Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Indium is part of the aluminum family in Group 13 (IIIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. Indium was discovered in 1863 by German chemists Ferdinand Reich (1799-1882) and Hieronymus Theodor Richter (1824-98).
Indium has a number of interesting properties. For example, it has a low melting point for metals, 156.6°C (313.9°F). When pure, it sticks very tightly to itself or to other metals. This property makes it useful as a solder. Solder is a material used to join two metals to each other. Other uses of indium are in the manufacture of batteries, electronic devices, and in research.
Group 13 (IIIA)
Discovery and naming
Between 1860 and 1863, indium, cesium, rubidium, and thallium were found using spectroscopy. Spectroscopy is the process of analyzing light produced when an element is heated. The light produced is different for every element. The spectrum (plural: spectra) of an element consists of a series of colored lines. Reich and Richter also produced the first impure sample of indium in 1863.
Scientists use a spectroscope in this process. The spectroscope was invented in 1814 by German physicist Joseph von Fraunhofer (1787-1826). Forty years later, German chemists Robert Bunsen (1811-99) and Gustav Robert Kirchhoff (1824-87) improved on the instrument. They showed how it could be used to study the chemical elements.
Reich and Richter suggested the name indium for the element they discovered because its main spectral lines are a brilliant indigo blue.
Indium is a silvery-white, shiny metal with a density of 7.31 grams per cubic centimeter. It is one of the softest metals known, even softer than lead. If drawn across a piece of paper, it leaves a mark like a "lead" pencil (which is actually carbon).
An unusual property of indium is that it produces a "tin cry." A tin cry is a scream-like sound made when the metal is bent.
Indium's main spectral lines are a brilliant indigo blue.
Indium has a melting point of 156.6°C (313.9°F) and a boiling point of 2,075°C (3,767°F). It has the unusual property of remaining soft and workable at very low temperatures. This property allows it to be used in special equipment needed for temperatures near absolute zero. Absolute zero is the coldest temperature possible. It is about —273°C (—459°F).
Indium metal dissolves in acids, but does not react with oxygen at room temperature. At higher temperatures, it combines with oxygen to form indium oxide (In2O3).
Occurrence in nature
Indium is relatively rare. Its abundance in the Earth's crust is estimated to be about 0.1 part per million. That makes it slightly more abundant than silver or mercury.
Indium is generally found in ores of zinc. The metal is not usually produced in the United States. It is imported from Canada, China, and Russia. In 1996, about 45 tons of the metal were used in the United States.
Two naturally occurring isotopes of indium exist, indium-113 and indium-115. 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.
Indium-115 is radioactive. A radioactive isotope is one that breaks apart and gives off some form of radiation. Indium-115 has a half life of about 440 trillion years. The half life of a radioactive element is the time it takes for half of a sample of the element to break down. Starting with 10 grams of indium-115 today, only 5 grams would be left 440 trillion years from now.
A number of artificial radioactive isotopes of indium also exist. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive. Two of these isotopes are used in medicine. Indium-113 is used to examine the liver, spleen, brain, pulmonary ("breathing") system, and heart and blood system. Indium-111 is used to search for tumors, internal bleeding, abscesses, and infections and to study the gastric (stomach) and blood systems. In both cases, the radioactive isotope is injected into the blood stream. Inside the body, the isotope gives off radiation. That radiation can be detected by means of a camera or other device. The radiation pattern observed provides information about the organ or system being studied.
Indium is obtained in its pure form by separating it from zinc and other elements in zinc ores.
Uses and compounds
The primary use of indium is in making alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals. Indium has been called a "metal vitamin" in alloys. That means that very small amounts of indium can make big changes in an alloy. For example, very small amounts of indium are sometimes added to gold and platinum alloys to make them much harder. Such alloys are used in electronic devices and dental materials.
Indium is also added to solders. It reduces the melting point of some solders, strengthens other solders, and prevents still other solders from breaking down too easily.
The single most important use of indium is in making coatings. For example, some aircraft parts are made of alloys that contain indium. The indium prevents them from wearing out or reacting with oxygen in the air.
Alloys and compounds of indium are also used in making optical (light) devices. For example, indium gallium arsenide (InGaAs) is able to convert pulses of light into electrical signals. One application of a device like this is in solar cells. A solar cell is a device used to change sunlight into electrical current. Many scientists think that solar cells may replace coal, oil, and natural gas for many purposes in the future.
In 1997, the Siemens Corporation announced the largest and most advanced solar conversion system ever developed. It uses copper indium diselinide (CuInSe2) in its solar cells. The system produces one million watts of electrical energy from sun-light. That is enough electricity to completely operate a large office building.
Very small amounts of indium are sometimes added to gold and platinum alloys to make them much harder.
The health effects of indium compounds are somewhat unusual. When taken by mouth, they are relatively harmless. When injected into the skin, however, they are very poisonous.
in·di·um / ˈindēəm/ • n. the chemical element of atomic number 49, a soft, silvery-white metal occurring naturally in association with zinc and some other metals. (Symbol: In)