General Characteristics of the Compounds of the Alkaline Earth Metals

For the alkaline earth metals, the dipositive oxidation state (M²⁺) is the sole, predominant valence. Their compounds, like those of the alkali metals, are predominantly ionic -- but less ionic than the corresponding alkali metal compounds, because of the alkaline earth metal cations' greater nuclear charge and smaller size. The paragraphs below survey how the group's oxides, hydroxides, halides and salts of oxo-acids (carbonates, sulphates, nitrates) behave as a family, before Sections 5.6.1-5.6.4 look at the specific, named compounds of calcium in detail.

1. Oxides. Alkaline earth metals generally form monoxides (and, for most of the family, peroxides too). Monoxides are obtained simply by heating the metal in oxygen. BeO and MgO are almost insoluble in water, while the oxides of the heavier members go on to form hydroxides. In terms of basicity, BeO is amphoteric, MgO is only weakly basic, and CaO, SrO and BaO are all strongly basic. BeO itself is covalent, on account of the small size of the Be²⁺ ion, while the other monoxides are ionic. Peroxides: every alkaline earth metal except beryllium forms a peroxide, made by heating the monoxide with oxygen at high temperature, e.g. 2BaO + O₂ → 2BaO₂.
2. Hydroxides. Every oxide except BeO is basic and reacts with water to give a sparingly soluble hydroxide: MO + H₂O → M(OH)₂. Solubility, thermal stability and basic character of these hydroxides all increase down the group; even so, the alkaline earth metal hydroxides are less basic and less stable than the corresponding alkali metal hydroxides. Beryllium hydroxide is the exception, being amphoteric -- it reacts with both acid and alkali: Be(OH)₂ + 2NaOH → Na₂BeO₂ + 2H₂O and Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O.
3. Halides. Alkaline earth metals form halides of general formula MX₂, prepared by heating the metal with the halogen: M + X₂ → MX₂. Beryllium's halides are covalent, on account of the small size of Be²⁺; they are hygroscopic, fume in moist air, and are soluble in organic solvents. Solid beryllium chloride has a chain structure (Figure 5.9); in the vapour phase, BeCl₂ tends to form a chloro-bridged dimer, which then dissociates into the linear monomer only at high temperatures around 1200 K. Every other alkaline earth metal halide, by contrast, is ionic -- the chlorides and fluorides of the remaining metals are ionic solids that conduct electricity well, both in the fused state and in aqueous solution. The tendency of these halides to form hydrates gradually decreases down the group, e.g. MgCl₂·8H₂O, CaCl₂·6H₂O, SrCl₂·6H₂O and BaCl₂·2H₂O. Salts of oxo-acids. Carbonates: every alkaline earth carbonate decomposes on heating to give carbon dioxide and the oxide, MCO₃ →(Δ)→ MO + CO₂. Their solubility in water decreases down the group, while thermal stability increases down the group as the cationic size grows (Table 5.13). …