Electron Gain Enthalpy Exception (Period 2 vs 3)

Anomalous Order (More Negative Energy Release):

$$\text{Cl} > \text{F} \qquad \text{and} \qquad \text{S} > \text{O}$$

Concept: Period 3 elements exhibit a more exothermic electron gain enthalpy than corresponding Period 2 elements because the extremely small size of Period 2 valence shells creates intense inter-electronic repulsion that opposes incoming electrons.

Notation:

• $$\text{Cl, S}$$ = Period 3 elements with larger, more spacious subshells;
• $$\text{F, O}$$ = Period 2 elements with dense valence shells and high local electron densities.