Cesium has a large valence electron shell and a low effective nuclear charge. The size of the valence shell affects how tightly bound the outermost electrons are to the nucleus. ... Both of these factors make cesium extremely reactive.

Why is cesium so much more reactive than potassium?

Francium is almost non-existent in nature so cesium is the most reactive metal of those observed. The reason for the trend of increasing reactivity with increasing atomic number within the alkali metals family has to do with the increasing number of electrons.

Is cesium very reactive?

Caesium metal is highly reactive and very pyrophoric. It ignites spontaneously in air, and reacts explosively with water even at low temperatures, more so than the other alkali metals (first group of the periodic table). ... Because of this high reactivity, caesium metal is classified as a hazardous material.