Reactivity of metals is generally determined by their position in the periodic table. Metals in Group 1 (also known as alkali metals) are highly reactive and show an increasing trend in reactivity as we go down the group. This means that Cs, being lower in the group than Na, is more reactive.

The reactivity of alkali metals is mainly due to their low ionization energy and large atomic radii. Cs has a lower ionization energy and a larger atomic radius than Na, making it easier for Cs atoms to lose their outermost electron and form positive ions. This increased tendency to lose electrons results in higher reactivity.