Alkaline earth metal
The
alkaline earth metals are the
series of
elements in
Group 2 (
IUPAC style) of the
periodic table:
beryllium (
Be),
magnesium (
Mg),
calcium (
Ca),
strontium (
Sr),
barium (
Ba) and
radium (
Ra) (though radium is not always considered an alkaline earth due to its
radioactivity).
The alkaline earth metals are named after their
oxides, the
alkaline earths, whose old-fashioned names were
beryllia,
magnesia,
lime,
strontia and
baryta. These were named
alkaline earths because of their intermediate nature between the
alkalis (oxides of the
alkali metals) and the
rare earths (oxides of rare earth metals). The classification of some apparently inert substances as 'earths' is millennia old. The earliest known system used by the ancient
Greeks consisted of four
elements, including earth. This system was later refined by philosophers and
alchemists such as
Aristotle (
4th century BC),
Paracelsus (first half of
16th century),
John Becher (mid
17th century) and
Georg Stahl (late 17th century), with later thinkers subdividing 'earth' into three or more types. The realization that '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
Substances simples salifiables terreuses, or 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.
The alkaline earth metals are silvery colored, soft, low-density
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. Beryllium is an exception: It does not react with water or steam, and its halides are covalent.For example, where
sodium and
potassium react with water at room temperature,
magnesium reacts only with
steam and
calcium with hot water. These elements all 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.
