Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
alkali metal[′al·kə‚lī ‚med·əl]
any one of several chemical elements of the major subgroup of Group I of Mendeleev’s periodic system of the elements: lithium, sodium, potassium, rubidium, cesium, and francium. The subgroup’s name is derived from the hydroxides of the alkali metals, called caustic alkalies. The atoms of alkali metals have one s electron in their outer shell and, with the exception of lithium, two 5 electrons and six p electrons in the next to the last shell. The alkali metals always exhibit an oxidation number of +1 in their compounds. They are very reactive chemically. They are oxidized rapidly by atmospheric oxygen and react vigorously with water to form alkalies, MeOH (where Me is a metal). Their reactivity increases from lithium to francium. (See alsoLITHIUM; SODIUM; POTASSIUM; RUBIDIUM; CESIUM; and FRANCIUM.)