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.
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,[n 1] or 200 or more. Atomic numbers of heavy metals are generally given as greater than 20; sometimes this is capped at 92 (U). 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."
The term borrows from ancient conceptions of metals as heavy, dense substances, in contrast to light metals—such as Na, K and Template:Strontium—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.
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 Pb, Pt or U, 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.
Some heavy metals, especially Cd, Hg and Pb, are potentially hazardous due to their intrinsic or selective toxicity, particularly in environmental contexts. Other commonly encountered toxic heavy metals are Cr, Template:Cobalt, Ni, Cu, Zn, As, Se, Ag, Sb and Tl.
Heavy metals regarded as essential for human health in small quantities include Template:Vanadium, Template:Manganese, Fe, Template:Cobalt, Cu, Zn, Se, Template:Strontium and Template:Molybdenum. A deficiency of these essential elements may increase susceptibility to heavy metal poisoning.
- ↑ Qian 2009, p. 496
- ↑ 2.0 2.1 Duffus 2002
- ↑ Rand 1995, p. 23
- ↑ Baldwin 1999, p. 267: "The term 'heavy metal'…should probably be reserved for those elements with an atomic mass of 200 or greater" i.e. mercury onwards.
- ↑ The United States Pharmacopeia 1985, p. 1189
- ↑ Hawkes 1997
- ↑ Hübner, Astin & Herbert 2010
- ↑ Habashi 2009, p. 31
- ↑ Gmelin 1849, p. 2
- Baldwin DR & Marshall WJ 1999, 'Heavy metal poisoning and its laboratory investigation', Annals of Clinical Biochemistry, vol. 36, no. 3, pp. 267–300, doi:10.1177/000456329903600301
- Duffus JH 2002, '"Heavy Metals"—A Meaningless Term?', Pure and Applied Chemistry, vol. 74, no. 5, pp. 793–807
- Gmelin L 1849, Hand-Book of Chemistry, vol. III, Metals, translated from the German by H Watts, Cavendish Society, London
- Habashi F 2009, 'Gmelin and his Handbuch', Bulletin for the History of Chemistry, vol. 34, no. 1, pp. 30–1
- Hawkes SJ 1997, 'What is a "Heavy Metal"?', Journal of Chemical Education, vol. 74, no. 11, p. 1374
- Hübner R, Astin KB & Herbert RJH 2010, 'Heavy metal'—time to move on from semantics to pragmatics?', Journal of Environmental Monitoring, vol. 12, pp. 1511–1514, doi:10.1039/C0EM00056F
- Qian Y 2009, 'Heavy metal-regulated gene expression', in K Ramos (ed.), Comprehensive Toxicology, vol. 2, Celluar and Molecular Toxicology, Elsevier, Amsterdam, ISBN 978-0-08-046868-6
- Rand GM, Wells PG & McCarty LS 1995, 'Introduction to aquatic toxicology', in GM Rand (ed.), Fundamentals Of Aquatic Toxicology: Effects, Environmental Fate And Risk Assessment, 2nd ed., Taylor & Francis, London, pp. 3–70, ISBN 1560320907
- The United States Pharmacopeia 1985, 21st Revision, The United States Pharmacopeial Convention, Rockville, Maryland, ISBN 0-913595-04-7