A fiery temperament of alkali metals

Chemistry Level 1

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?