Francium is the most reactive element in Group 1.

Explain why in terms of electronic structure

all group 1 elements have 1 electron in their outer shell, making this the electron that is involved in reactions. as you move down a group in The periodic table the number of shells increases, which means that the outer electron is further from the nucleus. as there are now more shells between the electron and the nucleus, the outer electron experiences more shielding and therefore less attraction to the nucleus. This means that the outer electron is more easily removed in francium than other group one element

Francium is hypothesized to be the most reactive metal, but so little of it exists or can be synthesized, and the longest half-life of its most abundant isotope is 22.00 minutes, so that its reactivity cannot be determined experimentally.

Explanation:

Francium is an alkali metal in group 1/IA. All alkali metals have one valence electron. As you go down the group, the number of electron energy levels increases - lithium has two, sodium has three, etc ..., as indicated by the period number. The result is that the outermost electron gets further from the nucleus. The attraction from the positive nucleus to the negative electron is less. This makes it easier to remove the electron and makes the atom more reactive.

Experimentally speaking, cesium (caesium) is the most reactive metal.