it was given in alkali metals if we go down the group the reactivity with water increase.it then turns out francium is the most reactive.but we know that potassium is the most reactive metal plus potassium occur before francium.why this kolaveri

Francium is the most active because the one outer electron is the farthest from the nucleus holding it.Francium is an alkaline metal that is it can easily give away its electron, making it highly reactive. Because the atom is so large, there is a shielding effect between the protons in the nucleus, and the valence electron. this blocks the attractive forces, and the other layers of electrons work to repel the valence electron, decreasing the ionization energy even further. the low ionization energy is what makes francium so reactive, that none of it can be isolated effectively.