If sodium is placed in water, it reacts vigorously, producing hydrogen gas and hydroxide. What property of alkali metals explains this reactivity?

Alkali metals have only one electron in their outermost shell, and this electron is held loosely because the atoms are large and the nuclear charge is spread over many inner electrons. Because the outer electron is far from the nucleus and shielded, it can be removed with very little energy, so the ionization energy is very low. When the metal touches water, that single electron is donated to a water molecule, forming a metal ion, hydrogen gas, and hydroxide ions. The ease of losing this electron explains why sodium, potassium, and the rest of the alkali metals react so vigorously with water.