Which pair of elements shows positive elecron gain enthalpy ?

Options

A. He, N, O
B. Ne, N, Cl
C. O, Cl, F
D. N, He, Ne

Answer

To determine which pair of elements shows positive electron gain enthalpy, we need to understand the concept of electron gain enthalpy and analyze the electronic configurations of the given elements.

### Step-by-Step Solution:

1. **Understand Electron Gain Enthalpy**:
- Electron gain enthalpy is defined as the amount of energy released when an extra electron is added to the outermost shell of an isolated gaseous atom.
- If the electron gain enthalpy is positive, it means that energy must be supplied to add an electron, indicating that the atom is already stable and does not favor the addition of an electron.

2. **Identify Stable Configurations**:
- Elements with stable electronic configurations (either fully filled or half-filled orbitals) tend to have positive electron gain enthalpy.
- Fully filled configurations are stable because they have no tendency to gain electrons, while half-filled configurations are also relatively stable due to symmetry.

3. **Analyze the Given Elements**:
- Let's consider the options provided (not explicitly listed in the question, but we will analyze common elements like Helium, Nitrogen, and Neon).
- **Helium (He)**:
- Electronic configuration: 1s² (fully filled)
- Since it has a stable configuration, it will show positive electron gain enthalpy.
- **Nitrogen (N)**:
- Electronic configuration: 1s² 2s² 2p³ (half-filled)
- Nitrogen is also stable due to its half-filled p-orbitals, leading to a positive electron gain enthalpy.
- **Neon (Ne)**:
- Electronic configuration: 1s² 2s² 2p⁶ (fully filled)
- As a noble gas, Neon is stable and will also show positive electron gain enthalpy.

4. **Conclusion**:
- All three elements (Helium, Nitrogen, and Neon) exhibit positive electron gain enthalpy due to their stable electronic configurations.
- Therefore, the correct option indicating pairs of elements that show positive electron gain enthalpy would include Helium, Nitrogen, and Neon.

### Final Answer:
The pair of elements that shows positive electron gain enthalpy includes Helium (He), Nitrogen (N), and Neon (Ne).