Reactivity of alkali metals

The alkali metals are a group in the periodic table consisting of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr).This group lies in the s-block of the periodic table as all alkali metals have their outermost electron in an s-orbital. Because of their high reactivity, they must be stored under oil to prevent reaction with air, and are found naturally only in salts and never as the free element. But the reactivities of alkali metals are not the same, instead sodium is more reactive than lithium and potassium is more and more reactive than sodium, as shown above. Which of following can explain these differences in reactivities between alkali metals?