It’s hard to believe in this day and age that nobody really knows what the heaviest element in the world is. This uncertainty caused me having to re-grade 30 chemistry tests one year and if there is anything a teacher cannot stand to do is to take back a test because you marked it incorrectly.

Depending on where you look it is a toss-up between osmium and iridium having the highest density. The two metals are very close and have changed first place over the last hundred years.

Wikipedia says osmium is 22.59 grams per cubic centimeter while iridium is a tad below that. The Hewitt physics book I used for years says iridium is the winner at 22.65 and — to make it worse — the old periodic table chart I would pull down in class was off on both counts. In 2018 there still seems to be no straight information.

Everyone agrees, however, that platinum is the third densest followed by rhenium, neptunium, plutonium and gold in that order.

What about lead? Forget it, lead doesn’t even make it to the top 20 when density is concerned. While a good sized car battery may weigh 60 pounds you couldn’t even think about picking one up if it were made of osmium or iridium.

Let’s take a look at these super-dense elements.

Osmium is a very rare silvery blue metal discovered in ores of platinum. In early days, chemists who studied platinum often dissolved it in aqua regia (a combination of hydrochloric and nitric acids) to create soluble nitrate salts. When all of the platinum was gone a small amount of a dark, insoluble residue would often result and many thought this was simply a form of graphite.

In 1803 the British scientist Smithson Tennant analyzed this residue and concluded it must contain a new metal the world had not known. Working the black residue he discovered two new elements at the same time, almost on the same day! He named one iridium after Iris, the Greek winged goddess of the rainbow because many of the salts he obtained were strongly colored. The second he named osmium after Greek word osme, meaning “a smell,” because of the ashy and smoky smell of the volatile osmium tetroxide. Discovery of both new elements was documented in a letter sent to the Royal Society on June 21, 1804.

Within Earth’s crust both osmium and iridium are found together at highest concentrations in either igneous deposits or impact craters. The largest known reserve is located at the Bushveld Igneous Complex in South Africa. The Sudbury Basin in Canada also offers some as well.

Since osmium alloys are very hard they are often used in the tips of fountain pens. Sometimes electrical relays that open and close contacts utilize a coating of osmium because they can resist wear from frequent operation.

Years ago, osmium was used as needles for phonographs to play 78 rpm records. Many a jukebox at the soda shop ran with osmium needles until they were replaced by sapphire and diamond tips during the 1970s.

Osmium forms many compounds. The tetroxide is a pungent and irritating gas that finds use in fingerprint detection due to its ability of reacting with the oils left by the skin to form black deposits of colloidal metal. It is also used in staining fatty tissue for optical and electron microscopy because it is a good oxidizer and because its atoms have plenty of electrons. Osmium staining greatly enhances image contrast in transmission electron microscopy.

The fact that osmium has such a high melting point, slightly over 3030 degrees Celsius, it was at one time used for light bulbs. The light bulb manufacturer Osram, founded in Germany in 1919, derived its name from the elements of osmium and wolfram, the last being German for tungsten.

You may see the name Osram Sylvania when you buy light bulbs since they still manufacture and market a wide range of lighting products for homes, business and vehicles and since the 1970s hold the largest share of the North American lighting market.

Devices that must withstand high temperatures and chemical erosion are often made from iridium. Its melting point of 2446 degrees Celsius and hardness are sometimes used for multi-pored spinnerets, through which a plastic polymer melt is extruded to form fibers, such as nylon or rayon.

Resistance to arc vaporization makes iridium alloys ideal for electrical points for high grade spark plugs, particularly in the aviation field. Sometimes the combination alloy of osmium–iridium is used for compass bearings and balances.

You may remember when a team led by Nobel Prize winner Luis Alvarez proposed that the thin stratum of iridium rich clay, found at the the Cretaceous-Paleogene boundary of 66 million years ago, was due to an asteroid or comet impacting the Earth. The theory, now known as the Alvarez hypothesis, is now widely accepted to explain the extinction of the non-avian dinosaurs.

Gary Hanington is Professor Emeritus of physical science at Great Basin College and chief scientist at AHV. He can be reached at garyh@ahv.com or gary.hanington@gbcnv.edu.

Outbrain