Heavy metal (chemical element)

A heavy metal, broadly speaking, is any relatively dense metal.[1] 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.[2] 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.[4] Atomic numbers of heavy metals are generally given as greater than 20; sometimes this is capped at 92 (uranium). 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."[5] 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."[6] 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."[2] 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."[7]

Etymology

The term borrows from ancient conceptions of metals as heavy, dense substances, in contrast to light metals—such as sodium, potassium and 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.[8] Light metals had densities of 0.860–5.0 gm/cm3; heavy metals 5.308–22.000.[9]

Uses

Cranberry red glass incorporating colloidal gold
A slab of neodymium-doped laser glass

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 lead, gold, palladium, platinum, or uranium are used to introduce electron density into a biological specimen by staining, negative staining or shadowing and to make conductive coatings. 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. The colours of glass, ceramic glazes and synthetic gemstones are produced by inclusion of heavy metals (or their compounds) such as iron, nickel, copper, chromium, manganese, cobalt, gold, silver and neodymium. Heavy metals, predominantly gold and silver, are used as the basis of currency in most countries of the world.[10]

Toxicity

Some heavy metals, especially cadmium, mercury and lead, are potentially hazardous due to their intrinsic or selective toxicity, particularly in environmental contexts. Other commonly encountered toxic heavy metals are chromium, cobalt, nickel, copper, zinc, arsenic, selenium, silver, antimony and thallium.

Essentiality

Heavy metals regarded as essential for human health in small quantities include vanadium, manganese, iron, cobalt, copper, zinc, selenium, strontium and molybdenum.[11] A deficiency of these essential elements may increase susceptibility to heavy metal poisoning.[12]

See also

Notes

  1. ↑ Excluding s- and f-block metals, hence starting with scandium[3]

Sources

Citations

  1. ↑ Qian 2009, p. 496
  2. 1 2 Duffus 2002
  3. ↑ Rand 1995, p. 23
  4. ↑ 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.
  5. ↑ The United States Pharmacopeia 1985, p. 1189
  6. ↑ Hawkes 1997
  7. ↑ Hübner, Astin & Herbert 2010
  8. ↑ Habashi 2009, p. 31
  9. ↑ Gmelin 1849, p. 2
  10. ↑ von Mises, =Ludwig (2010). The theory of money and credit. CreateSpace Independent Publishing Platform. p. 276. ISBN 9781451578171.
  11. ↑ Bánfalvi 2011, p. 12
  12. ↑ Chowdhury 1987

References

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