Discuss the factors affecting electron gain enthalpy and the trend in its variation in the periodic table.

Explanation

### Step-by-Step Text Solution:

**Step 1: Define Electron Gain Enthalpy**
Electron gain enthalpy, also known as electron affinity (EA), is defined as the amount of energy released when an isolated gaseous atom gains an electron. This process can be represented as:
\[ \text{A(g)} + e^- \rightarrow \text{A}^-(g) + \text{Energy} \]
The more negative the electron gain enthalpy, the greater the tendency of the atom to gain an electron.

**Step 2: Identify Factors Affecting Electron Gain Enthalpy**
There are several key factors that influence electron gain enthalpy:

1. **Effective Nuclear Charge (Z_effective)**:
- The effective nuclear charge is the net positive charge experienced by an electron in a multi-electron atom. A higher Z_effective means a stronger attraction between the nucleus and the incoming electron, leading to a more negative electron gain enthalpy.

2. **Atomic Size**:
- The size of the atom affects the distance between the nucleus and the outermost electrons. Smaller atoms have a higher Z_effective and a stronger attraction for electrons, resulting in a more negative electron gain enthalpy. However, if the atom is too small (like fluorine), the increased inter-electronic repulsion can lead to a lower electron affinity.

3. **Nature of Electronic Configuration**:
- Atoms with fully filled or half-filled subshells are more stable. Adding an electron to these configurations requires more energy, resulting in a less negative or even positive electron gain enthalpy.

**Step 3: Discuss the Trend in the Periodic Table**
The trend in electron gain enthalpy can be observed as follows:

- **Moving Down a Group**:
- As we move down a group in the periodic table, the atomic size increases due to the addition of electron shells. This leads to a decrease in Z_effective, resulting in a less negative electron gain enthalpy. Thus, the tendency to gain electrons decreases.

- **Moving Across a Period**:
- As we move from left to right across a period, the atomic size decreases while the Z_effective increases due to the increasing number of protons in the nucleus. This results in a more negative electron gain enthalpy, indicating a greater tendency to gain electrons.

**Step 4: Conclusion**
In summary, electron gain enthalpy is influenced by effective nuclear charge, atomic size, and the nature of electronic configuration. The general trend shows that electron gain enthalpy becomes less negative as we move down a group and more negative as we move across a period, with some exceptions.