What happens when an atom has more electrons than can fit in one energy level?

Answer : If any atom has more electrons than one energy level can hold, then automatically the electron is accommodated in the next energy level (shell). The remaining extra electrons starts to fill the next energy level. This produces the valency of that particular atom.

The electrons will start entering in higher levels.

Explanation:

When an atom has more electrons that can fit in one energy level then the extra electrons will start filling in next higher energy levels based on following rules of filling up of electrons

a) Auf bau Principle: The orbitals with lower energy will be filled first.

b) Hund's rule of maximum multiplicity: the electrons start pairing once they are filled singly in each orbital of same energy.

c) Paul's exclusion principle: no two electrons in an atom can have all the four quantum number same.