Alkaline Earth Metals Beryllium Magnesium Calcium Strontium and Barium

Magnesium and calcium ions are extremely common in natural water systems, with calcium carbonate (limestone) and dolomite [CaMg(CO3)2] being two widespread natural sources. Solubility in water is influenced by pH and CO2 content. The two ions are responsible for the hardness of water, which manifests itself by precipitation with soaps, the calcium carbonate deposits that form when water is heated (boiler scale), and so on. For many purposes (washing, waters for certain heat exchange processes), the precipitates formed by calcium and magnesium ions are obnoxious, and the ions must be held in solution by chelation or removed by ion exchange processes or precipitation in a way that prevents the formation of harmful products. Both magnesium and calcium are essential elements needed in significant amounts by living organisms. Except in the context of the problems just noted, they are not harmful either in solution or as particulate material.

Chemically, magnesium behaves similarly to calcium but generally is found in much lower concentrations in water. Both exist solely as the 2+ ions in solution, with interactions that are predominantly Coulombic. The ions favor coordination by oxygen donors; simple complexes are weak, but chelation can give moderate stability. Magnesium, being smaller, shows stronger bonding with solvent or ligand molecules and forms more stable complexes. Both ions are important in a variety of enzymatic and other biological processes where their weak complexing and rapid exchange determines their function. Calcium is the essential cation in structural features such as bones [as hydroxyapatite, Cai0(PO4)6(OH)2], and marine shells (calcium carbonate, usually in the form of aragonite) where the inorganic crystalline materials are bonded by an organic matrix. In spite of chemical similarities, calcium is not interchangeable with magnesium because of the significant ionic size difference. Calcium metal is extremely reactive and has little industrial use as such; magnesium metal on the other hand, is less reactive and is important structurally since it combines lightness and strength.

The heavier members of the alkaline earth family, strontium and barium, are comparatively rare. Both resemble calcium more than magnesium, and strontium in particular is close enough in size to Ca(II) that its ions can frequently replace those of calcium in solids such as bone. Since one radioactive isotope of strontium is an abundant fission product, this replacement is of considerable importance and is considered in Chapter 14. Nonradioactive strontium, however, is not particularly toxic. Soluble barium compounds, on the other hand, are highly toxic if ingested, although barium sulfate, a very insoluble compound, is commonly ingested during x-ray diagnostic procedures and is harmless.

The lightest alkaline earth, beryllium, is much different in its properties because its small size, high ionization energy, and greater electronegativity favor much more covalent bonding. It also differs in that it is highly toxic, as has been recognized for many years. Acute beryllium poisoning was recognized in the 1930s and 1940s, especially among workers in the fluorescent lamp industry, where beryllium phosphors were employed. Beryllium also exhibits chronic toxicity. Beryllium is employed for a number of high-level technological purposes, such as in the nuclear industry, and as an alloy with Cu having high corrosion resistance; people who work with such metals have also experienced toxicity. While these technologies offer at least the potential for industrial pollution, another source is the beryllium that may be released from burning coal, which usually contains up to 3 ppm Be but may contain much more in particular cases.

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Responses

  • dennis saenger
    Why does calcium shows more resemblance to magnesium than strontium?
    1 year ago

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