Gallium, Ga, Atomic number 31

General

Gallium is an extremely rare chemical element with the atomic symbol Ga and atomic number 31. In the periodic table it is in the 4. Period, and is the third element of the 3. Main group (group 13) or boron group. It is a silvery-white, easy-to-liquefy metal. Gallium does not crystallize in any of the other metals commonly encountered in crystal structures, but in its most stable modification in einerorthorhombischen structure with gallium dimers. In addition, six other modifications are known, is formed at a special crystallization conditions or under high pressure. In its chemical properties, the metal is very similar to aluminum.

In nature Gallium is only limited and mostly as an additive in aluminum, Zinc- or germanium ores; Galliumminerale are very rare. Accordingly, it is also recovered as a byproduct in the production of aluminum or zinc. The largest part of the gallium is further processed to the semiconductor gallium arsenide, which is mainly used for light-emitting diodes.

Occurrence

Gallium is a rare element on Earth, with a content of 19 ppm in the Earth's continental crust is its frequency comparable to that of lithium and lead. It is not elementary, but only attached before, mainly in aluminum, Zinc- or germanium ores. The galliumreichsten include bauxite ores, Sphalerite ores and Germanite.

The gallium budgets are usually low, says found in Suriname bauxite contains the highest known content only 0,008 % Gallium. The world to be present in bauxite Galliumreserven 1,6 · 106 Tons valued. Higher levels of up to 1 % Gallium occur in Germanite. Only in the Apex Mine in Utah so come before high contents in the ores, that degradation was trying to Galliumgewinnung. This failed, however, after a short time for reasons of profitability.

Galliumminerale are only a few known, these include the predominantly found in Tsumeb in Namibia Gallit (CuGaS2), Söhngeit (Ga(OH)3) and Tsumgallit (GaO(OH)).

Extraction and representation

Gallium is recovered as a by-product of aluminum production from bauxite in the Bayer process. Serves as a starting material, the mixture dissolved in caustic soda and vonNatriumaluminat Natriumgallat. By various methods thereof can be separated from the gallium aluminum. One possibility is the fractional crystallization with the help of carbon dioxide, first being preferably aluminum fails, while the more soluble Natriumgallat accumulates in the caustic soda. Only after further process steps like gallium, mixed with aluminum hydroxide. Then the mixture is dissolved in sodium hydroxide solution, and gallium by electrolysis win. Since this method of energy- and labor-consuming, It is only in countries with low costs for, about the People's Republic of China, applied.

Gallium can be recovered from the sodium hydroxide solution directly by electrolysis. These are mercury cathode used, wherein the electrolysis is one Galliumamalgam. Also adding to said solution sodium amalgam is possible.

Using special hydroxyquinolines as chelating ligands, it is possible, Gallium from the sodium hydroxide solution to extract kerosene and thus to separate the aluminum. Other elements of, which are extracted in this case likewise, can be removed with dilute acids. Subsequently, the remaining gallium compound is in a concentrated salt- or sulfuric acid and dissolved electrolytically reduced to the metal.

For many technical applications is very pure gallium requires, for semiconductors, for example, it may sometimes only one hundred millionth of impurities contained. Possible cleaning methods are vacuum distillation, fractional crystallization or melting zone.

The amount of produced gallium is low, 2008 was the world's primary production 95 Tons. Another important source is the re-processing of gallium-containing waste, it was 2008 more 135 Tons Gallium won. Main producing countries are the People's Republic of China, Germany, Kazakhstan and Ukraine, Galliumrecycling for the United States, Japan and the United Kingdom.

On a laboratory scale can be gallium by electrolysis of a solution of gallium in aqueous sodium hydroxide of platinum- or tungsten electrodes represent.

Properties

Physical Properties

Gallium is a silvery-white, soft (Mohs hardness: 1,5) Metal. It has an unusually low melting point for metals, at the 29,76 ° C.. It is thus for mercury and cesium the metal with the lowest melting point, which is also significantly lower than those of the adjacent elements aluminum and indium. Responsible for this is probably the most unusual crystal structure, in contrast to the structures of other metals does not have high symmetry and is therefore not very stable. Since the boiling point with 2204 ° C is comparatively high, Gallium has an unusually wide range, in which it is liquid. Due to the difficult crystallization, liquid gallium can easily be cooled below the melting point (Hypothermia) and suddenly crystallized in the formation of nuclei.

Gallium has as silicon, some other elements and water density anomaly, its density is in the liquid state by about 3,2 % higher than in the solid form. This is typical for substances, the solid state molecular bonds possess.

Gallium is in the solid state diamagnetic, However paramagnetic liquid state (\chi_{m} = 2,4 · 10−6 at 40 ° C)

Characteristic of its structure is the formation of gallium-gallium bonds. Various modifications are known, which under different crystallization conditions (four known modifications, a- δ-bis Gallium, under atmospheric pressure) and are under pressure (a total of three high-pressure modifications, Ga-II, Ga-III, Ga-IV). The most stable at room temperature modification is the α-gallium, which crystallizes in an orthorhombic layer structure. Thereby each form a covalent bond to each other via two atoms bonded dimer. Each gallium atom is adjacent to an additional six more atoms of other dimers. Between the dimers prevail metallic bonds. The Galliumdimere are so stable, that they remain so during melting and also received first in the gas phase are detected.

