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Heavy metal (chemical element)
A heavy metal, broadly speaking, is any relatively dense metal.〔Qian 2009, p. 496〕 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.〔Duffus 2002〕 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.〔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.〕 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."〔The United States Pharmacopeia 1985, p. 1189〕 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."〔Hawkes 1997〕 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."〔Hübner, Astin & Herbert 2010〕
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