Which of the following has the highest positive electron gain enthalpy ?

Options

A. F
B. `O^(-)`
C. `Na^(-)`
D. `Mg^(2+)`

Answer

To determine which of the given elements has the highest positive electron gain enthalpy, we need to analyze the electron gain enthalpy of each element provided in the options. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand Electron Gain Enthalpy
Electron gain enthalpy is the energy change that occurs when an electron is added to a neutral atom in the gaseous state. It can be either negative (energy is released) or positive (energy is absorbed). A more negative value indicates a greater tendency to gain an electron.

### Step 2: Analyze Each Element
1. **Fluorine (F)**:
- Atomic number: 9
- Electronic configuration: 1s² 2s² 2p⁵
- As a halogen, fluorine has a high tendency to gain an electron to achieve a stable noble gas configuration (1s² 2s² 2p⁶). Thus, the electron gain enthalpy is highly negative (energy is released).

2. **Oxygen (O)**:
- Atomic number: 8
- Electronic configuration: 1s² 2s² 2p⁴
- Oxygen also has a tendency to gain an electron, but when it does, it forms O⁻. The effective nuclear charge decreases due to the added electron, making it harder to add another electron. Therefore, the electron gain enthalpy is positive.

3. **Sodium (Na)**:
- Atomic number: 11
- Electronic configuration: 1s² 2s² 2p⁶ 3s¹
- Sodium is electropositive and prefers to lose an electron to achieve a stable configuration. Adding an electron to Na would be difficult, leading to a highly positive electron gain enthalpy.

4. **Magnesium ion (Mg²⁺)**:
- Atomic number: 12
- Electronic configuration: 1s² 2s² 2p⁶ (after losing 2 electrons)
- Adding an electron to Mg²⁺ would require overcoming the positive charge, making it energetically unfavorable. Thus, the electron gain enthalpy is also positive.

### Step 3: Compare the Values
- **Fluorine**: Highly negative (favorable)
- **Oxygen**: Positive (less favorable)
- **Sodium**: Highly positive (unfavorable)
- **Magnesium ion**: Positive (unfavorable)

### Conclusion
Among the options, **Oxygen (O)** has the highest positive electron gain enthalpy because, while it can gain an electron, it does so with a significant energy cost due to its already negative charge.

### Final Answer
**Oxygen (O)** has the highest positive electron gain enthalpy.

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