What is the most reactive element?
Due to its positioning on the periodic table, Francium should theoretically be the most reactive element, however because it is also radioactive, Cesium is instead.
An element's reactivity is based on how many electron shells it has. The more electron shells, the less magnetic attraction to the nucleus the electrons on the outer shells have. Therefore, they can easily travel to other chemically attractive atoms (chemically attractive meaning that their outer electron shells are incomplete and only need one or two more or less electrons to be complete.)
As you may know, the further down you go (along the y axis), the more electron shells the elements have and apart from the noble gases, the transitional metals are the least reactive group on the periodic table (due to having moderately balanced outer shells) which are situated towards the centre of the table. Therefore, the farther you go from those elements, the reactivity of the elements increase.
Alkali metals (situated far away from transitional metals and noble gases) are the most reactive elemental group. Cesium is second from the bottom of this group, with 6 electron shells, so it fits all the characteristics of a reactive atom, therefore making it the most reactive element.