The alkaline earth metals are the series of elements in Group 2 of the periodic table: beryllium, magnesium, calcium, strontium, barium and radium (not always considered due to its very short half-life).

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 realisation 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. 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 ion.