Which element is the most reactive?

fluorine is the most reactive element

A chemical reaction always includes a reshuffle of electrons (which are found in orbiting the positive protons and neutrons in the nucleus). Ionic reactions tend to be more vigorous than covalent. It is easiest for an atom to lose an electron (therefore easier for it to react/be more reactive) if there is only one electron in the outer shell and there is a big distance from the nucleus. Therefore, all group 1 elements are the very reactive (they all have 1 electron in their outer shell) but Francium is the most reactive (it has the most number of electron shells therefore there is a greater distance between the outer electron and the nucleus). Similarly, the elements that find it easiest to gain an electron are those with 7 electrons in their outer shell (all elements in group 7 as they really want to have 8 electrons in their outer shell). However, in this 'gaining electron' scenario, the most reactive element in group 7 is the one with the fewest electron shells. This is because the electron is attracted by the positive nucleus so the shorter the distance to the nucleus, the better. The most reactive group 7 element is Fluorine.

Francium is the most reactive element, it is at the bottom of group 1 in the periodic table. Because it is in group 1 this means it has 1 electron in its outer shell so to become stable it wants to lose that electron, it is in period (row) 7 it has 7 shells of electrons so that 1 electron that it wants to lose is very far away from the positive nucleus.