General characteristics of the compounds of the alkaline earth metals
The dipositive oxidation state (M2+) is the predominant valence of group 2 elements. The alkaline earth metals form compounds which are predominantly ionic. However, they are less ionic than the corresponding compounds of alkali metals. This is due to increased nuclear charge and smaller size. The general characteristics of some of the compounds of alkaline earth metals are described below.
Generally alkaline earth metals form monoxides and peroxides.
Monoxides are obtained by heating the metals in oxygen. BeO and MgO are almost insoluble in water. On the other hand, oxides of other elements form hydroxides. BeO is amphoteric; MgO is weakly basic while CaO, SrO and BaO are strongly basic.
BeO oxide is covalent due to the small size of Be2+ion,while other oxides are ionic in nature.
Except beryllium, all the remaining metals form peroxides. It is prepared by heating monoxides with oxygen at high temperature.
2 BaO +O2 → 2 BaO2
All the oxides except BeO are basic in nature and react with water to form sparingly soluble hydroxides.
MO + H2O →M(OH)2
The solubility, thermal stability and the basic character of the hydroxides increase down the group. The alkaline earth metal hydroxides are, however, less basic and less stable than alkali metal hydroxides. Beryllium hydroxide is amphoteric in nature as it reacts with both acid and alkali.
Be(OH)2 + 2 NaOH → Na2BeO2 +2H2O
Be(OH)2 + 2HCl → BeCl2 +2H2O
Alkaline earth metals form halides with general formula MX2. They can be prepared by heating metals with halogens on heating.
M +X2 → MX2
Beryllium halides are covalent on account of smaller size of Be+2. Beryllium halides are hygroscopic, fume in moist air and soluble in organic solvents. Beryllium chloride has a chain structure in the solid state as shown in figure 5.9 (structure-a). In the vapour phase BeCl2 tends to form chloro-bridged dimer (structure-c) which dissociates into the linear monomer at high temperatures of the order of 1200 K. (structure-b).
Except beryllium halides, all the other halides of alkaline earth metals are ionic in nature. Chloride and fluorides of the other metals are ionic solids. These are good conductors of electricity in fused state and in aqueous solutions. The tendency to form halide hydrates gradually decreases (for example, MgCl2..8H2O, CaCl2.6H2O, SrCl2.6H2O and BaCl2.2H2O) down the group.
The alkaline earth metals form salts of oxo acids. Some of these are given below:
All the carbonates decompose on heating to give carbon dioxide and the oxide.
MCO3 -- ∆→ MO + CO2
· The solubility of carbonates in water decreases down the group.
· The thermal stability increases down the group with increasing cationic size.
The sulphates of the alkaline earth metals are all white solids and stable to heat. BeSO4, and MgSO4 are readily soluble in water; the solubility decreases from CaSO4 to BaSO4. The greater hydration enthalpies of Be2+ and Mg2+ ions overcome the lattice enthalpy factor and therefore their sulphates are soluble in water.
The nitrates are made by dissolution of the carbonates in dilute nitric acid. Magnesium nitrate crystallises with six molecules of water, whereas barium nitrate crystallises as the anhydrous salt. This again shows a decreasing tendency to form hydrates with increasing size. All of them decompose on heating to give the oxide.