In hydrogen all orbitals in the same shell have the same energy: s p d

but once some orbitals contain electrons the energies of other orbitals are affected due to repulsion

so the p and d orbitals have higher energy than the s orbitals of the same shell: sp d You need to copy this diagram

Hydrogen 1s 1

Helium 1s 2

Lithium 1s 2 2s 1

Beryllium 1s 2 2s 2

Boron 1s 2 2s 2 2p 1

Carbon 1s 2 2s 2 2p 2

Nitrogen 1s 2 2s 2 2p 3

Oxygen 1s 2 2s 2 2p 4

Try to write down the electronic configurations of : Neon (z = 10) Sodium (z = 11) Silicon (z = 14) Potassium (z = 19) 1s 2 2s 2 2p 6 1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 3s 2 3p 2 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1

You may have expected the final electron in potassium to be in the 3d sub-shell 1s 2 2s 2 2p 6 3s 2 3p 6 3d 1 but potassium is so similar to sodium that the electronic structure should also be similar with a outer s electron The explanation is that the increase in energy of the 3d sub-shell puts it higher than the 4s. 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 1s 2 2s 2 2p 6 3s 1 Na K

1 2 3 s p d 4 4f NB 3d sub-shell higher than the 4s.

1 2 3 s p d 4 1s 2 2s 2 2p 6 3s 2 3p 6 4s Potassium

This accounts for the odd series of lengths of periods in the Periodic Table : 2, 8, 8, 18 when the maximum number of electrons per shell is 2, 8, 18, 36 Bromine for example is the 17 th element across the PT but has only 7 outer electrons.

1 2 3 s p d 4 Bromine 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p