Explain the trend in first ionisation energy as you go across Period 3

Overall, the first ionisation energies will increase. This is because as you go along the period, the atomic number increases and this causes a stronger electrostatic attraction between the outer electrons and the nucleus. There are 2 "dips" in this trend. There is a drop in energy from Magnesium to Aluminium. This is because in magnesium the outer electrons are in a 3s orbital, whereas in aluminium the outer electron is in a 3p orbital. The 3p orbital is further from the nucleus, and hence there is more electron shielding and distance from the nucleus so electrostatic attraction decreases. The second drop is from Phosphorus to Sulphur. This is because it is the first time electrons are paired up in 3p orbitals, so there is added electron-electron repulsion.