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any one of the chemical elements of the main subgroup of Group II of Mendeleev’s periodic system, comprising the calcium family: calcium, strontium, barium, and radium (beryllium and magnesium are also sometimes considered to be alkaline-earth metals). The origin of the name is related to the fact that the oxides of the alkaline-earth metals (”eerths” in alchemical terminology) were found to be alkaline in water. The outer electron shell of alkaline-earth metal atoms contains two 5 electrons; the preceding shell contains two s and six p electrons. The alkaline-earth metals exhibit an oxidation number of +2 in their compounds. They are chemically reactive, and their reactivity increases from calcium to radium. (See alsoCALCIUM; STRONTIUM; BARIUM; and RADIUM.)