Chemists can't agree which elements count as heavy metals
Heavy metals are everywhere in science writing, yet nobody agrees what they are. Published definitions sweep in anything up to 96 of the 118 known elements, and only three, mercury, lead and bismuth, satisfy every one of them. The world's chemistry naming authority calls the term both meaningless and misleading.
Authors pick different yardsticks. Density thresholds range from above 3.5 to above 7 grams per cubic centimetre, with 5 a common choice. Others use atomic weight, starting anywhere from just above sodium to mercury onwards, or atomic number, sometimes capped at uranium. Each approach has odd results: rubidium has a fairly high atomic number yet a density of only about 1.5, lighter than many so-called light metals. A chemical test in the United States Pharmacopeia, which precipitates impurities as coloured sulfides, would instead group together the transition and post-transition metals.
The idea of weight goes back a long way. Prehistoric people probably noticed how heavy and malleable gold, copper and iron were, which led to the first metal ornaments, tools and weapons. In 1817 Leopold Gmelin of Germany sorted the elements three ways, separating the nonmetals and then splitting metals into light and heavy at a density of roughly 5. Light metals such as magnesium, aluminium and titanium were only discovered from 1809 onwards.
The biggest problem is the link to poison. The International Union of Pure and Applied Chemistry argues that describing heavy metals as toxins has no scientific basis: density has almost no biological consequence, and pure metals are rarely the active form. The leading toxicology textbook, Casarett and Doull's, prefers toxic metal. Some dense metals, including iron, cobalt, copper and zinc, are essential nutrients in trace amounts, and others such as silver and indium are fairly harmless, while arsenic, cadmium, mercury and lead are highly poisonous. Exposure can come from mining, smelting, industrial waste, farm runoff, paints and treated timber.
An alternative scheme sorts metal ions by chemistry instead, into those preferring oxygen partners, those preferring nitrogen or sulfur, and a borderline group, but it has not caught on. Meanwhile these metals, rare in Earth's crust, turn up in golf clubs, cars, solar panels, mobile phones and particle accelerators.
Source: Heavy metals
