No Consensus on "Heavy Metals"

Articles Posted in January 2008

No Consensus on "Heavy Metals"

Postby utopiasi » Mon Jan 14, 2008 11:08 am

assembled by staff

"There is a tendency, unsupported by the facts, to assume that all so-called “heavy metals” and their compounds have highly toxic or ecotoxic properties. This has no basis in chemical or toxicological data. Thus, the term “heavy metals” is both meaningless and misleading."

- John H. Duffus, The Edinburgh Centre for Toxicology, Scotland, "Pure and Applied Chemistry" 74, 793–807

Bjerrum’s Inorganic Chemistry contains the earliest reference to the term "Heavy Metals" in chemistry, published in 1936. Bjerrum defined heavy metals based on density. Any metal with a density greater than 4 g/cm3 was thus considered to be a heavy metal. However, Bjerrum's definition was never accepted, nor used as a formal or official definition of a heavy metal.

However, there is no relationship between density and any reactive properties associated with metals, or any other element in the periodic table. Heavy metals were redefined based on a gram atomic weight, and also by the atomic number on the periodic table ( with many variations in "opinion" of the starting number ). By some of these "official" definitions of what a heavy metal is, both Magnesium and Potassium are classified as heavy metals.

The most widely used toxicology reference, Casarett and Doull’s Toxicology, never uses the term “heavy metal”, but only references toxic metals. This is most likely due to the fact that the heavy metal classification, in any historical or modern form, lacks scientific validity.

A few thoughts by Stephen J. Hawkes of Oregon State University, Department of Chemistry, demonstrate an interesting perspective:

"The metals that I have seen referred to as heavy metals comprise a block of all the metals in Groups 3 to 16 that are in periods 4 and greater. This seems to be a definition that should be generally useful. It may also be stated as the transition and post-transition metals. These acquired the name heavy metals because they all have high densities, but the usefulness of the term is related to their chemistry, not their density. It is not necessary to decide whether semimetals should be included as heavy metals, which is fortunate, since it is unlikely that any decision would be generally agreeable."

From a chemical standpoint, the above definition is satisfactory. However, it is clear that this definition of "heavy metal" may not in any way be related with suspected toxicity, but has meaning only pertaining to specific chemical properties.

Hawkes also places the whole "controversy" in rather comical perspective by sharing a chemistry teacher's answer to the question: What is a Heavy Metal?

"I asked this question of my introductory chemistry teacher over 50 years ago. He replied rather hesitantly, "A metal that behaves in a “heavy metal manner." A vague term requires a vague definition, but just what is a “heavy metal manner?”

There are also some articles in this week’s Silver Bulletin that may also shed some light on the subject.
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