Which among the following elements has the highest value for third ionisation energy ?

Options

A. Mg
B. Al
C. Na
D. Ar

Answer

To determine which element has the highest third ionization energy among the given options, we will analyze the electron configurations and the stability achieved after removing electrons from each element.

### Step-by-Step Solution:

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

2. **Identify the Elements**:
- The elements we need to consider are Magnesium (Mg), Aluminum (Al), Sodium (Na), and Argon (Ar).

3. **Determine Electron Configurations**:
- **Magnesium (Mg)**: Atomic number = 12
- Electron configuration: \(1s^2 2s^2 2p^6 3s^2\)
- After first ionization: \(1s^2 2s^2 2p^6 3s^1\)
- After second ionization: \(1s^2 2s^2 2p^6\) (Noble gas configuration)
- After third ionization: \(1s^2 2s^2 2p^5\) (Removing from a stable noble gas configuration)

- **Aluminum (Al)**: Atomic number = 13
- Electron configuration: \(1s^2 2s^2 2p^6 3s^2 3p^1\)
- After first ionization: \(1s^2 2s^2 2p^6 3s^2\)
- After second ionization: \(1s^2 2s^2 2p^6 3s^1\)
- After third ionization: \(1s^2 2s^2 2p^6\) (Noble gas configuration, very stable)

- **Sodium (Na)**: Atomic number = 11
- Electron configuration: \(1s^2 2s^2 2p^6 3s^1\)
- After first ionization: \(1s^2 2s^2 2p^6\) (Noble gas configuration)
- After second ionization: \(1s^2 2s^2 2p^5\) (Removing from a stable noble gas configuration)

- **Argon (Ar)**: Atomic number = 18
- Electron configuration: \(1s^2 2s^2 2p^6 3s^2 3p^6\)
- After first ionization: \(1s^2 2s^2 2p^6 3s^2 3p^5\)
- After second ionization: \(1s^2 2s^2 2p^6 3s^2 3p^4\)
- After third ionization: \(1s^2 2s^2 2p^6 3s^2 3p^3\)

4. **Analyze Stability and Ionization Energies**:
- **Magnesium**: After the third ionization, it removes an electron from a stable noble gas configuration, which requires a significant amount of energy.
- **Aluminum**: The third ionization also leads to a noble gas configuration, but it is less stable than that of magnesium.
- **Sodium**: The third ionization energy is lower because it achieves a noble gas configuration after the first ionization.
- **Argon**: Being a noble gas, the ionization energies are generally higher, but we are focusing on the third ionization energy.

5. **Conclusion**:
- Among the elements analyzed, **Aluminum (Al)** has the highest third ionization energy because it achieves a stable noble gas configuration after losing three electrons, making it energetically unfavorable to remove an additional electron.

### Final Answer:
**Aluminum (Al)** has the highest value for the third ionization energy.