Assertion : First ionisation enthalpy of N is higher than O.
Reason : Extra stability of fully filled up 2p subshell of N atom

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion.
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion.
C. If Assertion is true statement but Reason is false.
D. If both Assertion and Reason are false statements.

Answer

To solve the assertion and reason question regarding the first ionization enthalpy of nitrogen (N) and oxygen (O), we will analyze both statements step by step.

### Step-by-Step Solution:

1. **Understanding Ionization Enthalpy**:
- Ionization enthalpy (or ionization energy) is the energy required to remove an electron from an atom in its gaseous state. Higher ionization enthalpy means it is more difficult to remove an electron.

2. **Atomic Structure of Nitrogen and Oxygen**:
- Nitrogen (N) has an atomic number of 7, with the electronic configuration:
- 1s² 2s² 2p³
- Oxygen (O) has an atomic number of 8, with the electronic configuration:
- 1s² 2s² 2p⁴

3. **Evaluating the Assertion**:
- The assertion states that the first ionization enthalpy of nitrogen is higher than that of oxygen.
- Since nitrogen has a half-filled 2p subshell (2p³), it is relatively more stable compared to oxygen, which has a 2p⁴ configuration that is not half-filled or fully filled.

4. **Stability of Electron Configurations**:
- A half-filled subshell (like nitrogen's 2p³) provides extra stability due to symmetrical distribution of electrons and exchange energy.
- Oxygen's configuration (2p⁴) is less stable than nitrogen's because it has one more electron, which leads to increased electron-electron repulsion.

5. **Conclusion on Ionization Enthalpy**:
- Due to the extra stability of the half-filled 2p subshell in nitrogen, it is harder to remove an electron from nitrogen than from oxygen. Thus, nitrogen has a higher first ionization enthalpy than oxygen.

6. **Evaluating the Reason**:
- The reason states that the extra stability of the fully filled 2p subshell of nitrogen atom is the cause for its higher ionization enthalpy.
- This statement is incorrect because nitrogen has a half-filled 2p subshell, not a fully filled one. The stability arises from the half-filled configuration, not a fully filled one.

7. **Final Assessment**:
- The assertion is true (first ionization enthalpy of N is higher than O), but the reason is false (it incorrectly describes nitrogen's electron configuration).

### Final Answer:
- The assertion is true, and the reason is false.