Often I hear mention made of so-called “heavy metals”: metals that it is considered a bad idea to get into one’s body. So which metals fall into this category? Lead, mercury and cadmium are the three that I seem to hear mentioned the most, but I have heard that there are others as well. What determines whether a metal is “heavy” or not? Do the heavy metals all have some chemical property (or properties) in common, or are they designated as such simply due to their toxicity (or is it a bit of both: their toxicity being due to some chemical property they all have)?
There are a wide range of definitions, depending on the properties one wishes to group them by.
Link. As QED notes, I don’t believe there is a hard-and-fast definition. Zinc is common for bird poisoning.
Well, they’re heavy. That may sound like a flip answer, but one definition (Webster’s dictionary) is “metals with a specific gravity greater than 5g/cm[sup]3[/sup]”.
However, as I remember from my chemistry classes, they are the transition metals in rows 4 and below.Cite. Actually, that cite indicates that the post-transition metals, up to group 16, are also included.
By that definition, and looking at the [url=]periodic table, you have the elements coloured pink from 21 (scandium) down to 80 (mercury), and the first four of the orange-coloured ones after each of these rows (Ga-Se, In-Te, and Tl-Po).
It seems you’re not alone, either. From the bottom of my linked page:
Sorry, a bit more I should have added:
It’s both. Most of the heavy metals are transition metals, i.e. they have partially filled d-orbitals. They are therefore very good at forming “complexes” with organic compounds. This is useful in the body in the right place - e.g. haemoglobin is a complex of iron with a protein molecule - but also means that heavy metals can bind to proteins that they shouldn’t, which is why they are toxic. See here: http://www.luminet.net/~wenonah/hydro/heavmet.htm