How do you know which element is the most reactive in the periodic table?

The periodic trend of reactivity depends on whether the are metals or nonmetals.

For metals, reactivity increases down a group and from right to left across a period.

Nonmetals do the opposite. For nonmetals, reactivity increases up a group and from left to right across a period.

Francium is the most reactive metal, and fluorine is the most reactive nonmetal.