First ionisation energy

• the general trend is to increase, this is due to increasing number of protons as the electrons are being added to the same shell
• there is a small drop between Mg and Al as Mg has its outer electrons in the 3s subshell, while Al is starting to fill the 3p subshell, making Al's electron easier to remove because the 3p electrons are higher in energy
• there is a small drop between phosphorus and sulfur. sulfur's outer electron is being paired up with another electron in the same 3p orbital. when the second electron is added to an orbital there is a slight repulsion between the two negatively charged electrons which makes the second electron easier to remove