Heavy metal (chemical element)

A heavy metal, broadly speaking, is any relatively dense metal. The concept is sometimes extended to include metalloids such as arsenic and antimony. More specific definitions have been proposed; none have obtained widespread acceptance. Some heavy metals have niche uses, or are notably toxic; some are essential in trace amounts.

Definitions
There is no widely agreed criteria-based definition of a heavy metal. Criteria used to define heavy metals have included density, atomic weight, atomic number, aqueous chemistry or periodic table position. Density criteria range from above 3.5 g/cm3 to above 7 g/cm3. Atomic weight definitions start at greater than sodium (22.98) to greater than 40, or 200 or more. Atomic numbers of heavy metals are generally given as greater than 20; sometimes this is capped at 92. The United States Pharmacopeia includes a test for heavy metals, which is described as a test for "metallic impurities that are colored by sulfide ion." Hawkes, writing in 1997, and in the context of fifty years of experience with the term, said it referred to "metals with insoluble sulfides and hydroxides, whose salts produce colored solutions in water and whose complexes are usually colored." He suggested referring to heavy metals as "all the metals in Groups 3 to 16 that are in periods 4 and greater" or, in other words, the transition metals and post-transition metals, and commented that this definition "should serve the needs of most chemists and some others who use the term." In contrast, and writing in 2002, Duffus concluded that "over the 60 years or so in which it has been used in chemistry, it has been given such a wide range of meanings by different authors that it is effectively meaningless." Despite these inconsistencies, references to "heavy metals" appear regularly in scientific literature: a 2010 study found that "use of the term is still widespread and increasing" and that it "seems to have been 'vernacularised' in science."

Etymology
The term borrows from ancient conceptions of metals as heavy, dense substances, in contrast to light metals—such as, and —which were unknown before 1809. An early use dates from 1817, when the German chemist Leopold Gmelin divided the elements into nonmetals, light metals and heavy metals. Light metals had densities of 0.860–5.0 gm/cm3; heavy metals 5.308–22.000.

Uses
Heavy metals have niche uses in electron microscopy, nuclear science, mechanical engineering and soaps, that take advantage of either their density or chemistry. In electron microscopy, heavy metals such as, or , are used to introduce electron density into a biological specimen by staining, negative staining, or shadowing. In nuclear science and mechanical engineering, heavy metals, sometimes in the form of alloys, are used for radiation screening, or for balance weights on wheels, crankshafts, gyroscopes, propellers and centrifugal clutches, in situations requiring maximum weight in minimum space. In soap chemistry, heavy metals form insoluble soaps (unlike soluble sodium- or potassium-based soaps) that are used in lubricating greases, paint dryers, and fungicides.

Toxicity
Some heavy metals, especially, and , are potentially hazardous due to their intrinsic or selective toxicity, particularly in environmental contexts. Other commonly encountered toxic heavy metals are, , , , , , , , and.

Essentiality
Heavy metals regarded as essential for human health in small quantities include, , , , , , , and. A deficiency of these essential elements may increase susceptibility to heavy metal poisoning.