Which of the following process is not endothermic?

Options

A. `H(g) + e^- rarr H^-(g)`
B. `Ar(g) + e^- rarr Ar^-(g)`
C. `O^- + e^- rarr O^(2-)(g)`
D. `Na(g) rarr Na^+(g) + e^-`

Answer

To determine which of the following processes is not endothermic, we need to analyze each option provided in the question. An endothermic process is one that absorbs heat, while an exothermic process is one that releases heat.

### Step-by-Step Solution:

1. **Understanding Endothermic and Exothermic Processes**:
- An endothermic process absorbs heat from the surroundings.
- An exothermic process releases heat to the surroundings.

2. **Analyzing Each Option**:
- **Option 1: Adding an electron to hydrogen (H → H⁻)**:
- Hydrogen has an electronic configuration of 1s¹. When it accepts an electron, it becomes H⁻ (1s²), which is more stable. This process releases heat, making it exothermic.

- **Option 2: Adding an electron to argon (Ar → Ar⁻)**:
- Argon is a noble gas with a stable electronic configuration (Neon configuration: 3s² 3p⁶). Adding an electron to argon requires energy input (heat), as it is already stable. Thus, this process is endothermic.

- **Option 3: Adding an electron to O⁻ to form O²⁻**:
- Oxygen (O) has an electronic configuration of 1s² 2s² 2p⁴. When it gains an electron to become O⁻, it is stable. However, adding another electron to O⁻ (which has a negative charge) leads to electron-electron repulsion. This process also requires energy input (heat), making it endothermic.

- **Option 4: Removing an electron from sodium (Na → Na⁺)**:
- Sodium has an electronic configuration of 1s² 2s² 2p⁶ 3s¹. Removing the outermost electron to form Na⁺ requires energy (ionization energy), which means this process is endothermic.

3. **Conclusion**:
- The only process that is not endothermic is the first option, where hydrogen accepts an electron and releases heat, making it exothermic.

### Final Answer:
The process that is not endothermic is **adding an electron to hydrogen (H → H⁻)**.