Electron gain enthalpy will be positive in

Options

A. `O^(-2)` is formed from `O^(-1)`
B. `O^(-1)` is formed from O
C. `S^(-1)` is formed from S
D. `Na^(-)` is formed from Na

Answer

To determine when the electron gain enthalpy will be positive, we need to analyze the concept of electron gain enthalpy and the factors that influence it.

### Step-by-Step Solution:

1. **Understanding Electron Gain Enthalpy**:
- Electron gain enthalpy is defined as the amount of energy released or absorbed when an electron is added to a neutral atom in the gaseous state.
- If energy is released when an electron is added, the electron gain enthalpy is negative. Conversely, if energy is absorbed, the electron gain enthalpy is positive.

2. **Identifying Elements with Positive Electron Gain Enthalpy**:
- Generally, elements that have a stable electronic configuration (like noble gases) do not favor the addition of an electron.
- When an electron is added to such elements, they tend to absorb energy rather than release it, resulting in a positive electron gain enthalpy.

3. **Examples of Elements with Positive Electron Gain Enthalpy**:
- Noble gases (e.g., Helium, Neon, Argon) are classic examples where the electron gain enthalpy is positive because they have complete valence shells and are stable.
- Other elements that are less electronegative and have a tendency to resist gaining electrons can also exhibit positive electron gain enthalpy.

4. **Conclusion**:
- Therefore, electron gain enthalpy will be positive in noble gases and certain other elements that do not favor gaining an electron due to their stable electronic configuration.

### Summary:
Electron gain enthalpy is positive for noble gases and some other stable elements that resist gaining electrons.