Any of the monovalent metals of group I of the periodic table (lithium or sodium or potassium or rubidium or cesium or francium).
Any of a group of six metallic elements with similar chemical properties: lithium, sodium, potassium, rubidium, cesium, and francium. They form a linked group (Group One) in the periodic table of the elements. They are univalent (have a valence of one) and of very low density (lithium, sodium, and potassium float on water); in general they are reactive, soft, low-melting-point metals. Because of their reactivity they are only found as compounds in nature.