2.8 explain the relative reactivities of the elements in Group 1 in terms of distance between the outer electrons and the nucleus

All elements in Group 1 have 1 electron in their outer shell. 

The elements further down the group are more reactive. This is because as you go down the group the outer shell (with the single electron) gets further away from the nucleus (as there are complete shells inbetween them). This means that, because the electron is further away, it is more easily lost (given away).