Question

Magnesium is found to have a higher first ionization energy value than aluminum, how would you explain this exception to the general trend in terms of electron arrangements and attraction/repulsion?

Answer 1

Consider the valence electron configuration of the 2 metal atoms. A priori , we would expect that a p -electron would be easier to remove than a s -electron.

Explanation:

For Mg, `Z` `=` `12`. Therefore its electronic configuration is `1s^(2)2s^(2)2p^(6)3s^(2)`; versus Al, `Z` `=` `13`; `1s^(2)2s^(2)2p^(6)3s^(2)3p^(1)`. While aluminum metal has extra nuclear charge, `p` orbitals demonstrably have zero electron density at the nucleus, and should be easier to ionize than the `s` electrons of the alkaline earth (why?), given that `s` orbital electrons have some electron density at the nucleus.