A fiery temperament of alkali metals

The alkali metals are a group in the periodic table consisting of the chemical elements lithium ($\ce{Li}$), sodium ($\ce{Na}$), potassium ($\ce{K}$), rubidium ($\ce{Rb}$), cesium ($\ce{Cs}$), and francium ($\ce{Fr}$). Because of their high reactivity, they must be stored under oil to prevent reaction with air. They are found naturally only in salt and never as a free element. Which of following best explains their high reactivity?