Electron gain enthalpy becomes more negative across a period (left to right) and less negative down a group. However, there are significant anomalies due to atomic size.

1. Group Comparison: Group 17 elements (F, Cl) have higher effective nuclear charge and are closer to a stable octet than Group 16 elements (O, S). Thus, Halogens have more negative electron gain enthalpies than Chalcogens.
2. Size Anomaly:

• F vs Cl: Fluorine is very small (n=2 shell). The incoming electron faces strong inter-electronic repulsion. Chlorine (n=3 shell) is larger and accommodates the electron more easily. Thus, Cl has a more negative value than F.
• O vs S: Similarly, Oxygen is very small, leading to repulsion. Sulfur has a more negative value than Oxygen.

3. Overall Order of Negative Magnitude: Chlorine (Most negative: -349 kJ/mol) Fluorine (-328 kJ/mol) Sulfur (-200 kJ/mol) Oxygen (Least negative: -141 kJ/mol)
4. Increasing Order: O < S < F < Cl.