From looking at the graph we can see that there is a drop from neon, which has a full outer shell to sodium which has one electron in its outer shell. The only reason for this drop would be that the effect nuclear charge has dropped. The ENC has dropped, although the nuclear charge has increased, because the new electron must have joined a new shell, and is thus further away from the nucleus as well as experiencing more shielding. This is why less energy is required to make a gaseous atom of sodium into an ion, as the electron in its outer shells is easier to remove, due to less attraction between it and the nucleus.
As we move on, the next element up, magnesium, we can see that more energy is required to remove the extra electron added to the shell. This is because the nucleus has a higher effective nuclear charge and therefore, a greater force of attraction is felt between the electrons. As we move on to the next element, aluminium, we should be able to see a further rise in the energy required to remove the electron, as it should be in the same shell (note that the second shell holds up to 8 electrons). However, from the graph, we can see that there is a slight drop, although the nuclear charge has increased again. This indicates that the distance must have changed, as this is the only other factor which would cause the effective nuclear charge to drop. Therefore, one can see the evidence for sub shells as there is no other explanation for the drop in ionisation energy. This sub shell is called the “p-sub shell”. The first two electrons in the 2nd shell are in the “s-sub shell.”
The next two elements, silicon and potassium also require an increase in energy, because the effective nuclear charge has increased. But again, there is a further drop at sulphur. Now that we know the existence of sub shells, we know that the 1st shell has one sub shell (s), the 2nd sub shell has two sub shells (s and p) and the third will have 3 sub shells(s, p and d) and so on. What we also know is that the s sub shell can hold 2 electrons, the p sub shell can hold 6 electrons and the d sub shell can hold 10 electrons. Therefore, we have to take into consideration why there should be a drop at ionisation energies of sulphur when the electron should remain the same distance from the nucleus as the other electrons in the p-sub shell. For this we need to look at Afbau’s Principle which states that
From the table we can see that the 1st shell contains one orbital, where two electrons occupy the s-sub shell. The 2nd shell has 4 orbitals, one which is occupied by 2 electrons in the s-sub shell and a further 6 electrons occupying three more orbitals to make up the p-sub shell. As we can see from the diagram above, the electrons in magnesium, aluminium and silicon occupy one orbital in the p sub shell. However, each orbital can hold two electrons. Therefore, at sulphur, the electron has to join with another electron in one of the orbitals. Therefore it receives a larger repulsion from the electron that is already present in the orbital, as they are both negatively charged. Hence, it is easier for the electron to be removed as it is constantly facing repulsion against its neighbouring electron. This is why there is another dip at sulphur.
Chlorine and argon follow the same principle as sulphur and join orbitals that have already been occupied by other electrons. However, since the effective nuclear charge has raised, the force of attraction that is felt is larger, and so again a greater force of attraction from the nucleus is felt by them. The process ends with potassium where there is another large drop, where potassium has joined a new shell, sub shell, and orbital and because it is now further away from the nucleus as well as experiencing more shielding, the force of attraction is less.
