Chemistry - How 'heavy' should an element be, to be a "Heavy Metal"?

Solution 1:

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.

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

Solution 2:

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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Solution 3:

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.