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

1 Answer
Mar 14, 2018

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:

Aufbau Principle - Wikimedia CommonsAufbau 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.