Heavy Metals

In the fundamental review paper written by Duffus (2002), 13 different works were cited that used lower limits on the density of a "heavy" metal ranging from 3.5 to 7 g cm-3. The author stated that the threshold varied depending on the author, and that "it is impossible to come up with a consensus". Moreover, he concluded that "any idea of defining "heavy metals" on the basis of density must be abandoned as yielding nothing but confusion". However, this is beside the point; although half of the works cited suggested similar lower limits of 4.5 or 5 g cm-3, plants do not have the ability to detect the density of a metal. Thus, "heavy metal" remains an obscure term in the life sciences. It should also be noted that the review paper of Duffus (2002) was commissioned by the International Union of Pure and Applied Chemistry, and certainly represents a chemical point of view that is often neglected by biologists. Apart from the specific weight, the atomic weight, the atomic number, specific chemical properties, and the toxicity were all mentioned as a possible basis for classification - and then rejected for good reasons. So what should we base our definition of "heavy metals" upon? Indeed, is it necessary to use the term at all? Let us now consider what defining "heavy metals" according to the chemical properties of compounds can offer us.

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