The elements sodium to argon make up the third period of the Periodic Table - Scottish Highers Chemistry - Question 1 - 2017

The first ionisation energy increases across the period due to an increasing nuclear charge. As we move from left to right across a period, the number of protons in the nucleus increases, resulting in a stronger attraction between the nucleus and the electrons. This means that more energy is required to remove the outermost electron.

The equation for the second ionisation energy of magnesium is:

$\text{Mg(g)} \rightarrow \text{Mg}^{2+}(g) + e^-$

The large difference between the third and fourth ionisation energies is due to the removal of an electron from a new energy level. After removing three electrons, the fourth electron is removed from a much more stable, inner shell configuration that is closer to the nucleus. This requires significantly more energy because the remaining electrons are held more tightly by the nucleus, resulting in a substantial increase in ionisation energy.

Chlorine (Cl₂) molecules exhibit stronger Van der Waals forces compared to argon (Ar) atoms. This is due to the presence of London dispersion forces that are stronger in chlorine because it has a larger number of electrons compared to argon. Chlorine molecules have a significant surface area for attractions between them, contributing to stronger intermolecular forces. As a result, more energy is required to overcome these forces, leading to a higher boiling point for chlorine relative to argon.