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 solve the question regarding which element has a positive value of electron gain enthalpy among magnesium, neon, argon, or all of these, we can follow these steps:

### Step 1: Understand Electron Gain Enthalpy
Electron gain enthalpy (ΔH_eg) is defined as the amount of energy released when an electron is added to a neutral atom in the gas phase. A negative value indicates that energy is released (the process is exothermic), while a positive value indicates that energy must be supplied to add an electron (the process is endothermic).

**Hint:** Remember that elements with a stable electron configuration (like noble gases) generally have a positive ΔH_eg because they do not readily accept electrons.

### Step 2: Analyze Each Element
1. **Magnesium (Mg)**:
- Atomic number: 12
- Electron configuration: [Ne] 3s²
- Magnesium has two electrons in its outer shell and tends to lose these electrons to achieve a stable noble gas configuration (Neon). Adding an electron would disrupt its stable configuration, making it difficult to gain an electron. Thus, ΔH_eg for magnesium is positive.

2. **Neon (Ne)**:
- Atomic number: 10
- Electron configuration: 1s² 2s² 2p⁶
- Neon is a noble gas with a complete outer shell. Adding an electron would create an unstable anion, making the process endothermic. Therefore, ΔH_eg for neon is also positive.

3. **Argon (Ar)**:
- Atomic number: 18
- Electron configuration: [Ne] 3s² 3p⁶
- Similar to neon, argon has a complete outer shell. Adding an electron would also lead to instability, resulting in a positive ΔH_eg.

### Step 3: Conclusion
Since all three elements (magnesium, neon, and argon) require energy to add an electron, they all exhibit a positive value of electron gain enthalpy.

**Final Answer:** The positive value of electron gain enthalpy will be for **all of these** (Option D).