Energy Levels of Electrons

[Brian Huynh 3D]

How come the energy state is higher at 3d10 than 4s when Z(atomic number) < 20?

[Rachel_Smith_3D]

3d is higher energy than 4s when Z is less than or equal to 20, but is lower once Z is greater than 20. This shift seems sudden but is actually the result of a continuous change in the relative energies of 4s and 3d. You can see a graph of that change here:

http://www.scielo.br/img/revistas/qn/v36n6/27f02.jpg

The reason why Cu is [Ar] 3d10 4s1 instead of 3d9 4s2 is because of symmetry. Half-filled or fully-filled orbitals are more stable than partially-filled ones, so that electron is drawn into the 3d state.