Chemical Properties of Alkali Metals

Alkali metals are chemically very reactive. Reactivity increases from Li to Cs, tracking the fall in ionisation energy down the group, and every alkali metal reacts readily with the more electronegative elements -- especially oxygen and the halogens.

Reaction with oxygen. On exposure to air or oxygen, alkali metals burn vigorously, forming a surface layer of oxide -- but which kind of oxide depends on the metal. Lithium forms only the normal (simple) oxide: 4Li + O₂ → 2Li₂O. Sodium forms both the normal oxide and, predominantly, the peroxide: 2Na + O₂ → Na₂O₂. The heavier metals (K, Rb, Cs) go further still, forming the superoxide: M + O₂ → MO₂ (M = K, Rb, Cs). All these oxides are basic.

Reaction with hydrogen. All the alkali metals combine with hydrogen at about 673 K (lithium needs a higher temperature, 1073 K) to give the corresponding ionic hydride: 2M + H₂ → 2M⁺H⁻. Reactivity toward hydrogen actually decreases from Li to Cs, even as the ionic character of the hydride formed increases down the group and its thermal stability correspondingly falls. These hydrides are strong reducing agents, and their reducing power increases down the group.

Reaction with halogens. Alkali metals combine readily with the halogens to give ionic halides: 2M + X₂ → 2MX (M = Li, Na, K, Rb, Cs; X = F, Cl, Br, I). Reactivity toward the halogens increases down the group, tracking the fall in ionisation enthalpy. All the resulting metal halides are ionic crystals, with one notable exception: lithium iodide shows appreciable covalent character. This is because Li⁺, being the smallest alkali metal cation, exerts a strong polarising power on the iodide anion -- and I⁻, being the largest and most easily polarised halide ion, is particularly susceptible to this distortion.

Reaction with liquid ammonia. Alkali metals dissolve in liquid ammonia to give deep blue solutions that conduct electricity -- their specific conductivity is comparable to that of a pure metal (e.g. mercury's specific conductivity is about 10⁴ Ω⁻¹, and sodium dissolved in liquid ammonia gives about 0.5 × 10⁴ Ω⁻¹). This happens because the alkali metal readily loses its valence electron into solution, and both the resulting cation and the free electron become surrounded (ammoniated) by ammonia molecules:

M + (x + y)NH₃ → [M(NH₃)ₓ]⁺ + [e(NH₃)ᵧ]⁻

The deep blue colour comes from this "ammoniated electron," which absorbs energy in the visible region of light. Such solutions are paramagnetic (because of the unpaired electron) and, on standing, slowly liberate hydrogen gas as the electron and cation recombine with ammonia to form an amide: M⁺ + e⁻ + NH₃ → MNH₂ + ½H₂. In more concentrated solution, the blue colour instead changes to a bronze colour, and the solution becomes diamagnetic. …