What factor primarily influences the first ionisation energy of an element?

The first ionisation energy of an element is mainly influenced by the number of protons in the nucleus, also known as the nuclear charge. As the number of protons increases, the positive charge of the nucleus increases, which enhances the attraction between the nucleus and the outer electrons. This stronger attraction typically results in a higher amount of energy required to remove an outer electron, thus increasing the first ionisation energy.

Additionally, while the distance of outer electrons from the nucleus and the shielding effect by inner electrons also play significant roles in determining ionisation energy, they do so in a way that is often influenced by the nuclear charge. For instance, if the nuclear charge is relatively high, the impact of distance and shielding becomes less pronounced because the strong attraction from the nucleus can overcome these factors, making it harder to remove an outer electron.

Therefore, the nuclear charge, indicated by the number of protons, is the primary factor that directly influences the first ionisation energy, as it determines the strength of the electrostatic interactions in the atom.