Arrange the elements with the following electronic configuration in order of increasing electron gain enthalpy
`(i) 1s^(2) 2s^(2) 2p^(5) (ii) 1s^(2) 2s^(2) 2p^(4) (iii) 1s^(2) 2s^(2) 2p^(3) (iv) 1s^(2) 2s^(2) 2p^(6) 3s^(2) 3p^(4)`

Explanation

To arrange the elements with the given electronic configurations in order of increasing electron gain enthalpy, we need to analyze the configurations and understand the concept of electron gain enthalpy.

### Step-by-Step Solution:

1. **Identify the Elements**:
- (i) 1s² 2s² 2p⁵ corresponds to Fluorine (F).
- (ii) 1s² 2s² 2p⁴ corresponds to Oxygen (O).
- (iii) 1s² 2s² 2p³ corresponds to Nitrogen (N).
- (iv) 1s² 2s² 2p⁶ 3s² 3p⁴ corresponds to Selenium (Se).

2. **Understand Electron Gain Enthalpy**:
- Electron gain enthalpy is the energy change when an electron is added to a neutral atom in the gaseous state. A more negative value indicates a greater tendency to gain an electron.

3. **Consider Atomic Size and Electron Configuration**:
- Smaller atoms with higher effective nuclear charge tend to have more negative electron gain enthalpy because they can attract additional electrons more effectively.
- Half-filled and fully filled subshells are more stable, making it less favorable for these atoms to gain additional electrons.

4. **Analyze Each Element**:
- **Fluorine (F)**: 1s² 2s² 2p⁵ - Needs 1 electron to achieve a stable octet. Has a high electron gain enthalpy (very negative).
- **Oxygen (O)**: 1s² 2s² 2p⁴ - Needs 2 electrons to achieve a stable octet. Less negative than Fluorine.
- **Nitrogen (N)**: 1s² 2s² 2p³ - Half-filled p subshell, so it is relatively stable and does not favor gaining electrons. Thus, it has a less negative electron gain enthalpy than both F and O.
- **Selenium (Se)**: 1s² 2s² 2p⁶ 3s² 3p⁴ - Larger atomic size (third period) and less effective nuclear charge compared to the others, leading to the least negative electron gain enthalpy.

5. **Arrange in Order of Increasing Electron Gain Enthalpy**:
- Based on the analysis:
- Selenium (Se) has the least negative electron gain enthalpy.
- Nitrogen (N) is next.
- Oxygen (O) follows.
- Fluorine (F) has the most negative electron gain enthalpy.

### Final Order:
Increasing electron gain enthalpy:
**Se < N < O < F**