158 | The History and Use of Our Earth’s Chemical Elements
158 | The History and Use of Our Earth’s Chemical Elements
ingpercentageoftheelementonEarthisintheformofthetwonaturallyradioactive isotopes:Os-184=0.02%andOs-186=1.59%.Alltheotherisotopesofosmiumare radioactiveandartificiallyproducedinnuclearreactorsandparticleaccelerators.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=32
s2,p6,d10,f14
5-O=14
s2,p6,d6
6-P=2
s2
Properties One of the important properties of osmium is the formation of gases when the metal is
exposed to air. These fumes are extremely toxic, which limits osmium’s usefulness. Osmium is a hard, tough, brittle, bluish-white metal that is difficult to use except in a powder form that oxi-
dizes into osmium tetroxide (OsO 4 ), which not only has objectionable odor but also is toxic. Osmium has a relatively high melting point of 3,054°C and a boiling point of 5,500°C,
with a density of 22.61 g/cm 3 .
Characteristics Osmium is found in group 8 (VIII) of the periodic table and has some of the same chemi-
cal, physical, and historical characteristics as several other elements. This group of similar elements is classed as the platinum group, which includes Ru, Rh, and Pd of the second transi- tion series (period 5) and Os, Ir, and Pt of the third series of transition metals (period 6).
AbundanceandSource Osmium is the 80th most abundant element on Earth. As a metal, it is not found free in
nature and is considered a companion metal with iridium. It is also found mixed with plati- num- and nickel-bearing ores. It is recovered by treating the concentrated residue of these ores with aqua regia (a mixture of 75% HCl and 25% HNO). The high cost of refining osmium is made economically feasible by also recovering marketable amounts of platinum and nickel.
Osmium occurs along with iridium in nature as the mineral iridosmine. It is found in Canada, Russia, and parts of Africa.
History Osmium was discovered in 1803, at the same time as iridium, by Smithson Tennant
(1761–1815). Several researchers, including Tennant, were curious about a black metallic
159 substance that was produced when they refined platinum ore. At one time it was thought to
Guide to the Elements |
be graphite or possibly another allotropic form of carbon. Further research indicated that this substance was not a form of carbon but a mixture of two new elements. Tennant named one of the new elements “iridium” because of its brilliant colors and named the other “osmium” because of its repugnant smell.
CommonUses Because of its hard brittle nature, the metal osmium has few uses. However, the powdered
form can be sintered under high pressure and temperatures to form some useful products, despite its toxicity and malodor. Its main use is as an alloy to manufacture devices that resist wear and stand up to constant use. As an alloy, osmium loses both its foul odor and toxicity. Some of these products are ballpoint and fountain pen tips, needles for record players, and pivot points for compass needles. Osmium alloys are also used for contact points on special switches and other devices that require reduced frictional wear.
Another use is as a stain for animal tissues that are to be examined with a microscope to improve the contrast of the specimen.
ExamplesofCompounds The stable oxidation states of +3 and +4 of osmium are responsible for several different
compounds, including the following:
Osmium (III) tetrachloride: Os 3+ + 3Cl → OsCl 3 . Osmium (IV) dioxide: Os 4+ + 2O 2- → OsO 2 .
There is a special case where the +8 oxidation state of osmium is possible: Osmium (VIII) tetraoxide (Os 8+ + 4O 2- → OsO 4 ) is a yellow crystal and probably the most important compound used as an oxidizing agent, as a biological stain in microscopy, and to detect fingerprints.
Hazards Most of the oxides of osmium are not noxious, but are toxic if inhaled or ingested. The
compound OsO 4 is extremely poisonous. It is a powerful oxidizing agent that is soluble in water and will produce serious burns in skin as it oxidizes the various layers of tissues.
IRIDIUM SYMBOL:Ir PERIOD:6 GROUP:9(VIII) ATOMICNO:77
ATOMICMASS:192.217amu VALENCE:1,2,3,4,and6 OXIDATIONSTATE:+3and +4 NATURALSTATE:Solid ORIGINOFNAME: ThenameiridiumcomesfromtheLatinwordiris,meaning“rainbow,” becauseoftheelement’shighlycoloredsalts. ISOTOPES:Thereare55isotopesofiridium,twoofwhicharestableandaccountforthe element’stotalexistenceonEarth.ThosetwoareIr-191,whichmakesup37.3%ofthe amountintheEarth’scrust,andIr-193,whichconstitutes62.7%ofiridium’sexistence onEarth.Alltheother53isotopesofiridiumareradioactivewithhalf-livesrangingfrom afewmicrosecondstoafewhoursordaysanduptoafewhundredyears.These unstableisotopesareallartificiallyproduced.
