Which property of alkali metals primarily explains their vigorous reaction with water?

The key property is the very low ionization energy of alkali metals, which means they can lose their single valence electron almost effortlessly. Because that electron is held weakly, the metal atom readily donates it to water, forming a positively charged ion and hydroxide ions. This electron transfer releases a lot of energy, making the reaction rapid and sometimes explosive. For example, when sodium metal touches water it quickly loses its outer electron, producing sodium ions, hydrogen gas, and heat. Thus the low ionization energy explains the vigorous reaction.