Assertion: The electron gain enthalpy of N is +ve while that of P is -ve.
Reason: Smaller atomic size of N is which there is a considerable electron-electron repulsion and hence the additional electron is not accepted easily.

Options

A. If both assertion and reason are true annd the reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but the reason is false
D. if assertion is false but the reason is true

Answer

To analyze the assertion and reason provided in the question, we will break down the concepts step by step.

### Step 1: Understanding Electron Gain Enthalpy
- **Definition**: Electron gain enthalpy is the amount of energy released or absorbed when an electron is added to a neutral atom in the gas phase.
- **Exothermic vs. Endothermic**: If energy is released when an electron is added, the process is exothermic, and the electron gain enthalpy is negative. If energy is required to add an electron, the process is endothermic, and the electron gain enthalpy is positive.

**Hint**: Remember that exothermic processes release energy, while endothermic processes require energy input.

### Step 2: Analyzing Nitrogen (N)
- **Atomic Number**: Nitrogen has an atomic number of 7, with an electronic configuration of 1s² 2s² 2p³.
- **Electron Gain Enthalpy**: For nitrogen, the addition of an electron leads to a configuration that is less stable due to the increased electron-electron repulsion in the small 2p orbital. Therefore, the electron gain enthalpy for nitrogen is positive (+ve).

**Hint**: Consider the stability of the electron configuration after adding an electron and the effect of atomic size on electron repulsion.

### Step 3: Analyzing Phosphorus (P)
- **Atomic Number**: Phosphorus has an atomic number of 15, with an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p³.
- **Electron Gain Enthalpy**: For phosphorus, the addition of an electron results in a more stable configuration, and the larger size of phosphorus means that the added electron experiences less repulsion. Thus, the electron gain enthalpy for phosphorus is negative (-ve).

**Hint**: Larger atomic size often leads to reduced electron-electron repulsion, making it easier to gain an electron.

### Step 4: Comparing Nitrogen and Phosphorus
- **Assertion**: The assertion states that the electron gain enthalpy of nitrogen is positive while that of phosphorus is negative. This is correct based on the analysis above.
- **Reason**: The reason provided states that the smaller atomic size of nitrogen leads to considerable electron-electron repulsion, making it difficult for nitrogen to accept an additional electron. This is also correct and explains why nitrogen has a positive electron gain enthalpy.

**Hint**: Evaluate whether the reason logically supports the assertion based on the concepts of atomic size and electron repulsion.

### Conclusion
Both the assertion and the reason are true, and the reason correctly explains the assertion. Therefore, the final conclusion is that both the assertion and reason are valid.

**Final Answer**: Both assertion and reason are true, and the reason is the correct explanation of the assertion.