160 | The History and Use of Our Earth’s Chemical Elements
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=32
s2,p6,d10,f14
5-O=15
s2,p6,d7
6-P=2
s2
Properties Iridium is a hard, brittle, white, metallic substance that is almost impossible to machine.
It is neither ductile nor malleable. Iridium will only oxidize at high temperatures and is the most corrosive-resistant metal known. This is why it was used to make the standard meter bar that is an alloy of 90% platinum and 10% iridium.
At about the time of the French Revolution, it was decided to determine the length of the meter bar by first calculating the distance from the North Pole to the equator running through Paris. This distance was then divided into equal lengths of 1/10,000,000. A single unit of this distance was then called a “meter” (“measure” in Greek). This platinum-iridium meter bar, currently preserved in France, was for many years the standard unit of length in the metric system that is based on the decimal system. However, this metal bar is no longer used as the standard meter. Instead, the meter is now defined by scientists in terms of the length of the path traveled by light in a vacuum at the time of 1/299,792,458 of a second.
Iridium is highly resistant to attack by other chemicals and is one of the most dense ele- ments found on Earth. Its melting point is 2,410°C, its boiling point is 4,130°C, and its density is 22.560 g/cm 3 .
Characteristics Iridium is one of the so-called platinum group of 6 transition elements (Ru, Rh, and Pd of
period 5 and Os, Ir, and Pt of period 6). It is resistant to strong acids, including aqua regia. It is the only metal that can be used in equipment that must withstand temperatures up to 2,300°C or 4,170°F. Iridium can be poured into casts after it becomes molten. As it cools, it becomes crystalline and, while in this state, can be pulled into wires and formed into sheets. Unlike steel, which becomes more malleable (less brittle) after annealing (a process of heating followed by slowly cooling), iridium is just the opposite—it becomes more brittle and impos- sible to work into shapes after cooling.
161 AbundanceandSource
Guide to the Elements |
Iridium is the 83rd most abundant element and is found mixed with platinum, osmium, and nickel ores. The minerals containing iridium are found in Russia, South Africa, Canada, and Alaska.
Iridium metal is separated from its other metal ores when the combined minerals are dissolved with a strong acid know as aqua regia, which is a mixture of 25% nitric acid and 75% hydrochloric acid. Aqua regia is the only acid that will dissolve platinum and gold. Once the platinum and other metals are dissolved, the iridium, which is insoluble in this strong acid, becomes the residue. The refined iridium ends up in the form of either powder or crystals.
An interesting story as to how most of the iridium appeared on Earth was explained recently by scientists who discovered a thin layer of iridium in the sediments that were laid down in the Earth’s crust at the end of the Cretaceous period. This was a period about 65 mil- lion years ago when meteors and asteroids crashed into the Earth. These extraterrestrial bodies contained a high percentage of iridium. Dust from the impact spread around the Earth and blocked the sun for months, resulting in the extinction of many plants and animals, including the dinosaurs. This extensive dust cloud also deposited a thin coat of the element iridium that was contained in the fiery bolides.
History Iridium and its partner osmium were discovered in 1803 by the English chemist Smithson
Tennant (1761–1815). In essence, he employed the same technique to separate these elements from platinum ores that is used today to purify iridium. He dissolved the minerals with aqua regia, which left a black residue that looked much like graphite. After analyzing this shiny black residue, he identified two new elements—Ir and Os. Tennant was responsible for nam- ing iridium after the Latin word “iris” because of the element’s rainbow of colors.
CommonUses Iridium’s most common use is as an alloy metal that, when added to platinum, makes it
harder and more durable. It is also mixed with other metals to make electrical contacts, ther- mocouples (two dissimilar metals joined to form a special type of thermometer), and instru- ments that will withstand high temperatures without breaking down. It is also used to make special laboratory vessels because iridium will not react with most chemical substances. An alloy of iridium and platinum is used as the standard kilogram weight because it is noncor- rosive and will not oxidize and, thus, change its weight over long periods of time.
The radioisotope, iridium-192, is used to treat cancer and to take X-ray pictures of metal castings to detect flaws. Iridium is also used as a catalyst for several chemical reactions.
ExamplesofCompounds The oxidation states of +3 and +4 form the most stable compounds, examples of which
follow:
Iridium (III) chloride: Ir 3+ + 3Cl 1- → IrCl 3 . Iridium (IV) chloride: Ir 4+ + 4Cl 1- → IrCl 4 .