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Gallium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                31               zinc ← gallium → germanium
                Al
                ↑
                Ga
                ↓
                In

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                      Name, Symbol, Number gallium, Ga, 31
                                               Chemical series poor metals
                                             Group, Period, Block 13, 4, p
                                                  Appearance silvery white
                                      Typical (melted blob)   Crystallized
                                              Atomic mass 69.723 (1) g/mol
                               Electron configuration [Ar] 3d^10 4s^2 4p^1
                                           Electrons per shell 2, 8, 18, 3
                                                       Physical properties
                                                               Phase solid
                                       Density (near r.t.) 5.91 g·cm^−3
                                   Liquid density at m.p. 6.095 g·cm^−3
                                                 Melting point 302.9146  K
                                              (29.7646 ° C, 85.5763 ° F)
                                                      Boiling point 2477 K
                                                    (2204 ° C, 3999 ° F)
                                          Heat of fusion 5.59 kJ·mol^−1
                                     Heat of vaporization 254 kJ·mol^−1
                          Heat capacity (25 °C) 25.86 J·mol^−1·K^−1

   CAPTION: Vapor pressure

                                      P/Pa   1    10  100  1 k  10 k 100 k
                                     at T/K 1310 1448 1620 1838 2125 2518

                                                         Atomic properties
                                            Crystal structure orthorhombic
                                                        Oxidation states 3
                                                       ( amphoteric oxide)
                                    Electronegativity 1.81 (Pauling scale)
                                                       Ionization energies
                                           ( more) 1st: 578.8 kJ·mol^−1
                                                  2nd: 1979.3 kJ·mol^−1
                                                    3rd: 2963 kJ·mol^−1
                                                      Atomic radius 130 pm
                                              Atomic radius (calc.) 136 pm
                                                    Covalent radius 126 pm
                                               Van der Waals radius 187 pm
                                                             Miscellaneous
                                                 Magnetic ordering no data
                       Thermal conductivity (300 K) 40.6 W·m^−1·K^−1
                               Speed of sound (thin rod) (20 °C) 2740 m/s
                                                         Mohs hardness 1.5
                                                   Brinell hardness 60 MPa
                                             CAS registry number 7440-55-3
                                                         Selected isotopes

                 CAPTION: Main article: Isotopes of gallium

                                 iso    NA   half-life DM DE ( MeV)  DP
                                ^69Ga 60.11% Ga is stable with 38 neutrons
                                ^71Ga 39.89% Ga is stable with 40 neutrons

                                                                References

   Gallium ( IPA: /ˈgaliəm/) is a chemical element that has the symbol Ga
   and atomic number 31. A rare, soft silvery metallic poor metal, gallium
   is a brittle solid at low temperatures but liquefies slightly above
   room temperature and will melt in the hand. It occurs in trace amounts
   in bauxite and zinc ores. An important application is in the compound
   gallium arsenide, used as a semiconductor, most notably in
   light-emitting diodes (LEDs).

Notable characteristics

   Elemental gallium is not found in nature, but it is easily obtained by
   smelting. Very pure gallium metal has a brilliant silvery colour and
   its solid metal fractures conchoidally like glass. Gallium metal
   expands by 3.1 percent when it solidifies, and therefore storage in
   either glass or metal containers is avoided, due to the possibility of
   container rupture with freezing. Gallium shares the higher-density
   liquid state with only a few materials like water and bismuth.

   Gallium also attacks most other metals by diffusing into their metal
   lattice — another reason why it is important to keep gallium away from
   metal containers such as steel or aluminium. Gallium metal easily
   alloys with many metals, and was used in small quantities in the core
   of the first atomic bomb to help stabilize the plutonium crystal
   structure.

   The melting point temperature of 30 °C allows the metal to be melted in
   the hand. This metal has a strong tendency to supercool below its
   melting point, thus necessitating seeding in order to solidify. Gallium
   is one of the metals (with caesium, francium and mercury) which are
   liquid at or near normal room temperature, and can therefore be used in
   metal-in-glass high-temperature thermometers. It is also notable for
   having one of the largest liquid ranges for a metal, and (unlike
   mercury) for having a low vapor pressure at high temperatures. Unlike
   mercury, liquid gallium metal wets glass and skin, making it
   mechanically more difficult to handle (even though it is substantially
   less toxic and requires far fewer precautions). For this reason as well
   as the metal contamination problem and freezing-expansion problems
   noted above, samples of gallium metal are usually supplied in
   polyethylene packets within other containers.

   Gallium does not crystallize in any of the simple crystal structures.
   The stable phase under normal conditions is orthorhombic with 8 atoms
   in the conventional unit cell. Each atom has only one nearest neighbour
   (at a distance of 244 pm) and six other neighbors within additional 39
   pm. Many stable and metastable phases are found as function of
   temperature and pressure.

   The bonding between the nearest neighbors is found to be of covalent
   character, hence Ga[2] dimers are seen as the fundamental building
   blocks of the crystal. The compound with arsenic, gallium arsenide is a
   semiconductor commonly used in light-emitting diodes.

