Heavy metals

The heavy metals are a group of elements between copper and mercury on the periodic table of the elements -- having atomic weights between 63.546 and 200.590 and specific gravities greater than 4.0. Living organisms require trace amounts of some heavy metals, including cobalt, copper, iron, manganese, molybdenum, vanadium, strontium, and zinc, but excessive levels can be detrimental to the organism.

In medical usage, the definition is considerably looser, and "heavy metal poisoning" can include excessive amounts of manganese, aluminum, or beryllium as well as the true heavy metals.