Positive value of electron gain enthalpy (`Delta H_("eg")`) among the following will be of:

Options

A. Mg
B. Ne
C. Ar
D. all of these

Answer

To determine which element has a positive value of electron gain enthalpy (ΔH_eg), we need to analyze the electron configurations of the given elements: magnesium, neon, and argon.

### Step-by-Step Solution:

1. **Understanding Electron Gain Enthalpy**:
- Electron gain enthalpy is the energy change when an electron is added to a neutral gaseous atom. It can either be negative (energy is released) or positive (energy is absorbed).

2. **Analyzing the Elements**:
- **Magnesium (Mg)**:
- Atomic number: 12
- Electron configuration: 1s² 2s² 2p⁶ 3s²
- The outermost shell (3s²) has 2 electrons. Adding an electron would require energy because the new electron would need to occupy a higher energy level (3p), which is not fully filled.

- **Neon (Ne)**:
- Atomic number: 10
- Electron configuration: 1s² 2s² 2p⁶
- Neon has a complete outer shell (2s² 2p⁶). Adding an electron would require energy to accommodate it, leading to a positive electron gain enthalpy.

- **Argon (Ar)**:
- Atomic number: 18
- Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶
- Argon also has a complete outer shell (3s² 3p⁶). Similar to neon, adding an electron would require energy, resulting in a positive electron gain enthalpy.

3. **Conclusion**:
- All three elements (magnesium, neon, and argon) will have a positive value of electron gain enthalpy due to their electronic configurations. However, magnesium's outer shell is not fully filled, making it less stable compared to neon and argon, which are noble gases with fully filled outer shells.

### Final Answer:
All three elements (magnesium, neon, and argon) have a positive value of electron gain enthalpy, but the most significant positive value will be observed in the noble gases (neon and argon).