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Heavy metals
The term heavy metal may have various more general or more specific meanings. According to one definition, the heavy metals are a group of elements between copperand leadon the periodic tableof the elements?having atomic weightsbetween 63.546 and 200.590 and specific gravitiesgreater than 4.0. Living organisms require trace amounts of some heavy metals, including cobalt, copper, manganese, molybdenum, vanadium, strontium, and zinc, but excessive levels can be detrimental to the organism. Other heavy metals such as mercury, leadand cadmiumhave no known vital or beneficial effect on organisms, and their accumulation over time in the bodies of mammalscan cause serious illness. A stricter definition restricts the term to those metals heavier than the rare earthmetals, at the bottom of the periodic table. None of these are essential elements in biological systems; all of the more well-known elements with the exception of bismuthand goldare horribly toxic. Thoriumand uraniumare sometimes included as well, but they are more often called simply "radioactivemetals". In medical usage, the definition is considerably looser, and "heavy metal poisoning" can include excessive amounts of iron, manganese, aluminium, or beryllium(the second-lightest metal) as well as the true heavy metals. Also, often the elements beyond mercury, e.g., the actinidessuch as uraniumand plutonium, are not excluded from the heavy metals. In the context of nuclear power plants, tHM means tons of heavy metal. External links
de:Schwermetall es:Metal pesado eo:Pezmetalo fr:Métal lourd ko:??? lt:Sunkusis metalas nl:Zwaar metaal ja:??? no:Tungmetall pl:Metale ci??kie fi:Raskasmetalli sv:Tungmetall zh:??? Categories: Chemical element groups| Toxicology| Periodic table
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