Heavy metals
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- For other uses, see Heavy metal (disambiguation).
A heavy metal is any of a number of higher atomic weight elements, which has the properties of a metallic substance at room temperature. There are several different definitions concerning which elements fall in this class designation:
- According to one definition, heavy metals are a group of elements between copper and bismuth on the periodic table of the elements—having specific gravities greater than 4.0.
- A more strict definition increases specificity to metals heavier than the rare earth metals, which are at the bottom of the periodic table. None of these are essential elements in biological systems and additionally, most of the better known elements sans bismuth and gold, are toxic [citation needed]. Thorium and uranium are occasionally included in this classification as well, but they are more often referred to as "radioactive metals". Please see actinides in the environment for further details of these 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.
- In astronomy, which defines any element heavier than helium a metal, a heavy metal or heavy element includes all elements that were not formed in the big bang; all but only hydrogen (and deuterium), helium, and lithium.
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[edit] Relationship to living organisms
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. The pathway for toxic effects on humans is normally: (i) for the entry of heavy metals into the atmosphere as industrial stack gas; (ii) or to enter the soil as a soil contaminant; (iii) or to enter groundwater as a water pollutant; or to be deposited in ocean bottoms or bay mud, which materials at a later time be dredged to the surface.
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.
[edit] Sources
- Kuhn, Karl F. and Koupelis, Theo (2004) In Quest of the Universe, Fourth Edition. Jones and Bartlett Publishers, Canada. ISBN 0-7637-0810-0
