The third ionisation energy will be maximum for:

Options

A. nitrogen
B. phosphorus
C. aluminium
D. boron

Answer

To determine which element has the maximum third ionization energy, we need to analyze the electronic configurations and the stability of the resulting ions after the removal of electrons.

### Step-by-Step Solution:

1. **Understanding Ionization Energy**:
- Ionization energy is the energy required to remove an electron from an atom. The third ionization energy (E3) refers to the energy needed to remove the third electron from an atom.

2. **Identify the Elements**:
- We will analyze the third ionization energies for the following elements: Nitrogen (N), Phosphorus (P), Aluminium (Al), and Boron (B).

3. **Electronic Configurations**:
- **Nitrogen (N)**: Atomic number = 7, Electronic configuration = 1s² 2s² 2p³
- **Phosphorus (P)**: Atomic number = 15, Electronic configuration = 1s² 2s² 2p⁶ 3s² 3p³
- **Aluminium (Al)**: Atomic number = 13, Electronic configuration = 1s² 2s² 2p⁶ 3s² 3p¹
- **Boron (B)**: Atomic number = 5, Electronic configuration = 1s² 2s² 2p¹

4. **Removing Electrons**:
- For each element, we will consider the removal of three electrons:
- **Nitrogen**:
- 1st ionization: 1s² 2s² 2p²
- 2nd ionization: 1s² 2s² 2p¹
- 3rd ionization: 1s² 2s²
- **Phosphorus**:
- 1st ionization: 1s² 2s² 2p⁶ 3s² 3p²
- 2nd ionization: 1s² 2s² 2p⁶ 3s² 3p¹
- 3rd ionization: 1s² 2s² 2p⁶ 3s²
- **Aluminium**:
- 1st ionization: 1s² 2s² 2p⁶ 3s² 3p¹
- 2nd ionization: 1s² 2s² 2p⁶ 3s²
- 3rd ionization: 1s² 2s² 2p⁶
- **Boron**:
- 1st ionization: 1s² 2s² 2p¹
- 2nd ionization: 1s² 2s²
- 3rd ionization: 1s²

5. **Stability of Resulting Ions**:
- The stability of the resulting ions after the removal of electrons is crucial. The more stable the ion, the higher the ionization energy required to remove the next electron.
- After the third ionization:
- **Nitrogen**: 1s² 2s² (stable)
- **Phosphorus**: 1s² 2s² 2p⁶ 3s² (stable)
- **Aluminium**: 1s² 2s² 2p⁶ (stable)
- **Boron**: 1s² (inner shell, stable)

6. **Comparison of Ionization Energies**:
- The third ionization energy will be highest for the element that has the most stable ion after the removal of the third electron.
- Boron, after losing three electrons, has a stable inner shell configuration (1s²), which experiences a high effective nuclear charge, thus requiring more energy to remove the next electron.

7. **Conclusion**:
- Therefore, the third ionization energy will be maximum for Boron (B).

### Final Answer:
The third ionization energy will be maximum for Boron (B).