Heavy metals are the term used for a group of elements that are on the "heavier" end of the periodic table of elements. Some heavy metals - such as copper, iron, manganese, and zinc - are essential to health in trace amounts. Others are non-essential and can be harmful to health especially in excessive amounts. These include cadmium, thallium, antimony, mercury, lead, and arsenic - the last three being the most common in cases of heavy metal toxicity.