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"? In my day job I routinely conducted a test for heavy metals, which was and is described ( 1 ) as a test for "metallic impurities that are colored by sulfide ion". It may mean different things to different people, but as I have used, heard, and interpreted the term over the last half-century it refers to metals with insoluble sulfides and hydroxides, whose salts produce colored solutions in water, and whose complexes are usually colored. During a project to establish a pre-engineering chemistry curriculum, I found several references to heavy metals, together with two attempts to define them. I consulted a number of chemical dictionaries and found that most did not mention them at all. One ( 2 ) defined them as "metals of atomic weight greater than sodium that form soaps on reaction with fatty acids, e.g., aluminum, lead, cobalt." This definition must have been culled from a book about soaps, but clearly is not a useful definition in any other field. Turning to regular dictionaries (as opposed to chemical dictionaries): Webster ( 3 ) gives "A metal of high specific gravity; esp: a metal having a specific gravity of 5.0 or over." All the metals that I think of as heavy metals have densities above 5 g/cm 3, while no metals with densities below 5 have heavy metal chemistry. This supports Webster's definition, but it is deceptive. Being a heavy metal has little to do with density, but rather concerns chemical properties. 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. The definition in bold type above should serve the needs of most chemists and some others who use the term. When it refers to a musical genre, the user is unlikely to be concerned about its chemical meaning.