Gallium (revised)
GALLIUM (REVISED)
Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Overview
In the late 1860s Russian chemist Dmitri Mendeleev (1834-1907) made one of the greatest discoveries in modern chemistry: the periodic law. The periodic law describes how chemical elements are related to each other. These elements are in the periodic table. This is a chart that lists all of the chemical elements and sorts them into groups based on similarities. Elements in vertical columns are similar to each other in many ways.
When Mendeleev first proposed the periodic law, he made a troubling discovery. There were a few empty spots in his table. For example, the box set aside for element number 31 was empty. No element had been found that belonged in that box.
SYMBOL
Ga
ATOMIC NUMBER
31
ATOMIC MASS
69.72
FAMILY
Group 13 (IIIA)
Aluminum
PRONUNCIATION
GA-le-um
Part of Mendeleev's genius was what he did next. He said that an "element number 31" did exist. Scientists simply had not found it yet. But Mendeleev described what the element would be like. He based his prediction on elements on all sides of the box for element number 31. He said it would be similar to aluminum (in box 13, above 31) and indium (in box 49, below 31). He named this missing element eka-aluminum.
Using Mendeleev's periodic law, the element was soon found. It was discovered by French chemist Paul Emile Lecoq de Boisbaudran in 1875.
Until recently, gallium had few applications. Then, some of its compounds were discovered to have unusual properties when exposed to light. These properties make gallium an important and essential element in many electronic devices.
Discovery and naming
Lecoq de Boisbaudran did not discover gallium by accident. For 15 years, he had been studying the spectra of the chemical elements. Spectra (singular: spectrum) are the lines produced when chemical elements are heated. Each element produces its own distinctive set of lines, or spectra. An element can be identified in a sample by the spectrum it produces.
Lecoq de Boisbaudran knew that the element between aluminum and indium was missing. He also knew about Mendeleev's prediction. Lecoq de Boisbaudran wanted to learn more about the spectra of elements. He thought that element number 31 might be found in zinc ores. Zinc has an atomic number of 30, so it is next to gallium on the periodic table.
Lecoq de Boisbaudran had to work through a large amount of zinc ore. But his hunch turned out to be correct. The missing element was present in the ore, but only in very small amounts. Finally, in August 1875, Lecoq de Boisbaudran reported that "the new substance gave under the action of the electric spark a spectrum composed chiefly of a violet ray, narrow, readily visible, and [located at] about 417 on the scale of wave lengths."
Later in the same year, Lecoq de Boisbaudran isolated gallium metal. He was given several tons of zinc ore by miners for his research. Out of this ore, he was able to produce a few grams of nearly pure gallium.
Lecoq de Boisbaudran proposed the name gallium for the new element. The name was given in honor of the ancient name for France, Gallia.
Physical properties
Gallium is a soft, silvery metal with a shiny surface. In some ways, however, it is very un-metal-like. It is so soft that it can be cut with a knife. It has a very low melting point of only 29.7°C (85.5°F). A sample of gallium will melt if held in the human hand (body temperature, about 37°C.
Another unusual property is that gallium can be supercooled rather easily. Supercooling is the cooling of a substance below its freezing point without it becoming a solid. Gallium is a liquid at 30°C, so one would expect it to become a solid at 29.7°C. Instead it is fairly easy to cool gallium to below 29.7°C without having it solidify.
Gallium's boiling point is about 2,400°C (4,400°F) and its density is 5.9037 grams per cubic centimeter.
Chemical properties
Gallium is a fairly reactive element. It combines with most non-metals at high temperatures, and it reacts with both acids and alkalis. An alkali is a chemical with properties opposite those of an acid. Sodium hydroxide (common lye, such as Drano) and bleach are examples of alkalis.
Occurrence in nature
Gallium is a moderately abundant element in the Earth's crust. Its abundance has been estimated to be about 5 parts per million. It is found primarily in combination with zinc and aluminum ores. It is also found in germanite, an ore of copper sulfide (CuS).
The United States produces no gallium. The largest producers are Australia, Russia, France, and Germany.
Isotopes
Two naturally occurring isotopes of gallium are known: gallium-69 and gallium-71. 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 a dozen radioactive isotopes of gallium are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
One radioactive isotope of gallium, gallium-67, has long been used in medicine. This isotope has a tendency to seek out cancer cells in the body. Its presence in a cell can be detected by the radiation it gives off. By giving a patient a dose of gallium-67, a doctor can determine whether the patient has cancer. Gallium-67 has been used to look for cancer in the liver, spleen, bowels, breasts, thymus, kidneys, and bones.
