Hope this helps
Atomic radius increases down the group, due to the addition of shells and thus greater electron shielding = less electrostatic attraction between electrons and the nucleus. Atomic radius also increases as atomic number increases. Less energy is needed to remove an outer electron and thus make the group 1 element an ion.
For the highest element of group 1, lots of energy is needed to seperate the valence electron to make a +1 ion due to this greater electrostatic attraction between the electron and nucleus as a result of the smaller atomic radius and smaller nucleus.