The first ionization energies (in kJ mol⁻¹) of four successive elements in the same period are: 496, 738, 578, 786. Which of the elements has the lowest first ionization energy because it has a half-filled 3p subshell that is less stable than expected?

The anomalously low ionization energy of 578 kJ mol⁻¹ belongs to aluminium (Al), which comes after magnesium (738 kJ mol⁻¹). Al has the configuration [Ne]3s²3p¹; the 3p electron is easier to remove than the 3s² electrons of Mg because the 3p subshell is higher in energy and more shielded. This explains why Al has a lower first ionization energy than Mg despite having a higher atomic number.