Any of a group of six metallic elements with similar chemical properties:
lithium,
sodium,
potassium,
rubidium,
caesium, and
francium. They form a linked group (Group 1) in the
periodic table of the elements. They each have a valency of one and have very low densities (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.
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