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I've come across the term "Heavy metals" innumerable times in articles, mostly pertaining to environmental issues.

Is there a weight range (of sorts) against which an element (metal) is classified as a "Heavy metal"?

Often I see "Heavy metals" refer to Cadmium (Atomic number 48), Mercury (Atomic number 80) and Lead (Atomic number 82), but does this term also apply to metals of intermediate atomic masses [such as Tungsten (Atomic number 50), Gold (Atomic number 79) and Tin (Atomic number 81)]?

I can't seem to find any chemical literature that deals with this, so I suspect that the term "Heavy metal" has no real serious scientific definition.

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    $\begingroup$ Beryllium, the second lightest metal, is pretty toxic. It should therefore (ironically) be included in many definitions of "heavy" metals based on toxicity. Note that even lithium is bioactive, and is used (normally as the carbonate) in appropriate doses to treat mental illness. $\endgroup$
    – Level River St
    Commented Jan 1, 2017 at 19:28
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    $\begingroup$ For some people all elements with an atomic number greater than helium are metals, and all elements with an atomic number greater than iron are heavy metals. $\endgroup$
    – aventurin
    Commented Jan 1, 2017 at 21:28
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    $\begingroup$ That's the astronomer's definition of metals. $\endgroup$
    – Russell Borogove
    Commented Jan 2, 2017 at 4:08
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    $\begingroup$ According to Wikipedia, there is no universally accepted definition. $\endgroup$
    – Dmitry Grigoryev
    Commented Jan 2, 2017 at 11:46
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    $\begingroup$ @Russell Borogove: Yes. The funny thing is that even hydrogen and helium exist as metallic phases. $\endgroup$
    – aventurin
    Commented Jan 2, 2017 at 14:52

4 Answers 4

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There is no true, accepted definition of heavy metal. I was taught to apply the option a metal that has density equal to or over $5.0\ \mathrm{g/cm^3}$.

Other variants include a different density range, specific gravity over density, environmental impact, atomic number, toxicity, or atomic mass, even chemical properties. See here$^{[1]}$ for further information and references.

$[1]$ John H. Duffus. '"Heavy metals"—a meaningless term?'. IUPAC technical report. Pure and Applied Chemistry, $(2002)$, 74(5), pp 793$-$807.

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There is no set criteria, as far as I am aware. I tend to think of iron as the median.

Thus, I would happily refer to metals above iron as being 'heavy', i.e. density in excess of $\pu{7.8 g cm-3}$.

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I don't know of a single agreed way. The way I use relates to the way elements are created.

The way elements are created in the universe is through fusion in stars. Fusion releases energy when elements are light and heavy elements require energy ( soak up energy ) in order to fuse.

Iron is the element at which marks the boundary between releasing energy and soaking up energy. Elements above Iron are consider heavy, so I would describe metals with an atomic number above Iron as heavy metals.

When a star begins to create Iron ( and heavier elements ) it starts to die ( rapidly, because energy generated is less than that required to resist gravitational collapse ), so it's a pretty definite marker for how to separate light and heavy elements.

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I think it should be a bio-chemical term, because they act as toxic in biological forms and they block biological processes in a cell or an organ.

The term "heavy metal" should be an interactivity of a metal classified element on bioorganic reactions.

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