|62144. Mon Mar 27, 2006 4:38 pm
Another question on Density now.
What is the densest of the elements?
MERCURY, URANIUM, GOLD, TUNGSTEN (-10)
Any other likely suggestions?
It's either osmium or iridium. Depends how you measure it.
The two metals are extremely close in density and have changed places several times over the years in vying for top place.
Under the traditional method of determining a substances 'specific gravity' or 'relative density'– by comparing it to water– modern measurements show that osmium just has the edge. Using the very latest technique for determining density, however – the 'space lattice' method, which measures how tightly packed the atomic structure of an element is – iridium wins.
The third densest element is platinum, followed by rhenium, neptunium, plutonium and gold. Lead is way, way down the list – it's only half as dense as either osmium or iridium.
Osmium is a very rare, very hard, silvery-blue metal discovered (along with iridium) in 1803 by the English chemist Smithson Tennant (1761-1815).
Tennant was a vicar’s son from Richmond who studied medicine at Edinburgh University where he attended the lectures oF Joseph Black, the father of modern chemistry, before going on to study both chemistry and botany at Cambridge. In 1796 he received his medical degree and the following year (1797) was the first man to show that diamond is a form of pure carbon. For the last two years of his life he was Professor of Chemistry at Cambridge. During a trip to France, he was crossing a drawbridge in Boulogne on horseback when it broke and he drowned.
Smithson named osmium from osmé, Greek for smell. It gives off highly toxic osmium tetroxide which has a pungent, irritating odour and can damage the lungs, skin and eyes and cause intense headaches. Osmium tetroxide has been used in fingerprinting because its vapour reacts with minute traces of oil left by the fingers to form black deposits. For the same reason it has been used to stain fatty tissue for microscope slides.
At one time, the extreme hardness and resistance to corrosion of osmium made it useful in the manufacture of long-life gramophone styluses, compass needles and the nibs of quality fountain-pens – hence the trade name Osmiroid.
Osmium also has an unusually high melting point of 3,054 C. In 1897, this inspired Karl Auer to create an osmium electric light-bulb filament to improve on that used by Edison. It worked, but the filament was extremely fragile and would break if the bulb was turned upside-down.
However, it was a better bet than Edison’s bamboo filaments and enjoyed some success until 1905. Osmium filaments were replaced tantalum and eventually by tungsten, which has an even higher melting point (3407C) and is still used today. Auer’s most successful legacy was the invention of an alloy of cerium called Auer metal (or Mischmetal ‘mixed metal’ in German) which is used for flints in cigarette lighters.
The name Osram was registered by Auer in 1906. It derives from OSmium and WolfRAM, the German for tungsten. The company had been a pioneer in the development of both osmium and tungsten filament lamps. In 1919, Auer merged with the lighting divisions of AEG and Siemens to become Osram GmbH. Today, it is one of the two leading lighting manufacturers in the world. With headquarters in Munich, it employs 38,000 people at 49 sites in 19 different countries, has customers in about 150 countries and in 2005 had sales worth 4.3 billion Euros.
An alloy of 90% platinum and 10% osmium is used in surgical implants such as pacemakers. Osmium is also used in dating both terrestrial and meteoric rocks, and, like iridium, is used to analyze the KT boundary.
Osmium has few other modern industrial uses. Less than 100 kilos are produced worldwide every year.
Iridium is a yellowish white metal which, like osmium, is closely related to platinum. It is the most corrosion-resistant of all metals and can withstand attack by hot aqua regia (a mix of concentrated hydrochloric and nitric acids) which dissolves even gold. Iridium (and osmium) were both discovered by Smithson Tennant in the residue left by dissolving crude platinum in aqua regia. The name Iridium comes from iris, Greek for rainbow, because of the many beautiful colours its compounds produce.
Iridium has has an extremely high melting point (2446C) and is routinely used at temperatures up to 2000C. It is mainly used as a hardening agent for platinum but is also used to make crucibles and other devices able to withstand extremely high temperatures.
Because of its sturdiness, it was used to make the original standard metre bar in Paris, which is 90% platinum and 10% iridium. This metre bar was replaced as the standard unit of length in 1960.
Iridium is one of the rarest elements on earth (84th out of 92) but improbably large amounts of it are found in the thin geological layer known as the KT boundary laid down about 65 million years ago. Apparently, this can only have come from space and thus adds to support for the theory that an asteroid may have caused the extinction of the dinosaurs.
Most iridium comes from South Africa, where it is obtained as a by-product of platinum. Only about 3 tonnes are produced every year.
The commonest use for iridium is as the contacts in spark plugs.
NICE PRETTY "TABLE OF THE ELEMENTS" CHART FOR THE QUESTION
PICTURES OF LUMPS OF OSMIUM AND IRIDIUM FOR THE ANSWER
Measuring the density of an element isn't as straightforward as you might think. For example, it can be different for different forms of the same element. Diamond is much denser than graphite, but which is the true density of carbon? Both, apparently. There are numerous other more technical difficulties and figures for the supposed densities vary quite widely, both on the internet and in print.
The Royal Society of Chemistry, sticking by the tradtional methods, gives the relative densities of osmium as 22590 kg per cubic metre and iridium as 22420 kg/m3.
Wikipedia (in a brilliantly researched article which spotted a crucial typing error in the official site of the Los Alamos National Laboratory) cites the 'space lattice' method and comes out in favour of iridium, with a given density of 22650 kilograms per square metre, placing osmium second at 22610 kg/m3.
Osmium has a crystal structure that is "hexagonal"; iridium is a "cubic face centred" crystal.
s: NBB s: BES
s: per s: rsc s: thg