Chemical Reactivity of Alkali Metals

Alkali metals are the most chemically reactive of all metals, on account of their very low ionization enthalpies, and this reactivity increases steadily on descending the group from lithium to caesium.

Reaction with oxygen. Every alkali metal tarnishes rapidly in moist air and must be stored under kerosene oil to protect it from atmospheric oxygen and moisture. On burning in an excess supply of dry oxygen, the three lightest members give three different kinds of oxide, in a pattern that is one of the most striking features of Group 1 chemistry:

and rubidium and caesium likewise give superoxides, $\text{RbO}_2$ and $\text{CsO}_2$. The reason is a balance between lattice energy and the size of the anion formed: the very small $\text{Li}^+$ ion has a strong preference for the small $\text{O}^{2-}$ ion (giving the highest lattice energy with the oxide), while the much larger $\text{K}^+$, $\text{Rb}^+$ and $\text{Cs}^+$ ions are better matched in size to the larger peroxide ($\text{O}_2^{2-}$) and, even more so, the still larger superoxide ion ($\text{O}_2^{-}$), so these bigger, less-tightly-packed anions become the thermodynamically favoured product as the cation grows.

Reaction with water. Every alkali metal reacts with water to liberate dihydrogen gas and form the metal hydroxide, by the general equation

Lithium reacts comparatively gently (though the reaction is strongly exothermic); sodium reacts vigorously enough that the heat generated often melts the metal into a moving globule; and potassium reacts so violently that the hydrogen evolved usually catches fire spontaneously. This increasing vigour down the group tracks the falling ionization enthalpy directly -- it becomes progressively easier for the metal to lose its valence electron to water.

Reaction with hydrogen. On heating in a stream of dihydrogen, every alkali metal combines directly to form a white, crystalline ionic hydride, containing the hydride ion, $\text{H}^-$: …