General Characteristics of the Compounds of Alkali Metals

All the common compounds of the alkali metals are, as a family, generally ionic in nature -- a direct consequence of how readily these highly electropositive metals give up their single valence electron. The paragraphs below survey how the oxides/hydroxides, halides and salts of oxo-acids behave across the whole group, before Section 5.3.1 looks at four specific, industrially important sodium compounds in full detail.

Oxides and hydroxides. On combustion in excess air, alkali metals form normal oxides of general formula M₂O, which react with water to give the corresponding (basic) hydroxide: M₂O + H₂O → 2MOH. Every alkali metal except lithium additionally forms a peroxide on combustion with excess air; these peroxides react with water to give the hydroxide plus hydrogen peroxide: M₂O₂ + 2H₂O → 2MOH + H₂O₂ (M = Na, K, Rb, Cs). Going further still, every alkali metal except lithium and sodium also forms a superoxide, which similarly gives the hydroxide (plus H₂O₂ and O₂) with water: 2MO₂ + 2H₂O → 2MOH + H₂O₂ + O₂ (M = K, Rb, Cs). Under the right conditions, the pure compound M₂O, M₂O₂ or MO₂ can each be isolated.

Properties: the pure oxides and peroxides are colourless, while the superoxides are yellow or orange. The peroxides are diamagnetic, but the superoxides -- having an unpaired electron -- are paramagnetic. Sodium peroxide, Na₂O₂, is widely used as an oxidising agent. The hydroxides obtained from these oxides are all white crystalline solids and strong bases; they dissolve in water with the evolution of heat, on account of the intense hydration the small, highly-charged-density M⁺/OH⁻ ions undergo.

Halides. The alkali metal halides, MX (X = F, Cl, Br, I), are colourless crystalline solids with high melting points. They can be prepared by reacting the appropriate oxide, hydroxide or carbonate with the aqueous hydrohalic acid (HX). As the electropositive character of the metal increases from Li to Cs, the ease of halide formation also increases from Li to Cs. All these halides are ionic except LiBr and LiI, which show appreciable covalent character. Similarly, all the halides are soluble in water except LiF, whose low solubility comes from its unusually high lattice enthalpy (both Li⁺ and F⁻ being very small ions). Because of their partial covalent character, LiBr and LiI are additionally soluble in organic solvents. …