“Heavy metal? Like Rock of Ages?” — Isis Hainsworth in Metal Lords
It’s hard to pin down a definition of heavy metal, whether you are talking about the rock-n-roll genre, or elements from the periodic table. Serious fans (of either the music or the elements) can argue at length about what is meant by the term.
In 2002, John Duffus published a paper he called “Heavy Metals”—A Meaningless Term?, where he concludes that the term “is both meaningless and misleading.” So, while there is no standard definition of the term “heavy metal,” there is general agreement that the term is too useful to set aside.
I recently encountered the term on a safety data sheet, in Section 10, Stability and Reactivity. The list of incompatible materials included, along with copper and iron, heavy metals. The result would be hazardous decomposition. But then the SDS recommended stainless steel 316, an alloy of iron, chromium, nickel, and molybdenum, as a material of construction.
When you encounter the term “heavy metal”, however, you have to wonder what exactly is meant. It’s not just semantics.
An Early Encounter
The first time I confronted my ignorance of the definition of “heavy metal” was in college. I was in an elective course on the environment and oddly, perhaps, I was the only chemical engineering student in the class. One day, the class was discussing heavy metals and the professor (not a chemical engineer or a chemist) turned to me and asked, “What, exactly, is a heavy metal?” I was mortified to realize that I didn’t know the definition. I could give examples, but then, so could every other student in the class, including the one major in the class. But a definition? I had no idea. So, immediately after class I went to the library to look it up. This is what I found:
Heavy metal: Any metal that reacts with a fatty acid to form a soap that is not soluble in water.
Ah. Heavy metals result in bathtub rings. The answer.
Actually, an answer. There are several ways to define the term and the ability to form a water-insoluble soap isn’t even one of the most commonly used ways.
A leading definition is based on the density of the metal. After all, we’re talking about heavy metals. The density criteria varies depending on the user, but a specific gravity of 5 is often used.
Another definition is based on the atomic number of the metal. Specifically, the definition is any metal with an atomic number of 20 (calcium) or higher. Except, of course, for those who define a heavy metal as any metal with an atomic number higher than 20 (scandium).
Then there are those who define a heavy metal based on its electronegativity. Metals with low electronegativity favor oxygen as an electron donor and form ionic bonds. Metals with higher electronegativity favor nitrogen and sulfur as electron donors and form covalent bonds.
Toxicity is yet another basis for defining a metal as a heavy metal. This seems to be regardless of the form the metal atom takes or the route by which it is ingested. This is a bit circular, though: if it is a heavy metal, then it causes toxic effects, OR if it causes toxic effects, then it is a heavy metal.
One of the most useless definitions, at least in this context, is the definition that astronomers use: a heavy metal is any metal with an atomic number or density greater than that of hydrogen and helium.
It’s tempting to refer to a metal that is not a heavy metal as a light metal. Based on all the definitions of heavy metals (except the astronomers’ definition), there are two unambiguously light metals: potassium and sodium. Likewise, (including the astronomers’ definition) there are three unambiguously heavy metals: bismuth, lead, and mercury. All other metals may or may not be heavy metals, depending on the definition used.
If you are simply arguing semantics in the lunchroom, then it doesn’t really matter. Enjoy your discussion. However, if your concern is catastrophic decomposition of a material you’ll be using in your facility, understanding the definition is much more important.
Make Sure You Know
When you encounter the term “heavy metal,” don’t assume that you know what it means. In fact, it doesn’t matter what you think it means. It doesn’t matter if your rock-n-roll roots go back to the sixties and you think of heavy metal as Led Zeppelin and Black Sabbath, or the term makes you think of Linkin Park and Five Finger Death Punch. What matters is what the person using the term thinks it means. Make sure you know.