Explain the trend in 1st ionisation energy across the period 3 elements, explaining the anomalies of aluminium and sulfur.

The overall trend is that the 1st ionisation energy increases. This is because as the nuclear charge increases, the attraction of the nucleus to the surrounding electrons becomes stronger. This means that more energy is required to break this electrostatic attraction.Both sulfur and aluminium deviate from the trend and show lower 1st IEs than expected. For sulfur, it is the first element with an electron in the 3p orbital, and as such it lies slightly further from the nucleus and as such is easier to remove.For aluminium, it is the first element to put 2 electrons in one subshell (the first 2p). As such, there is slight repulsion between the two electrons and this weakens the electrostatic interaction with the nucleus, making it easier to remove.