The element with positive electron gain enthalpy is

Options

A. hydrogen
B. sodium
C. oxygen
D. neon

Answer

To determine the element with a positive electron gain enthalpy, we need to understand what electron gain enthalpy means and analyze the electron configurations of the given elements.

### Step-by-Step Solution:

1. **Understanding Electron Gain Enthalpy**:
- Electron gain enthalpy is the amount of energy released when an atom in the gaseous state gains an electron. If the value is positive, it means that energy is required to add an electron to the atom, indicating that the atom does not favor gaining an electron.

2. **Analyzing the Given Elements**:
- We will look at the electron configurations of the elements mentioned:
- **Hydrogen (H)**: 1s¹
- **Sodium (Na)**: 1s² 2s² 2p⁶ 3s¹
- **Oxygen (O)**: 1s² 2s² 2p⁴
- **Neon (Ne)**: 1s² 2s² 2p⁶

3. **Identifying the Stability of Electron Configurations**:
- Neon has a completely filled outer shell (1s² 2s² 2p⁶), which makes it very stable and inert. Atoms with a filled outer shell do not tend to gain electrons because they are already in a stable state.
- In contrast, hydrogen, sodium, and oxygen have unfilled outer shells and can easily accept an electron to achieve a more stable electronic configuration.

4. **Conclusion**:
- Since neon has a filled outer shell and does not favor gaining an electron, it has a positive electron gain enthalpy. Therefore, the element with positive electron gain enthalpy is **Neon (Ne)**.

### Final Answer:
The element with positive electron gain enthalpy is **Neon (Ne)**.

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