Ionization Enthalpy Exception (Group 2 vs 13)

Anomalous Order: $$\text{Be} > \text{B} \qquad \text{and} \qquad \text{Mg} > \text{Al}$$

Concept: Group 2 elements possess a higher first ionization energy than Group 13 elements in the same period because it is more difficult to remove an electron from a stable, fully filled $$ns^2$$ subshell compared to a lone $$np^1$$ electron.

Notation:

• $$\text{Be}$$ = Beryllium ($$1s^2 2s^2$$);
• $$\text{B}$$ = Boron ($$1s^2 2s^2 2p^1$$);
• $$\text{Mg}$$ = Magnesium ($$3s^2$$);
• $$\text{Al}$$ = Aluminum ($$3s^2 3p^1$$).