What element in the fourth period is an exception to the Aufbau principle?
Copper and chromium
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
Writing down the electron configuration of vanadium, the element just before chromium, we have:
Obviously, chromium should be same, except that we have
Similarly, for nickel, the element just before copper, the config is:
For the same reasons as before, copper's config is:
It all ties up.