Further modifications are formed during the crystallization of supercooled, liquid gallium. At -16.3 ° C to form β-gallium, having a monoclinic crystal structure. In the structure are arranged in parallel zigzag chains of Ga ago. Delivers the crystallization at a temperature of -19.4 ° C., trigonal forms δ-gallium, present in the comparable distorted with α-boron icosahedron of twelve gallium. These are individual gallium atoms linked. At -35.6 ° C, finally produced γ-gallium. In this orthorhombic modification to form tubes of interconnected Ga7-Rings, lies in the middle of a linear chain of other gallium.

Is gallium at room temperature placed under high pressure, so as to form when pressure increases successively various high-pressure polymorphs. From 30 kbar is the cubic gallium II modification stable, is surrounded in every atom of each of eight other. If the pressure on 140 increased kbar, crystallized as the metal now tetragonal gallium III in a structure, corresponds to that of indium. The pressure further to about 1200 increased kbar, to form after the face-centered cubic structure of the gallium-IV.

Chemical Properties

The chemical properties of gallium are similar to those of aluminum. How this is gallium passivated by the formation of a dense oxide layer on the air and does not react. Only in pure oxygen at high pressure, the metal burns with a bright flame, forming the oxide. Similarly, it does not react with water, since in this case the insoluble gallium hydroxide forms. However, is alloyed with aluminum and gallium liquid by lowering the melting point at room temperature, it reacts violently with water. Reacts with halogen gallium rapidly to form the corresponding salts GaX3.

Gallium is amphoteric and soluble in both acids and bases in the evolution of hydrogen. Acids are formed in analogy to aluminum salts with Ga3+-Ion, in base Gallate where Form [Ga(OH)4]. In dilute acids it dissolves slowly, quickly in aqua regia and concentrated sodium hydroxide. By nitric acid passivated gallium.

\mathrm{2\ NaOH + 2\ Ga + 6\ H_2O \rightarrow 2\ Na[Ga(OH)_4] + 3\ H_2 \uparrow}

Reaction of gallium with sodium hydroxide solution

Most metals are attacked by molten gallium, so it only in containers made of quartz, Glass, Graphite, Alumina, Tungsten to 800 ° C und Tantal bus 450 ° C can be stored.

Isotope

A total of 30 Galliumisotope between 56Ga and 86Ga and another seven Kernisomere known. Of these two, 69Ga and 71Ga stable and occur in nature. Predominates in the natural isotopic composition 69Mit Ga 60,12 %, 39,88 % are 71Ga. Features of the unstable isotopes 67Mit Ga 3,26 Days, the longest half-life, the other half lives ranging from seconds to a maximum 14,1 Hours at 72Ga.

Two Galliumisotope, 67Ga and with 67,71 Minute half-life short-lived 68Ga can be used in nuclear medicine as tracers for positron emission tomography. 67Ga is generated in a cyclotron, while for the production of 68Ga cyclotron is not required. Instead, the longer-lived isotope of germanium 68Ge by irradiation of 69Ga produced by protons. This to disintegrate 68Ga, wherein the resulting 68Ga can be extracted in a gallium-68-generator. For investigations, the gallium is usually in a complex with a strong chelating ligands such as 1,4,7,10-tetraazacyclododecane-1 ,4,7,10-tetraacetic acid (DOTA) bound.

Use

Due to the rarity of the element gallium is used only to a limited extent. For the most part of the produced gallium gallium compounds produced are different. The economically most important by far are the elements with the 5. Main group, gallium especially, which is required for, inter alia, solar cells and LEDs. In 2003 were 95 % the produced processed this gallium. In addition, it also serves as a material for the doping of silicon (p-doping).

The wide temperature range, in which the element is liquid, and the low vapor pressure at the same time can be utilized for the construction of thermometers. Gallium can be up to temperatures of 1200 Insert ° C. Liquid gallium may be used as a barrier fluid for flow measurement of gases at high temperatures, as well as the liquid electrode material in the recovery of pure metals such as indium.

Gallium has a high wettability and a good reflectivity, and therefore is used as a coating for mirrors. Furthermore, it is in melting alloys, used for heat exchangers in nuclear reactors and as a substitute for mercury in lamps.

Of gallium alloys with other metals have different purposes. Magnetic materials produced by alloying with gadolinium, Require, Yttrium, Lithium and magnesium. The alloy with vanadium in the composition V3Ga is a superconductor having a comparatively high transition temperature of 16,8 K. In nuclear weapons, it is alloyed with plutonium, to prevent phase transformations. Many gallium as Galinstan are liquid at room temperature and can replace toxic mercury or the highly reactive sodium-potassium alloy.

