Define the phrase first ionization energy of an element. Write an equation for the first ionization energy of chlorine.

(a)

(i) The first ionization energy of an element is the minimum energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous $1+$ ions.

(ii) The equation for the first ionization energy of chlorine is:

$$\text{Cl}_{(\text{g})} \rightarrow \text{Cl}^{+}_{(\text{g})} + \text{e}^-$$

(b)
1. Magnesium has a higher first ionisation energy than aluminium.
Magnesium has an electron configuration of $[\text{Ne}] 3\text{s}^2$, while aluminium has $[\text{Ne}] 3\text{s}^2 3\text{p}^1$. The electron removed from magnesium is from a filled and stable 3s subshell. The electron removed from aluminium is from a 3p subshell, which is at a slightly higher energy level and is shielded by the 3s electrons. This makes the 3p electron in aluminium easier to remove than a 3s electron in magnesium, resulting in aluminium having a lower first ionization energy.

2. Sulphur has a lower first ionisation energy than phosphorus.
Phosphorus has an electron configuration of $[\text{Ne}] 3\text{s}^2 3\text{p}^3$, which corresponds to a half-filled and stable 3p subshell. Sulphur has an electron configuration of $[\text{Ne}] 3\text{s}^2 3\text{p}^4$. The electron removed from sulphur is a paired electron in a 3p orbital. This paired electron experiences electron-electron repulsion, making it easier to remove compared to an electron from the more stable, half-filled 3p subshell of phosphorus.