Heavy metals

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). There are nearly 40 known definitions. Among them:

• According to one definition, heavy metals are a group of elements between periodic table of the elements—having specific gravities greater than 4.0.

• A more strict definition increases specificity to metals heavier than the 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 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 lithium.

• Any toxic metals may be called "heavy metals", irrespective of their atomic mass or density.