The reactivity of the alkali metals (group 1) increases as the atomic number increases.
Which one of these statements is the correct explanation of this fact?

Answer:

23) E

As we go down group 1, the number of shells increases by 1. Since the number of shells increases, the atomic radius also increase. The attraction between the nucleus and the outer electron decreases. Therefore, it becomes easier to lose the outer electron as you go down group 1. Hence, the alkali metals become more reactive.

The other options are correct statements but they do not explain why the reactivity increases.