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 and identify whether it is exothermic (releases energy) or endothermic (absorbs energy).

### Step-by-Step Solution:

1. **Understanding Endothermic and Exothermic Processes**:
- An endothermic process absorbs energy, resulting in a positive change in enthalpy (ΔH > 0).
- An exothermic process releases energy, resulting in a negative change in enthalpy (ΔH < 0).

2. **Analyzing Option 1**:
- The process involves hydrogen gas (H) gaining an electron to form H⁻.
- This process is known as electron affinity, where energy is released when an electron is added to a neutral atom.
- Therefore, this process is exothermic (ΔH < 0).

3. **Analyzing Option 2**:
- The process involves argon (Ar) gaining an electron to form Ar⁻.
- Noble gases, like argon, have a complete octet and do not readily gain electrons. The electron affinity for noble gases is effectively zero.
- Thus, this process does not release energy and is not exothermic.

4. **Analyzing Option 3**:
- The process involves O⁻ gaining an electron to form O²⁻.
- The first electron affinity of oxygen is negative (exothermic), but the second electron affinity is positive (endothermic) because of increased inter-electronic repulsion.
- Therefore, this process is endothermic (ΔH > 0).

5. **Analyzing Option 4**:
- The process involves sodium (Na) losing an electron to form Na⁺.
- This process is known as ionization energy, which requires energy to remove an electron from an atom.
- Hence, this process is endothermic (ΔH > 0).

6. **Conclusion**:
- Among the options, only Option 1 (H + e⁻ → H⁻) is exothermic, meaning it is not an endothermic process.
- Therefore, the correct answer is **Option 1**.

### Final Answer:
**Option 1** is the process that is not endothermic.