10 julio, 2024

Gallium: properties, structure, obtaining, uses

He gallium It is a metallic element that is represented by the symbol Ga and that belongs to group 13 of the periodic table. Chemically it resembles aluminum in its amphotericism; however, both metals end up exhibiting properties that make them differentiable from one another.

For example, aluminum alloys can be worked to give them all kinds of shapes; while those of gallium have very low melting points, consisting practically of silvery liquids. Also, the melting point of gallium is lower than that of aluminum; the former can melt from the heat of the hand, while the latter cannot.

The chemical similarity between gallium and aluminum also group them together geochemically; that is, minerals or rocks rich in aluminium, such as bauxites, have estimable concentrations of gallium. Apart from this mineralogical source, there are others of zinc, lead and carbon, widely disseminated throughout the earth’s crust.

Gallium is not popularly a well-known metal. Its mere name can evoke the image of a rooster in the mind. In fact, graphic and general representations of gallium are usually found with the image of a silver rooster; painted with liquid gallium, a highly wettable substance on glass, ceramics and even the hand.

Experiments in which pieces of metallic gallium are melted by hand are frequent, as is the manipulation of its liquid and its tendency to stain everything it touches.

Although gallium is not toxic, as is mercury, it is a destroying agent for metals, since it makes them brittle and useless (in the first instance). On the other hand, pharmacologically it intervenes in the processes where the biological matrices use iron.

For those in the world of optoelectronics and semiconductors, gallium will be held in high esteem, comparable and, perhaps, superior to silicon itself. On the other hand, with gallium thermometers, mirrors and objects based on its alloys have been manufactured.

Chemically, this metal still has a lot to offer; perhaps in the field of catalysis, nuclear energy, in the development of new semiconductor materials, or «simply» in the clarification of its confusing and complex structure.

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History

Predictions of its existence

In 1871, the Russian chemist Dmitri Mendeleev had already predicted the existence of an element whose properties resembled those of aluminium; which he named as ekaluminium. This element had to be located just below the aluminum. Mendeleev also predicted the properties (density, melting point, formulas of its oxides, etc.) of ekaluminium.

discovery and isolation

Surprisingly, four years later, the French chemist Paul-Emili Lecoq de Boisbaudran had found a new element in a sample of sphalerite (zinc blende) from the Pyrenees. He was able to discover it thanks to a spectroscopic analysis, in which he observed the spectrum of two violet lines that did not match that of another element.

Having discovered a new element, Lecoq carried out experiments on 430 kg of sphalerite, from which he was able to isolate 0.65 grams of it; and after a series of measurements of its physical and chemical properties, he concluded that it was Mendeleev’s ekaluminium.

To isolate it, Lecoq carried out the electrolysis of its respective hydroxide in potassium hydroxide; probably the same with which he dissolved the sphalerite. By certifying that it was ekaluminium, and for being also the discoverer of it, he gave it the name ‘gallium’ (galium in English). This name derived from the name ‘Gallia’, which in Latin means France.

However, the name presents another curiosity: ‘Lecoq’ in French means ‘rooster’, and in Latin ‘gallus’. Being a metal, ‘gallus’ became ‘gallium’; although in Spanish the conversion is much more direct. Thus, it is no coincidence that a rooster is thought of when talking about gallium.

Physical and chemical properties

Appearance and physical characteristics

Gallium is a silvery, glassy-surfaced, odorless metal with an astringent taste. Its solid is soft and brittle, and when it fractures it does so in a conchoidal way; that is, the pieces formed are curved, similar to seashells.

When melted, depending on the angle at which it is viewed, it may show a bluish glow. This silver liquid is non-toxic on contact; however, it «clings» too much to surfaces, especially if they are ceramic or glass. For example, a single drop of gallium can permeate the inside of a glass beaker to coat it with a silver mirror.

If a solid piece of it is deposited in liquid gallium, it serves as a nucleus where sparkling gallium crystals develop and grow rapidly.

Atomic number (Z)

31 (31Ga)

molar mass

69.723 g/mol

Melting point

29.7646ºC. This temperature can be reached by holding a gallium crystal between the two hands until it melts.

