Electron Gain Enthalpy ($$\Delta_{\text{eg}} H$$) Anomalies

Anomalous Orders (Magnitude of Negative Energy Released):

$$\text{Cl} > \text{F} > \text{Br} > \text{I} \qquad \text{and} \qquad \text{S} > \text{Se} > \text{Te} > \text{O}$$

Concept: Period 2 non-metals ($$\text{F, O}$$) possess an exceptionally compact volume where severe inter-electronic cloud repulsion weakens incoming electron affinity compared to roomier Period 3 subshells ($$\text{Cl, S}$$).

Notation:

• $$\Delta_{\text{eg}} H$$ values are heavily exothermic (negative) for halogens and endothermic (positive) for filled closed-octet noble gases ($$ns^2 np^6$$).