Science Fair Project Encyclopedia
For other meanings, see heavy metal
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 copper and lead on the periodic table of the elements -- having atomic weights between 63.546 and 200.590 and specific gravities greater 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, lead and cadmium have no known vital or beneficial effect on organisms, and their accumulation over time in the bodies of mammals can cause serious illness.
A stricter definition restricts the term to those metals heavier than the rare earth metals, 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 bismuth and gold are horribly toxic. Thorium and uranium are sometimes included as well, but they are more often called simply "radioactive metals".
Also, often the elements beyond mercury, e.g. the actinides such as uranium and plutonium, are not excluded from the heavy metals. In the context of nuclear power plants, tHM means tons of heavy metal.
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