# An atom contains a total of 25 electrons. When the atom is in the ground state, how many different principal energy levels will contain electrons?

##### 1 Answer

Well, with

With

#1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^2 -> [Ar] 3d^5 4s^2#

And you can convince yourself that

This lands us in the **first-row transition metals**, and so we have involved

#n = 1# :#1s#

#n = 2# :#2s, 2p#

#n = 3# :#3s, 3p, 3d#

#n = 4# :#4s#

We ignore the