Group IIA metals

Group IIA metals inelude Be, Mg, Ca, Sr, Ba and Ra whieh are grey, moderately-hard, high melting-point substanees. Like the alkali metals they attaek water to liberate hydrogen but with less vigour. The salts of the alkaline earths are generally less stable towards heat and water than those of alkali metals, and less water soluble. 

The decomposition of dolomite shows many points of similarity with the reactions of calcite and of other single carbonates of Group IIA metals (Sects. 3.1.1 and 3.1.2) the reaction is reversible, occurs at an interface, and both apparent kinetic parameters and reactivity are influenced by the prevailing C02 pressure. 

This chapter describes the formation of bonds of the group-IA and group-IIA metals to the group-IA, group-IIA, group-IB and group-IIB metals, as well as to the transition and inner transition metals. 

The binary mixtures lack miscibility in the liquid and solid states with any of the group-IIA metals or with the heavier group-IA metals, K, Rb and Cs (Fig. 1). There is, however, a progressive increase in the miscibility of Li and Na with the group-IIA metal as the atomic number of the latter increases. This is accompanied by compound formation Li with Ca, Sr and Ba, and Na with Sr and Ba " . 

A major problem is the reactivity of the metals, particularly the group-IIA metals, to the atmosphere and the containment vessel. Products containing Ca, Sr and Ba are often as reactive as the metals Be- and Mg-containing products, however, are generally chemically inert. As a result, much of the original work was undertaken on impure samples and products (quoted metal purities of 98% were not uncommon). 

Typically, Be-containing alloys and intermetallic phases have been prepared in beryllia or alumina crucibles Mg-containing products have been synthesized in graphite, magnesia or alumina crucibles. Alloys and compounds containing Ca, Sr and Ba have been synthesized in alumina , boron nitride, zircon, molybdenum, iron , or steel crucibles. Both zircon and molybdenum are satisfactory only for alloys with low group-IIA metal content and are replaced by boron nitride and iron, respectively, for group-IIA metal-rich systems . Crucibles are sealed in silica, quartz, iron or steel vessels, usually under either vacuum or purified inert cover gas in a few cases, the samples were melted under a halide flux . 

As a result of their reactivity, particular attention must be given to preparation and purification of the metals, the conditions under which the metals, alloys and compounds are handled and the choice of material for the containment vessel. Ultrapure group-IIB metals may be used without further purification, but it is advisable to purify the group-IIA metals by a multidistillation process, the final distillation preferably being carried out in situ. The reactants and products are best handled in an atmosphere of a purified inert gas, usually He or Ar (N2 cannot be used because of the ready formation of group-IIA metal nitrides) alternatively, they can be handled under vacuum or, in rare cases, under halide fluxes. The containment vessel is normally fabricated from a refractory. 

Compounds containing Mg bonded to an inner transition metal or other group-IIA metals bonded to transition metals are not known. Compounds such as I, in which there are only weak interactions between Mg and M, are excluded from this discussion. 

Hydrogen sulfide gas is used in analytical chemistry laboratories to detect certain metal ions in a solution. If the solution contains a Group IIA metal, such as calcium, a precipitate will form when an acidic solution containing hydrogen sulfide is added to it. The precipitate forms because one of the products of the chemical reaction is insoluble (which means it does not dissolve). 

For most atoms, the addition of an electron occurs with the release of energy, so the value of AE is negative. There are some exceptions, most notably the noble gases and group IIA metals. These atoms have completely filled shells, so any additional electrons would have to be added in a new, empty shell. Nitrogen also has virtually no tendency to accept an additional electron because of the stability of the half-filled outer shell. 

The oxides of the group IIA metals are ionic, so they react with water to produce the hydroxides. 

The sulfides of the group IIA metals generally have the sodium chloride structure, but those of the group IA metals have the antifluorite structure because the ratio of anions to cations is 2. Solutions of the sulfides are basic as a result of the hydrolysis reaction... 

Most oxides (O2-) are insoluble except for those of Group IA and Group IIA metals, which react with water. (These will produce strong bases.)... 

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