2. In hydrogen all orbitals in the same shell have the same energy: 1 2 3 s p d

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

4. so the p and d orbitals have higher energy than the s orbitals of the same shell: 1 2 3 sp d You need to copy this diagram

5. Hydrogen 1s 1

6. Helium 1s 2

7. Lithium 1s 2 2s 1

8. Beryllium 1s 2 2s 2

9. Boron 1s 2 2s 2 2p 1

10. Carbon 1s 2 2s 2 2p 2

11. Nitrogen 1s 2 2s 2 2p 3

12. Oxygen 1s 2 2s 2 2p 4

13. 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

14. 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

15. 1 2 3 s p d 4 4f NB 3d sub-shell higher than the 4s.

16. 1 2 3 s p d 4 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 2 8 8 1 Potassium

17. 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.

18. 1 2 3 s p d 4 Bromine 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 5 2 8 18 7