Ionization Enthalpy Exception (Group 15 vs 16)

Anomalous Order: $$\text{N} > \text{O} \qquad \text{and} \qquad \text{P} > \text{S}$$

Concept: Group 15 elements display an unexpectedly higher ionization energy than Group 16 elements due to the enhanced thermodynamic stability associated with a symmetric, exactly half-filled $$np^3$$ electronic subshell configuration.

Notation:

• $$\text{N}$$ = Nitrogen ($$2p^3$$half-filled);
• $$\text{O}$$ = Oxygen ($$2p^4$$);
• $$\text{P}$$ = Phosphorus ($$3p^3$$half-filled);
• $$\text{S}$$ = Sulfur ($$3p^4$$).