A "heavy metal" is a loose definition for a group of chemical elements that contain metallic properties. Heaviness of a metal is measured differently depending on whether the term refers to density, atomic weight or "relative atomic mass" that alludes to force rather than weight, or toxicity in medicine such as beryllium, which is the fourth lightest element in density. All heavy metals exist naturally on Earth with large variations in concentration.
Dense Metals of the Periodic Table.
Measuring the density of an element in grams per cubic centimeter, there are several metals that hit double digits. To give an idea, the lightest metal, lithium, registers a density of 0.53 grams per cubic centimeter. Silver measures 10.5, lead 11.35, mercury 13.55, uranium 18.95, gold 19.32. tungsten 19.35 and plutonium 19.84.
Platinum measures a density of 21.45 grams per cubic centimeter. It is a malleable transition metal --- hard with high melting and boiling points. It is an extremely rare element in the Earth's crust found mostly in South Africa, which accounts for 80 percent of platinum production worldwide. The element's name comes from the Spanish word "platina," meaning little silver. Although platinum was discovered by Spanish astronomers Ulloa and Santacilia in 1735 in South America, natives in the 16th century first recorded discovering the strange metal in Colombian mines and that it would not burn in fire. Because it does not combine easily with other elements, pure platinum is used in catalytic converters, laboratory equipment, electrical contacts and electrodes, dentistry equipment and jewelry. Its rarity makes it a highly valuable precious silvery-white metal.
Iridium's density measures 22.4 grams per cubic centimeter. Iridium is considered to be of low toxicity but in powder form is an irritant and a fire hazard. Like platinum the white-silver-yellow transition metal iridium is rare and hard, and even more corrosion-resistant than platinum. Iridium is generally thought to be the second densest metallic element. Unusually high amounts of iridium have been found in rocks dating from over 65 million years ago which has led to a widely held view that an iridium-containing comet struck the Earth at that time, causing mass life extinction. Iridium is mainly used as a hardening agent for platinum alloys, for equipment that needs to be used at high temperatures.
Osmium registers a density of 22.6 grams per cubic centimeter, the densest metallic element of the periodic table. During his platinum ore research in the 19th century, English chemist Smithson Tennant discovered in 1803 both Iridium and osmium. The name comes from the Greek "osme' meaning a smell associated with toxic osmium tetroxide which is released by the powdered metal in air. Highly pungent and toxic, it can cause lung, skin and eye damage and is rarely used in pure form. The brittle and rare bluish-gray transition metal osmium has the highest melting point and the lowest vapor pressure of the three. Osmium is used to produce very hard and expensive alloys. An alloy of 90 percent platinum and 10 percent osmium is used in surgical apparatus such as implants like pacemakers and replacement heart valves. Osmium is the natural companion to platinum, found most often together in the same rocks.