IRIDIUM (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Indium is in Group 9 (VIIIB) of the periodic table. The periodic table is a chart that shows how elements are related to one another. Indium is a transition metal that is also part of the platinum family.

The metals in the platinum family are also known as the noble metals. They have this name because they do not react well with other elements and compounds. They appear to be "too superior" to react with most other substances. In fact, iridium is the most corrosion-resistant metal known. It is not affected by high temperatures, acids, bases, or most other strong chemicals. That property makes it useful in making objects that are exposed to such materials.

Iridium may be a key element in the puzzle of dinosaur extinction. Scientists search for iridium in the soil to track the impact of a giant meteor with the Earth 65 million years ago.

SYMBOL
Ir

ATOMIC NUMBER
77

ATOMIC MASS
192.217

FAMILY
Group 9 (VIIIB)
Transition metal
Platinum group

PRONUNCIATION
i-RI-dee-um

Discovery and naming

The platinum metals posed a difficult problem for early chemists. These metals often occurred mixed together in the earth. When a scientist thought that he was analyzing a sample of platinum, the sample often contained iridium, rhodium, osmium, and other metals as well. The work of French chemist Pierre-François Chabaneau is an example. In the late 1780s, the Spanish government gave its entire supply of platinum to Chabaneau to study. But Chabaneau's experiments puzzled him. Sometimes the platinum he worked with could be hammered into flat plates easily. At other times, it was brittle and shattered when hammered. Chabaneau did not realize that the "platinum" he was studying included various amounts of other noble metals.

In the early 1800s, a number of chemists worked to separate the platinum metals. One of those chemists was an Englishman named Smithson Tennant (1761-1815). Like so many others, Tennant became interested in chemistry at an early age. He is said to have made gunpowder to use in fireworks when he was only nine years old!

In 1803, Tennant attempted to dissolve platinum in aqua regia. Aqua regia is a mixture of two strong acids—nitric acid and hydrochloric acid. He found that most of the platinum metal dissolved, leaving a small amount of black powder. Other chemists had not bothered to study the powder. But Tennant did. He discovered that it had properties very different from those of platinum. He realized he had discovered a new element. He named it iridium, from the Greek goddess Iris, whose symbol is a rainbow. Tennant chose this name because the compounds of iridium have so many different colors. For example, iridium potassium chloride (K₂IrCl₆) is dark red, iridium tri-bromide (IrBr₃) is olive-green, and iridium trichloride (IrCl₃) is dark green to blue-black.

Physical properties

Iridium metal is silvery-white with a density of 22.65 grams per cubic centimeter. A cubic centimeter of iridium weighs 22.65 times as much as a cubic centimeter of water. It is the most dense element known. Iridium has a melting point of 2,443°C (4,429°F) and a boiling point of about 4,500°C (8,130°F). Cold iridium metal cannot be worked easily. It tends to break rather than bend. It becomes more ductile (flexible) when hot. Ductile means capable of being drawn into thin wires. Therefore, it is usually shaped at high temperatures.

Chemical properties

Iridium is unreactive at room temperatures. When exposed to air, it reacts with oxygen to form a thin Layer of iridium dioxide (IrO₂).

At high temperatures, the metal becomes more reactive. Then it reacts with oxygen and halogens to form iridium dioxide and iridium trihalides. For example:

Occurrence in nature

Iridium is one of the rarest elements in the Earth's crust. It is thought to exist in two parts per billion. Interestingly, it is more abundant in other parts of the universe. Iron meteorites, for example, generally contain about 3 parts per million of iridium. Stony meteorites contain less iridium, about 0.64 parts per million.

Iridium usually occurs in combination with one or more other noble metals. Two common examples are osmiridium and iridosmine, combinations of iridium and osmium. The most important sources of iridium metal are Canada, South Africa, Russia, and the state of Alaska.

Tracking the fate of the dinosaurs

W hy did the dinosaurs die out? This question has long been one of the most interesting and puzzling issues in science. What happened to make these huge reptiles disappear in such a short period of geological time?

