The first ionization energy of sulfur is lower than that of phosphorus because of the presence of an additional 3d electron in sulfur. This 3d electron shields the outermost 3p electrons from the nucleus, making them easier to remove.

Aluminium

The first ionization energy of aluminium is lower than that of magnesium because of the presence of an additional 3d electron in aluminium. This 3d electron is not as effective at shielding the outermost 3p electrons as the 2p electrons in magnesium, but it does provide some shielding. Additionally, the aluminium nucleus has a higher charge than the magnesium nucleus, which also makes the 3p electrons easier to remove.