The alkali metals become more reactive as you move down in the group's order in the periodic table starting with lithium, and francium is the bottom element of the group. These metals get more reactive because each successive member has more electrons, meaning that the reactive valence electron is further away from the nucleus. The nucleus' protons therefore have less pull on the valence electron, which allows it to react more freely. Francium reacts violently with water, and even a small vial of it in water creates an enormous explosion.