One answer might be found in the Asteroid Disaster Theory. According to this theory, a huge asteroid struck the Earth's surface about 65 million years ago. The exploding asteroid threw enormous amounts of dust into the air. The dust blocked out sunlight for more than a year. Plants on the Earth's surface died. Dinosaurs who lived on those plants died out. So did the meat-eating dinosaurs who lived off the plant eaters.

But how is it possible to know if an asteroid really did hit the Earth's surface 65 million years ago? Scientists have now found an answer. In some parts of the Earth, they have found a layer of the Earth's crust that contains an unusually high level of iridium metal. Iridium rarely occurs on Earth. But it occurs much more commonly in meteors and asteroids. Scientists believe the iridium-rich layer was formed when an asteroid struck the Earth's surface. They believe the event occurred 65 million years ago. This "iridium clue" is a key, therefore, to understanding how dinosaurs disappeared from the Earth.

Iridium is one of the rarest elements in the Earth's crust. But it is more abundant in other parts of the universe. Iridium is found in meteorites.

Isotopes

Two naturally occurring isotopes of iridium exist, iridium-191 and iridium-193. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

About ten radioactive isotopes of iridium exist. A radioactive isotope is one that gives off radiation and changes into a new form. The only important radioactive isotope of iridium is iridium-192. This isotope has a half-life of 74 days. A half-life is the time it takes for one half of a sample to break down. Iridium-192 is used to make X-ray photographs of metal castings and to treat cancer.

Extraction

Iridium and the other platinum metals tend to occur together. A series of chemical reactions is used to separate one metal from the other. The other metals are then removed by other techniques. Very little iridium is produced each year, probably no more than a few metric tons.

Uses

The primary use of iridium is in the manufacture of alloys. An alloy is made by melting and mixing two or more metals. An alloy's properties differ from those of the elements that make it up. Iridium is often combined with platinum, for example, to provide a stronger material than the platinum itself. These alloys are very expensive and are used for only special purposes. For instance, the sparkplugs used in helicopters are made of a platinum-iridium alloy. Such alloys are also used for electrical contacts, special types of electrical wires, and electrodes.

The standard kilogram

B utter comes in one-pound or one-kilogram packages. But who decides how much "one pound" or "one kilogram" of butter is?

Every nation has a governmental office for weights and measures. The office maintains an "official" pound or kilogram. It is usually a piece of metal known to weigh exactly one pound or one kilogram. But how does each nation know exactly what size its official weight should be?

The official world standard for the kilogram is kept at the International Bureau of Weights and Measures in Paris. The standard is a piece of platinum-iridium metal stored in an airtight jar. The standard is made of platinum and iridium to protect it from reacting with oxygen and other chemicals in the air. In this way, the standard's weight will always remain exactly the same.

One kind of iridium catalyst is able to capture sunlight and turn it into chemical energy.

Iridium metal is increasingly being used as catalysts. Catalysts are substances that speed up a reaction without changing themselves. Iridium catalysts have been used in amazing new products. For example, one kind of iridium catalyst is able to capture sunlight and turn it into chemical energy. That process is similar to the one used by plants in photosynthesis. Finding a synthetic (artificial) way to make photosynthesis happen is one of the great goals of modern chemistry.

Space technology often uses alloys that are too expensive for everyday use. An example is the propulsion systems used for keeping satellites in place. Some of these systems use alloys made of indium and another platinum metal, rhenium. These alloys remain strong at high temperatures and are not attacked by fuels used in the systems.

Compounds

The compounds of indium have almost no practical applications. A few are used in coloring ceramics because of their striking colors.

Health effects

Scientists are not aware of any health benefits or risks associated with iridium.

