Assertion: The formation of `F_((g))^(-)` from `F_((g))` is exothermic, whereas that of `O_((g))^(2-) from `O_((g))` is endotherrmic.
Reason: The addition of second electron to a monovalent anion is difficult because both have the same charge and experience more repulsion.

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 solve the question, we will analyze the assertion and reason provided, and then evaluate their correctness step by step.

### Step 1: Analyze the Assertion
The assertion states that the formation of \( F^{-} \) from \( F \) is exothermic, while the formation of \( O^{2-} \) from \( O \) is endothermic.

- **Formation of \( F^{-} \)**:
- Fluorine (\( F \)) has the electronic configuration of \( 1s^2 2s^2 2p^5 \).
- When it gains one electron, it achieves a stable electronic configuration of \( 1s^2 2s^2 2p^6 \) (the configuration of neon).
- The process of gaining an electron releases energy because the product is more stable than the reactant, making it exothermic.

- **Formation of \( O^{2-} \)**:
- Oxygen (\( O \)) has the electronic configuration of \( 1s^2 2s^2 2p^4 \).
- When it gains one electron, it becomes \( O^{-} \) (with configuration \( 1s^2 2s^2 2p^5 \)), which is still not fully stable.
- Adding a second electron to \( O^{-} \) to form \( O^{2-} \) requires energy because both the \( O^{-} \) and the incoming electron have negative charges, leading to repulsion. Therefore, this process is endothermic.

### Conclusion for Step 1:
The assertion is true: the formation of \( F^{-} \) is exothermic, while the formation of \( O^{2-} \) is endothermic.

### Step 2: Analyze the Reason
The reason states that the addition of a second electron to a monovalent anion is difficult because both have the same charge and experience more repulsion.

- **Monovalent Anion**:
- \( O^{-} \) is a monovalent anion, meaning it has a single negative charge.
- When a second electron is added to \( O^{-} \), both the anion and the incoming electron repel each other due to their like charges.

### Conclusion for Step 2:
The reason is also true: the addition of a second electron to \( O^{-} \) is difficult due to the repulsion between the negatively charged anion and the incoming electron.

### Final Conclusion:
Both the assertion and the reason are true, and the reason correctly explains the assertion. Thus, the answer to the question is that both the assertion and reason are true, and the reason is the correct explanation for the assertion.