Question

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

Answer 1

Copper and chromium

Explanation:

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:

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.