Can somebody please help me answer this : why are the 4th and the 12th ionisation values so high? Why does sulphur have a lower 1st ionisation value than oxygen or phosphorous?
For the 4th and 12th element, if you look at their electronic configurations;
4: 1S2, 2S2
In both of these cases the sub-levels are all filled, which gives them greater stability, which means higher values of ionisation.
As for sulfur, its similar.
Sulfur is 16, so the config is : 1S2,2S2,2P6,3S2,3PX2,3PY1, 3PZ1
Phosphorous before it, has 15, : 1S2,2S2,2P6,3S2,3PX1,3PY1, 3PZ1
The important thing to look at is the "3PX" sub level, in Phosphorous, all the "3P" sub levels are half-filled (Half-filled gives greater stability).
Whereas in Sulfur, the "3PX" sub level has an extra electron, meaning the 3P sub level is just over half-full.
In summary, if an element has all of its sub-levels half-filled or filled then it will have greater stability, and therefore higher ionisation values.