Order of Electrons Lost by Ions

[Christina Cen 2J]

Why is it that 4s is filled before 3d but when cations are formed, electrons are taken from 4s before 3d and not 3d first?

[Sammy Thatipelli 1B]

There is a special rule for the 3d orbital. the lowest energy state for the electron configuration is made when the d orbital has either 5 electrons or 10 electrons. This is why you would remove from the 4s orbital first if you had to on a homework problem. when there are 5 electrons in the d orbital or 10 electrons in the d orbital, you are writing the most "stable" configuration.

[Kyra LeRoy 1E]

The 4s energy level is higher than 3d (and 5s higher than 4d, etc.), and you remove electrons from the highest energy level first, therefore you remove from the 4s first. You remove from the higher energy levels so that the ion is more stable.