   High-purity gallium is attacked slowly by mineral acids.

History

   Gallium (Latin Gallia meaning Gaul (essentially modern France); also
   gallus, meaning "rooster") was discovered spectroscopically by Lecoq de
   Boisbaudran in 1875 by its characteristic spectrum (two violet lines)
   in an examination of a zinc blende from the Pyrenees. Before its
   discovery, most of its properties had been predicted and described by
   Dmitri Mendeleev (who called the hypothetical element eka-aluminium) on
   the basis of its position in his periodic table. Later, in 1875,
   Boisbaudran obtained the free metal through the electrolysis of its
   hydroxide in KOH solution. He named the element "gallia" after his
   native land of France and, in one of those multilingual puns so beloved
   of men of science of the early 19th century, after himself, as 'Lecoq'
   = the rooster, and Latin for rooster is "gallus".

Occurrence

   Gallium does not exist in free form in nature, nor do any high-gallium
   minerals exist to serve as a primary source of extraction of the
   element or its compounds. Gallium is rather found and extracted as a
   trace component in bauxite, coal, diaspore, germanite, and sphalerite.
   The USGS estimates gallium reserves based on 50 ppm by weight
   concentration in known reserves of bauxite and zinc ores. Some flue
   dusts from burning coal have been shown to contain as much as 1.5
   percent gallium.

   Most gallium is extracted from the crude aluminium hydroxide solution
   of the Bayer process for producing alumina and aluminium. A mercury
   cell electrolysis and hydrolysis of the amalgam with sodium hydroxide
   leads to sodium gallate. Electrolysis then gives gallium metal. For
   semiconductor use, further purification is carried out using zone
   melting, or else single crystal extraction from a melt ( Czochralski
   process). Purities of 99.9999% are routinely achieved and commercially
   widely available.

Applications

   As a component of the semiconductor Gallium arsenide, the most common
   application for gallium is analog integrated circuits, with the second
   largest use being optoelectronic devices (mostly laser diodes and
   light-emitting diodes.)

   Other uses include:
     * Since gallium wets glass or porcelain, gallium can be used to
       create brilliant mirrors.
     * Used widely as a dopant to dope semiconductors and produce
       solid-state devices like transistors.
     * Gallium readily alloys with most metals, and has been used as a
       component in low-melting alloys. The plutonium used in nuclear
       weapon pits is machined by alloying with gallium to stabilize the
       allotropes of plutonium. Much research is being devoted to gallium
       alloys as substitutes for mercury dental amalgams, but such
       compounds have yet to see wide acceptance.
     * Gallium added in quantities up to 2% in common solders can aid
       wetting and flow characteristics.
     * Gallium is used in some high temperature thermometers.
     * A eutectic alloy of gallium, indium, and tin, is widely available
       in medical thermometers (fever thermometers), replacing problematic
       mercury. This alloy, with the trade name Galinstan, has a freezing
       point of −20 °C.
     * Magnesium gallate containing impurities (such as Mn^+2), is
       beginning to be used in ultraviolet-activated phosphor powder.
     * It has been suggested that a liquid gallium-tin alloy could be used
       to cool computer chips in place of water. As it conducts heat
       approximately 65 times better than water it can make a comparable
       coolant. Gallium has a specific heat capacity of 0.37 J/(g K).
       Gallium's high specific gravity of 5.91 gives it a volumetric heat
       capacity of 0.37 x 5.91 = 2.187 J/cm³, meaning that a volume of
       gallium will heat by 4.184/2.187 = 1.9 times more than an equal
       volume of water in a cooling device. However given water's benign
       handling characteristics and plentiful abundance in most developed
       countries, gallium alloys are only really likely to see use in
       specialised applications such as cooling supercomputers.
     * Gallium salts such as gallium citrate and gallium nitrate are used
       as radiopharmaceutical agents in nuclear medicine imaging. (The
       form or salt is not important, since it is the free dissolved
       gallium ion Ga^+3 which is active). For these applications, a
       radioactive isotope such as ^67Ga is used. The body handles Ga^+3
       in many ways as though it were iron, and thus it is bound (and
       concentrates) in areas of inflammation, such as infection, and also
       areas of rapid cell division. This allows such sites to be imaged
       by nuclear scan techniques. See gallium scan. This use has largely
       been replaced by FDG PET scan.
     * Gallium is the rarest component of new photovoltaic compounds (such
       as copper indium gallium selenium sulphide or Cu(In,Ga)(Se,S)2,
       recently announced by South African researchers) for use in solar
       panels as an alternative to crystalline silicon, which is currently
       in short supply.

Precautions

   While not considered toxic, the data about gallium is inconclusive.
   Some sources suggest that it may cause dermatitis from prolonged
   exposure; other tests have not caused a positive reaction. Like most
   metals, finely divided gallium loses its luster. Powdered gallium
   appears gray. When gallium is handled with bare hands, the extremely
   fine dispersion of liquid gallium droplets which results from wetting
   skin with the metal may appear as a gray skin stain.

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