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

1 Answer
Mar 14, 2018

Answer:

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 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.