with a stable electron configuration (like noble gases) have higher ionization energies.

4. **Evaluate the Third Ionization Energies**:
- **Beryllium**: After removing two electrons (from 2s²), the next electron removed will be from the 1s² core, which requires a lot of energy due to the stability of the filled shell.
- **Boron**: After removing two electrons (from 2s²), the next electron will be removed from 2p¹, which is less stable than beryllium's configuration.
- **Magnesium**: After removing two electrons (from 3s²), the next electron will be removed from the stable 2p⁶ core, requiring significant energy.
- **Aluminium**: After removing two electrons (from 3s²), the next electron will also be removed from the 2p⁶ core, similar to magnesium.

5. **Comparison**:
- Beryllium will have the highest third ionization energy due to the removal of an electron from a filled 1s² shell, which is very stable.
- Magnesium and aluminium will have lower third ionization energies as they will be removing electrons from a more stable core (2p⁶).

### Conclusion:
The third ionization energy is highest for **Beryllium (Be)**.