What is the ionisation energy trend as we go down the group and across the period?

Ionisation energy decreases down the group. This is because the atomic radii also inrease down the group, making the removal of an electron from the valence shell of an atom easier, as electrons located further away form the nucleus aren't attracted by it as strongly as those on shells closer to the nucleus.Across the period ionisation energy increases, because every consecutive element has one more proton in its nucleus (also called 'increase in nuclear charge') and consequently attracts valence electrons more strongly, making the removal of an electron from the outer shell more difficult.