heavy metal (chemistry)

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. Alternative terms are 'metal' or 'semi-metal' (according to the element in view). Some of the nearly 40 known definitions are: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 are toxic in fairly low concentrations. Thorium and uranium are occasionally included in this classification as well, but they are more often referred to as "radioactive metals". 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 hydrogen (and deuterium), helium, and lithium.Any toxic metals may be called "heavy metals", whether or not they are heavy.
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