Answer to Question #71366 in General Chemistry for Eeyore

Question

Why the reason first ionisation energy of Al is less than Mg, is because 3p in Al is further than 3s in Mg? If atomic radius decreases across the group then shouldnt 3p in Al be closer than the 3s in Mg? Thanks!

Answer 1

The radius of the aluminum atom is smaller than the radius of the magnesium atom, it's true. But when filling electronic levels, there is a rule. Semi-filled (except s level) and filled sublevels are almost always more stable than other options. Therefore, the atoms of metals of the first groups easily give off electrons and acquire the configuration of an inert gas,
preceding them. In the case of magnesium, when one electron is taken away, it goes into an unstable state s1. And when ionizing aluminum, it will go to the state s2, which is a filled sublevel and is more advantageous than s1 magnesium.
Also, 3 aluminum electrons are shielded by a filled orbital.