Explain why potassium has a greater first ionisation energy than rubidium.

The outer electron of potassium is closer to its nucleus than the outer electron for rubidium, as it has a stronger attractive force between the electron and the potassium nucleus. The outer electron for K also has less shielding from other electrons than for Rb. Although Rb has a greater nuclear charge, the distance and the shielding its outer electron faces means it has a weaker effective nuclear force attracting it than for K.