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

These metals have one electron in the outer shell, which they lose when they react. This electron is kept in place by the attraction of the positive nucleus.

As we descend the group the atoms get larger (an extra electron shell has been added), so the outer electron gets further from the nucleus and the force of the attraction becomes less.

Therefore, the outer electron is lost more easily as we descend the group, which is why the reactivity increases.