182 | The History and Use of Our Earth’s Chemical Elements Properties
182 | The History and Use of Our Earth’s Chemical Elements Properties
Gallium is soft and bluish off-white when solid and silvery in color as a liquid. It is soft enough to cut with knife and has an extremely low melting point. When held in the hand, it will melt from body heat as it becomes mirror-like in color. It expands when changing back from a liquid to a solid. When cold, it becomes hard and brittle. Of all the metals, gallium exhibits the largest range of temperatures from its liquid phase to its solid phase, and, like water, it expands when it freezes. Its melting point is 29.76°C, its boiling point is 2,204°C,
and its density is 5.903 g/cm 3 ,
Characteristics Gallium is truly an “exotic” element in that it has so many unusual characteristics. It can
form monovalent and divalent as well as trivalent compounds. It is considered a “post-tran- sitional metal” that is more like aluminum than the other elements in group 13. It has few similar characteristics to the two elements just below it in group 13 (In and Ti).
Gallium reacts strongly with boiling water, is slightly soluble in alkali solutions, acids, and mercury, and is used as an amalgam. It has some semiconductor properties but only if “doped” with elements in group 14, such as As, P, and Sb. It is also used as a “dope” for other semiconducting elements.
Gallium is easy to mix with several other metals to produce alloys with low melting points.
AbundanceandSource Gallium is the 34th most abundant element, but it is not widely distributed as an elemental
metal. It is usually combined with other elements, particularly zinc, iron, and aluminum ores. It is found in diaspore, sphalerite, germanite, gallite, and bauxite. Although small amounts are recovered from burning coal used for heating or generation of electricity, it is mostly recovered as a by-product from the production of ores of other metals. Gallium is about as abundant as lead in the Earth’s crust.
Since 1949, the Aluminum Company of America has extracted gallium metal from alu- minum bauxite ore. In the past gallium had few uses. Only recently, with the development of microprocessors, chips, computer, and the like, has gallium found many profitable uses.
History Gallium is one of the elements Mendeleev predicted to fill the space just below aluminum.
He named it “eka-aluminum” and even gave it the chemical symbol Ea because, when found, it would mostly resemble aluminum. He also suggested that it would combine with oxygen
with the formula Ea 2 O 3 .
In 1875 Paul-Emile Lecoq de Boisbaudran (1838–1912), a French chemist, used Mendeleev’s clues along with the aid of a spectroscope to identify the metal missing between aluminum and indium. He found gallium’s spectral lines in the mineral sphalerite, which is a zinc/sulfide mineral that at one time was thought to be “useless lead.” Sometime later,
the scientist used the process of electrolysis with molten gallium hydroxide [Ga(OH) 3 ] and potassium hydroxide (KOH) to produce a pure sample of gallium metal. Paul-Emile Lecoq
de Boisbaudran is given credit for the discovery of gallium, which he named after his native
183 CommonUses
Guide to the Elements |
The compound gallium arsenide (GaAs) has the ability to convert electricity directly into laser-light used as the laser beam in compact disc players. It is also used to make light-emit- ting diodes (LEDs) for illuminated displays of electronic devices such as watches. Gallium is also a semiconductor that when used in computer chips generates less heat than silicon chips, making it a viable option for designing supercomputers that otherwise would generate exces- sive heat.
The radioisotope of gallium-67 is one of the first to be used in medicine. It has the ability to locate and concentrate on malignant tissue, such as skin cancers, without harming normal tissue in the same area.
One of the more recent uses of gallium is based on the fact that normal gallium, when bombarded by neutrinos, is converted into the radioisotope germanium-71, which can be detected by sensitive instruments. Neutrinos are subatomic particles that “bathe” the Earth as
a product of the sun’s thermonuclear activity and, from outer space, and can easily go through miles of solid rock. Neutrinos are classed as leptons, which are somewhat like electrons, but with no electrical charge and with no or very little mass. Two large gallium detectors are bur- ied deep underground, one in the tunnel known as Gran Sasso in Italy, and the other, named SAGE, under the Caucasus Mountains in Russia. Scientists from the United States also run this neutrino detector in cooperation with the Russians, thus the name Soviet-American Gallium Experiment (SAGE). Buried deep underground is a deposit of 250,000 pounds of gallium that has a market price of about $400 a pound. Reportedly, attempts have been made to steal this stash of gallium. The purpose of this experiment is to indirectly identify the elusive
neutrinos as they convert the gallium trichloride (GaCl 3 ) to the radioactive isotope germa- nium-71, which is then exposed to sensitive instruments that detect radiation, thus revealing the existence of neutrino activity.
Gallium makes a safe substitute for mercury amalgams in dental fillings when it is com- bined with tin or silver. Because of its high range of temperatures as a liquid (from 29.8°C to 2,403°C), it is used in special types of high-temperature thermometers. It is also alloyed with other metals to make alloys with low temperature melting points.
Because of the unique property of some of its compounds, gallium is able to translate a mechanical motion into electrical impulses. This makes it invaluable for manufacturing tran- sistors, computer chips, semiconductors, and rectifiers.
A unique use of gallium metal is to “glue” gemstones to metal jewelry. ExamplesofCompounds
Gallium can combine with –1, –2, and –3 ions as follows:
Gallium chloride: Ga 3+ + 3Cl 1- → GaCl 3 . Gallium oxide: Ga 3+ + 3O 2- → Ga 2 O 3 .
Gallium arsenide (Ga 3+ + As 3- → GaAs) is electroluminscent in infrared light and is used for telephone equipment, lasers, solar cell, and other electronic devices. Galllium phosphide (GaP) is a light-colored highly pure crystal form used as “whiskers” and crystals in semiconductor devices. Gallium antimonide (GaSb), when in pure form, is used in semiconductor industries.