Explain the trend of first ionisation energy down a group.

The first ionisation energy is the energy required to remove one electron from an atom. Down a group, there are more electron shells surrounding the nucleus, which results in a greater shielding from the nuclear charge on the outermost electron shells. This makes it easier to remove an electron, as there is a weaker nuclear attraction, so a lower energy is required to overcome it. This is why the first ionisation energy decreases down a group.