Proof

Gallium can be detected with various color reactions typical high. These include the reaction of Rhodamine B in benzene, fluoresces orange-yellow to red-violet on the addition of gallium, Morin, which shows how in the reaction with aluminum, a green fluorescent, und Kaliumhexacyanidoferrat(III), with a white precipitate of gallium Galliumhexacyanidoferrat(III) forms. Furthermore, a spectroscopic detection of the characteristic violet spectral lines at 417,1 and 403,1 nm possible.

Quantitative evidence can complexometric titrations, for example with ethylene diamine or atomic absorption spectrometry.

Toxicology and biological significance

For gallium, there are no toxicological data; it does, however, corrosive to skin and mucous membranes. The gallium compounds(III)-Nitrate Ga(NO3)3 and gallium(III)-oxid Ga2The3 possess oral LD50-Values ​​in the gram range: 4,360 g / kg for the nitrate, and 10 g / kg for the oxide. Gallium is therefore considered to have low toxicity and plays, if known, no role as a trace element for humans.

Connections

Gallium compounds comes almost exclusively in the oxidation state +3 before. In addition, are rare and usually very unstable gallium(I)-Such compounds are known and, both a- contain both trivalent gallium (formal Gallium(II)-Connections).

Compounds with elements of the nitrogen group

The technically most important compounds of gallium, those are the elements of the nitrogen group. Gallium Nitride, Galliumphosphid, Gallium arsenide and gallium are typical semiconductors (III-V semiconductor) and for transistors, Diodes and other components used derElektronik. In particular light-emitting diodes of different colors can be produced as a gallium-nitrogen group compounds. Which depends on the band gap of color can be adjusted by the ratio of different anions, or by the substitution of gallium by aluminum or indium. Gallium arsenide is also used for solar cells. In particular, they are used in satellites, as gallium arsenide is more resistant to ionizing radiation than silicon.

Halides

Galliumhalogenide to Form GAX3 similar in many properties the corresponding aluminum compounds. With the exception of gallium(III)-fluorides they come as a dimer in a Aluminiumbromidstruktur. Is the only gallium halide(III)-chloride little economic significance. It is used as a Lewis acid in the Friedel-Crafts reaction.

Other connections

Gallium(III)-oxide such as aluminum oxide, a colorless, high melting solid. It comes in five different modifications, of which the cubic β-modification is most stable.

Organic gallium compounds exist as Gallane GAR3, Gallylene GAR and as higher gallanes, The gallium-gallium bonds. You are like many other organometallic compounds unstable to air and hydrolysis. One of the few galliumorganischen connections with economic impact is trimethylgallium, which is used as Dotierungsreagenz and for the production of thin layers of gallium arsenide and gallium nitride in the organometallic vapor phase epitaxy.

General
Name, Symbol,Atomic number Gallium, Ga, 31
Series Metals
group, Period, Block 13, 4, p
Appearance silvery white
CAS-Nummer 7440-55-3
Mass fraction of derErdhülle 14 ppm
Atomic
Atomic mass 69,723 you
Atomradius (calculated) 130 (136) pm
Kovalenter Radius 122 pm
Van-der-Waals-radius 187 pm
Electron configuration [It] 3d10 4s2 4p1
1. Ionization 578,8 kJ / mol
2. Ionization 1979,3 kJ / mol
3. Ionization 2963 kJ / mol
Physically
Physical State fixed
Modifications seven
Density 5,904 g/cm3
Mohs 1,5
Magnetism diamagnetisch (\chi_{m} = −2,3 · 10−5)
Melting point 302,91 K (29,76 ° C)
Boiling point 2477 K (2204 ° C)
Molar Volume 11,80 · 10−6 m3/mol
Heat of vaporization 256 kJ / mol
Schmelzwärme 5,59 kJ / mol
Speed ​​of sound 2740 m / s at 293,15 K
Specific heat capacity 371 J/(kg · K)
Electrical conductivity about 7,14 · 106 A/(In · m)
Thermal conductivity 29 W /(m · K)
Chemical
Oxidation states 3
Normalpotential -0.53 V (Ga3+ + 3 and → Ga)
Electronegativity 1,81 (Pauling-Scale)
Isotope
Isotope NH t1/2 ZA ZE (MeV) ZP
67Ga {a son.} 3,2612 d e 1,00 67Zn
68Ga {a son.} 67,629 my b+, e 2,921 68Zn
69Ga 60,1 % Stable
70Ga {a son.} 21,14 my b 1,656 70Ge
71Ga 39,9 % Stable
72Ga {a son.} 14,10 h b 4,001 72Ge
73Ga {a son.} 4,86 h b 1,593 73Ge
Safety
GHS Hazard Identification
05 – Ätzend

GefahrH- and P-SätzeH: 314EUH: no EUH setsP: 280-305 351 338- 310 for materials

Ätzend
Corrosive
(C)

R- and S-SätzeR: 34S: 26-36/37/39-45

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