Electron Configuration

[WUng_1D]

Can someone explain to me why an element such as Scandium ([Ar]3d14s2) has the 3d orbital come before the 4s orbital?

[Harry Zhang 1B]

Normally, the order of electron-filling takes the following sequence: 1s 2s 2p 3s 3p 4s 3d.... However, when the electrons contain more than 20 electrons, that is, after potassium starting with scandium, the 4s orbital is complete and electrons start going into the 3d orbital but once there is electron in the 3d orbital, for some reason in the quantum world, the 3d electron becomes lower energy and therefore is placed before the 4s electron. This rule is true for all atoms having more than 20 electrons in the subsequent period.

[ABombino_2J]

The 3d orbital has less energy than the 4s orbital so it must come before because electron configurations are written in order based on amounts of energy.

[Chris Tai 1B]

You can also think of this in terms of valence electrons. The 4s shells will contain electrons that are more likely to be lost, thus ionizing a particular atom. Since it's more probable for a 4s electron to be given up by an atom than a 3d electron, the 3d shell should come first in the sequence. This makes it easier to write ions too, so for instance

Zn: [Ar] 3d^10 4s^2
Zn^2+: [Ar] 3d^10

[Michelle N - 2C]

When the d-orbital is not full, they are generally lower in energy. Plus, it would then follow the trend and order of the numbers.

Sorry this is vague, but that's how I was able to get it..