Iridium

melting point: 2,454°C
boiling point: 3,590°C
density: 22.6 g/cm³ (at 25°C)
most common ions: Ir³⁺

Iridium was discovered by English chemist Smithson Tenant. It was named after the Greek goddess Iris (a goddess related to the rainbow) because of the variety of color in its compounds. A very rare element (0.0001 ppm of Earth's crust), iridium is found in the naturally occurring alloys osmiridium (approximately 50% iridium) and iridiosmium (approximately 70% iridium) in Alaska and South Africa. It is obtained by igniting ammonium chloroiridate, (NH₄)₃IrCl₆, in hydrogen atmosphere. Its ground state electronic configuration is [Xe]4p¹⁴5d⁷6s².

Iridium is the densest of all elements and its metal is lustrous, silvery, and very hard. The metal has the face-centered cubic crystal structure. Iridium has two stable isotopes : ¹⁹¹Ir (37.3%) and ¹⁹³Ir (62.7%).

Iridium is extremely inert to acids, but reacts slowly with oxygen and halogens at high temperatures. It forms the fluoride compounds IrF₆, (IrF₅)₄, and IrF₄. There is some doubt of the existence of other halides with the composition IrX₄. The most stable halides are the trihalides. Compounds that have the composition [IrX₆]²⁻ are strongly colored.

Ir₂O₃ · n H₂O (a brown solid) is obtained via the addition of alkali to IrCl₆²⁻ in the presence of H₂O₂. This hydrated oxide is oxidized in air to IrO₂ · n H₂O. IrO₂ (black), with rutile structure, is the product of the reaction of Ir with O₂ at approximately 1,100°C (2,012°F); it dissociates at higher temperatures (>1,100°C, or >2,012°F).

Iridium may form complexes in its several oxidation states. Table 1 contains some examples of these complexes.

Iridium oxide, IrO₂, is used in the fabrication of thin films for stable electrochromic materials and as an electrode material. Iridium metal is used in the manufacture of fountain pen points, airplane spark plugs, and hypodermic needles.

see also Inorganic Chemistry.

Lea B. Zinner

Bibliography

Allred, A. L. (1961). Journal of Inorganic Nuclear Chemistry 17:215.

Cotton, F. Albert, and Wilkinson, Geoffrey (1972). Advanced Inorganic Chemistry: A Comprehensive Text, 3rd edition. New York: Interscience.

Greenwood, Norman N., and Earnshaw, A. (1984). Chemistry of the Elements. Oxford, U.K.: Pergamon.

Livingstone, Stanley E. (1973). "The Platinum Metals." In Comprehensive Inorganic Chemistry, Vol. 3, edited by J. C. Bailar Jr., et al. Oxford, U.K.: Pergamon.

iridium anomaly The anomalously high (typically 50 p.p.b.) concentrations of iridium, relative to typical crustal abundances of less than 1 p.p.b., observed world-wide in sediments straddling the Cretaceous—Tertiary boundary. The anomaly is attributable to fall-out resulting from a massive asteroidal or cometary impact, that is thought by many to have been responsible for the mass extinctions that define the Cretaceous—Tertiary boundary.

iridium anomaly The occurrence of unusually high concentrations of the relatively scarce metal iridium at the boundaries of certain geological strata. Two such layers have been discovered, one at the end of the Cretaceous, 65 million years ago, and the second at the end of the Eocene, 34 million years ago. One theory to account for these suggests that on each occasion a huge iridium-containing meteorite may have collided with the earth, producing a cloud of dust that settled out to form an iridium-rich layer. The environmental consequences of such an impact, notably in causing a general warming of the earth by the greenhouse effect, may have led to the extinction of the dinosaurs at the end of the Cretaceous and the extinction of many radiolarians at the end of the Eocene. See Alvarez event.

iridium anomaly The anomalously high (typically 50 p.p.b.) concentrations of iridium, relative to typical crustal abundances of less than 1 p.p.b., observed worldwide in sediments straddling the Cretaceous–Tertiary boundary. The anomaly is attributed to fallout resulting from a massive asteroidal or cometary impact, which is thought by many to have been responsible for the mass extinctions that define the Cretaceous–Tertiary boundary.