Why does the 1st ionisation energy of the atoms decrease down group 1? What does this tell us about their reactivity?

As you descend the group, the nuclear charge increases with the addition of further protons. Despite this, the atomic radius from the nucleus to the valence electron increases with more electron shells as well as an increase in shielding of the valence electron from these additional shells. As a result, the outer electrons experience a reduced nuclear charge down the group and hence a reduced force so less energy is required to remove them. We know group 1 metals react by losing their outer electron (as seen in 2Na + 2H2O -> 2NaOH + H2) so as you descend the group, the reduction in ionisation energy means it is easier for the metals to lose their electron and react.