Contrast the chemical reactivity of potassium lithium and sodium

Answer is: lithium is the least reactive and potassium is the most reactive of the three alkali elements. 
The reactivity of the alkali metals increases down the group because one valence electron (I group of Periodic system of elements) is farther away from nucleus and attraction force between electron and proton is weaker.

Iwould go with i'm not positive because i'm dumb that