WAEC Chemistry 2014 Theory — Question 14

Model answer

Higher stability is associated with an orbital that is either fully filled or exactly half-filled. Magnesium's 3s orbital is fully filled (3s²), making it more stable, whereas Aluminium's outer electron is in the 3p subshell (3p¹), a not-so-stable orbital. There is therefore less difficulty removing the single 3p electron from Al than removing an electron from the fully-filled, more stable 3s orbital of Mg, so Mg has a higher first ionization energy than Al.