alkaline-earth metal

alkaline earth metal

[al-kuh-lahyn-urth, -lin-]

Any of the six chemical elements in the second leftmost group of the periodic table (beryllium, magnesium, calcium, strontium, barium, and radium). Their name harks back to medieval alchemy. Their atoms have two electrons in the outermost shell, so they react readily, form numerous compounds, and are never found free in nature.

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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 characterized homologous behavior down the group.

The alkaline earth metals are silvered 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 valence 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.

Biological occurrences

  • 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, magnesium functioning as the active center in some enzymes, and calcium salts taking a structural role (e.g. bones).
  • Strontium and barium have a lower availability in the biosphere. Strontium plays an important role in marine aquatic life, Especially hard corals. They use strontium to build their exoskeleton. These elements have some uses in medicine, for example "barium meals" in radio graphic imaging, whilst strontium compounds are employed in some toothpastes.
  • Radium has a low availability and is highly radioactive, making it toxic to life.


References

Explanation of above periodic table slice:
Alkaline earth metals Atomic numbers in indicate solids Solid borders indicate primordial elements (older than the Earth) Dashed borders indicate natural radioactive elements with no isotopes older than the Earth

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