What element in the fourth period is an exception to the Aufbau principle?

The aufbau principle states that electrons are placed in orbitals of lower energy levels before placing themselves in higher energy levels. It goes like this:

Aufbau Principle - Wikimedia Commons

So first, we have `1s`, then `2s`, then `2p`, then `3s`, followed by `3p, 4s, 3d`, and so on and so forth.

Writing down the electron configuration of vanadium, the element just before chromium, we have:

`1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^3`, or `["Ar"]4s^2 3d^3`.

Obviously, chromium should be same, except that we have `3d^4`, right? Well, no. Since `3d^4` is very unstable, as one orbital is left unfilled while all others are (partly) filled. So one electron is taken from the `4s` orbital, and since the orbital still has one electron, it's okay to do so. So chromium's electron configuration will be:

`["Ar"]4s^1 3d^5`, not `["Ar"]4s^2 3d^4`. So in manganese, the next element, the `4s` orbital is completely filled.

Similarly, for nickel, the element just before copper, the config is:

`["Ar"]4s^2 3d^8`

For the same reasons as before, copper's config is:

`["Ar"]4s^1 3d^10`, instead of `["Ar"]4s^2 3d^9`. Zinc, the element after copper, has this config:

`["Ar"]4s^2 3d^10`

It all ties up.