How and why does the reactivity of the alkali
metals change?

A simpler The reactivity of group 1 elements increases as you go down the group because:

the atoms become larger.

the outer electron becomes further from the nucleus.

the force of attraction between the nucleus and the outer electron decreases

Explanation:

Similar Explanation to the The reactivity of alkali metals increases as we move down the group. As the atomic number increases, the atomic racfcus also increases and the single electron in the valence shell is located further away from the nucleus. Hence, Li is least reactive and Francium is the most reactive.