Gallium-67 has been used to look for cancer in the liver, spleen, bowels, breasts thymus, kidneys, and bones.
Extraction
Pure gallium metal can be prepared by passing an electric current through a gallium compound, such as gallium oxide (Ga2O3):
Uses and compounds
About 95 percent of all gallium produced is used to make a single compound, gallium arsenide (GaAs). Gallium arsenide has the ability to convert an electrical current directly into light. The lighted numbers on hand-held calculators, for example, are produced by a device known as a light-emitting diode (LED). Gallium arsenide is used to make LEDs. An LED allows an electric current to flow in one side, but not the other. When it flows into a piece of gallium arsenide, a flash of light is produced. When a button is pushed on a calculator, a circuit is closed. The electric current flows into an LED and produces a light.
Similar devices are used in making lasers. An electric current passes into a piece of gallium arsenide. The current produces an intense beam of laser light. A laser is a device for producing very bright Light of a single color. Gallium arsenide lasers are used in a number of applications. The laser that operates a compact disc (CD) player, for example, may contain a piece of gallium arsenide.
Gallium arsenide is also used to make transistors. A transistor is a device used to control the flow of electricity in a circuit. Gallium arsenide has many of the properties of a semiconductor. A semiconductor is a material that conducts an electrical current, but not as well as a metal, such as silver or copper. Gallium arsenide has one big advantage over silicon, another element used in transistors. Gallium arsenide produces less heat. Therefore, more transistors can work together at the same time to produce a higher computing capacity.
Gallium arsenide is also used in photovoltaic cells. These devices turn sunlight into electricity. Many people believe that photovoltaic cells will someday replace coal-fired generating and nuclear power plants as the major source of electricity.
Health effects
Gallium and its compounds are somewhat hazardous to the health of humans and animals. They produce a metallic taste in the mouth, skin rash, and a decrease in the production of blood cells. Gallium and its compounds should be handled with caution.
Gallium
Gallium
melting point: 29.78°C
boiling point: 2,250°C
density: 5.907 g/cm 3
most common ions: Ga 2+, Ga 3+
In 1870 Dimitri Mendeleev predicted many of the properties of an unknown element that he called eka-aluminum. The element was discovered in 1875 by Paul-Émile Lecoq de Boisbaudran who named it gallium from Gaul, the Latin name for France. The properties of the new element were those predicted by Mendeleev and helped to validate his Periodic Table of the elements.
Gallium can be obtained as a by-product of zinc and alumina production. The metal has an unusually low melting point but a very high boiling point. The liquid range is the largest known for any element, allowing gallium to be used in high-temperature thermometers. The liquid metal has a number of unusual properties: It has a tendency to supercool; to expand on crystallization ; and does not crystallize in any of the common closely packed or body-centered cubic structures.
The chemistry of gallium is very similar to that of aluminum, its congener . Compounds of gallium almost always have a +3 oxidation state. While a few compounds with a +1 and +2 state have been postulated, these are controversial.
One of the most important uses of gallium is in electronic devices, usually in the form of gallium arsenide, which, together with other group-3 or group-5 elements, converts electrical energy to light, and is the basis of the light-emitting diode.
Metallic gallium and its salts have little or no toxicity, compared to the very toxic thallium salts. The toxicity of the aluminum ion is controversial. The gallium ion has been investigated as a possible antitumor agent, but no clinically useful compounds have been produced.
see also Inorganic Chemistry; Mendeleev, Dimitri; Semiconductors.
Gus J. Palenik
Bibliography
Emsley, John (2001). Nature's Building Blocks: An A–Z Guide to the Elements. New York: Oxford University Press.
Krebs, Robert E. (1998). The History and Use of Our Earth's Chemical Elements: A Reference Guide. Westport, CT: Greenwood Press.
Weeks, Mary E., and Leicester, Henry M. (1968). "Discovery of the Elements." In Journal of Chemical Education, 7th edition. Easton, PA: Journal of Chemical Education.
gallium
gal·li·um / ˈgalēəm/ • n. the chemical element of atomic number 31, a soft, silvery-white metal that melts at about 30°C, just above room temperature. (Symbol: Ga)