Alkaline earth metal
2008/9 Schools Wikipedia Selection. Related subjects: Chemical elements
The alkaline earth metals are a series of elements comprising Group 2 ( IUPAC style) of the periodic table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra). The alkaline earth metals provide a good example of group trends in properties in the periodic table, with well characterised homologous behaviour down the group.
The alkaline earth metals are silvery colored, soft metals, which react readily with halogens to form ionic salts, and with water, though not as rapidly as the alkali metals, to form strongly alkaline ( basic) hydroxides. For example, where sodium and potassium react with water at room temperature, magnesium reacts only with steam and calcium with hot water:
- Mg + 2 H2O → Mg(OH)2 + H2
Beryllium is an exception: It does not react with water or steam, and its halides are covalent.
All the alkaline earth metals have two electrons in their outermost shell, so the energetically preferred state of achieving a filled electron shell is to lose two electrons to form doubly charged positive ions.
The alkaline earth metals are named after their oxides, the alkaline earths, whose old-fashioned names were beryllia, magnesia, lime, strontia and baryta. These oxides are basic (alkaline) when combined with water. "Earth" is an old term applied by early chemists to nonmetallic substances that are insoluble in water and resistant to heating--properties shared by these oxides. The realization that these earths were not elements but compounds is attributed to the chemist Antoine Lavoisier. In his Traité Élémentaire de Chimie (Elements of Chemistry) of 1789 he called them salt-forming earth elements. Later, he suggested that the alkaline earths might be metal oxides, but admitted that this was mere conjecture. In 1808, acting on Lavoisier's idea, Humphry Davy became the first to obtain samples of the metals by electrolysis of their molten earths.
- Beryllium's low aqueous solubility means it is rarely available to biological systems - it has no known role in living organisms, and when encountered by them, is generally highly toxic.
- Magnesium and calcium are ubiquitous and essential to all known living organisms. They are involved in more than one role, with for example Mg/Ca ion pumps playing a role in some cellular processes such as modern day batteries, magnesium functioning as the active centre in some enzymes, and calcium salts taking a structural role (e.g. bones).
- Strontium and barium have a lower availability in the biosphere. They generally have no natural role in biological systems, (perhaps the only documented example is the primitive marine organism Acantharea, which uses strontium sulphate to build its exoskeleton). These elements have some uses in medicine, for example " barium meals" in radiographic imaging, whilst strontium compounds are employed in some toothpastes.
- Radium has a low availability and is highly radioactive, making it toxic to life.