How does the reactivity of alkali metals change as you descend the group?

As you descend the group of alkali metals in the periodic table, their reactivity increases with each period. This trend is primarily due to the increase in atomic size and the decrease in ionization energy as you move down the group.

The outermost electron in alkali metals is held less tightly to the nucleus because it is farther away from the positively charged protons as the atomic number increases. With the outer electron being more shielded by the inner electron layers, it becomes easier for the atom to lose this electron and participate in chemical reactions, leading to greater reactivity. Additionally, the alkali metals tend to react with water and halogens, and this propensity grows stronger as one moves down the group.

The other choices suggest opposing observations or suggest constant behavior, which do not accurately reflect the established trend in reactivity of alkali metals. Thus, the increase in reactivity as one goes down the group is a well-documented trend in chemistry.