Heavy metals are here treated as light metals and, in general, are not further considered. The heaviness of naturally occurring metals such as gold, copper, and iron may have been noticed in prehistory and, in light of their malleability, led to the first attempts to craft metal ornaments, tools, and weapons. All metals discovered from then until 1809 had relatively high densities; their heaviness was regarded as a singularly distinguishing criterion. From 1809 onwards, light metals such as sodium, potassium, and strontium were isolated. Their low densities challenged conventional wisdom and it was proposed to refer to them as "metalloids" (meaning "resembling
Period 6 element oxide. Bismuth metal has been known from ancient times, although until the 18th century it was often confused with lead and tin, which each have some of the metal's bulk physical properties. The etymology is uncertain but possibly comes from Arabic "bi ismid" meaning having the properties of antimony or German words "weisse masse" or "wismuth" meaning"white mass." Bismuth is the most naturally diamagnetic of all metals, and only mercury has a lower thermal conductivity. Bismuth has classically been considered to be the heaviest naturally occurring stable element, in terms of atomic mass. Recently, however, it has been found to