Boiling point

2400ºC. Note the large gap between 29.7 ºC and 2400 ºC; that is, liquid gallium has a very low vapor pressure, and this fact makes it one of the elements with the greatest temperature difference between the liquid and gaseous state.

Density

-At room temperature: 5.91 g/cm3

-At the melting point: 6.095 g/cm3

Note that the same thing happens with gallium as with water: the density of its liquid is greater than that of its solid. Therefore, its crystals will float on liquid gallium (gallium icebergs). In fact, such is the volume expansion of the solid (three times), that it is inconvenient to store liquid gallium in non-plastic containers.

heat of fusion

5.59kJ/mol

heat of vaporization

256kJ/mol

molar heat capacity

25.86 J/(mol K)

Vapor pressure

At 1037 ºC just its liquid exerts a pressure of 1 Pa.

electronegativity

1.81 on the Pauling scale

ionization energies

-First: 578.8 kJ/mol (Ga+ gas)

-Second: 1979.3 kJ/mol (Ga2+ gas)

-Third: 2963 kJ/mol (Ga3+ gas)

Thermal conductivity

40.6 W/(m K)

electrical resistivity

270 nΩ m at 20ºC

Mohs hardness

1.5

Goo

1,819 cP at 32ºC

Surface tension

709 dynes/cm at 30ºC

amphotericism

Like aluminum, gallium is amphoteric; It reacts with both acids and bases. For example, strong acids can dissolve it to form gallium(III) salts; if they are treated with H2SO4 and HNO3, Ga2(SO4)3 and Ga(NO3)3 are produced, respectively. While when reacting with strong bases, gallate salts are produced, with the Ga(OH)4– ion.

Note the similarity between Ga(OH)4– and Al(OH)4– (aluminate). If ammonia is added to the medium, gallium (III) hydroxide, Ga(OH)3, is formed, which is also amphoteric; when reacting with strong bases, it again produces Ga(OH)4–, but if it reacts with strong acids, it releases the aqueous complex [Ga(OH2)6]3+.

Reactivity

Gallium metal is relatively inert at room temperature. It does not react with air, since a thin layer of oxide, Ga2O3, protects it from oxygen and sulfur. However, when it is heated, the oxidation of the metal continues, transforming completely into its oxide. And if sulfur is present, at high temperatures it reacts to form Ga2S3.

There are not only gallium oxides and sulphides, but also phosphides (GaP), arsenides (GaAs), nitrides (GaN) and antimonides (GaSb). Such compounds can originate through the direct reaction of the elements at elevated temperatures, or through alternative synthetic routes.

Likewise, gallium can react with halogens to form their respective halides; such as Ga2Cl6, GaF3 and Ga2I3.

This metal, like aluminum and its congeners (members of the same group 13), can covalently interact with carbon atoms to form organometallic compounds. In the case of those with Ga-C bonds, they are called organogalles.

The most interesting thing about gallium is not any of its previous chemical characteristics, but its enormous ease with which it can be alloyed (similar to that of mercury and its amalgamation process). Its Ga atoms “nudge” rapidly between metallic crystals, giving rise to gallium alloys.

Electronic structure and configuration

Complexity

Gallium is not only unusual in that it is a metal that melts in the heat of the palm of your hand, but also that its structure is complex and uncertain.

On the one hand, it is known that its crystals adopt an orthorhombic structure (Ga-I) under normal conditions; however, this is just one of the many possible phases for this metal, for which the exact arrangement of its atoms is not specified. It is therefore a more complex structure than it might appear at first glance.

It seems that the results vary according to the angle or direction in which its structure is analyzed (anisotropy). Also, these structures are very susceptible to the slightest change in temperature or pressure, which means that gallium cannot be defined as a single type of crystal at the time of data interpretation.

dimers

The Ga atoms interact with each other thanks to the metallic bond. However, a certain degree of covalence has been found between two neighboring atoms, so the existence of the Ga2 (Ga-Ga) dimer is assumed.

In theory, said covalent bond should be formed by the overlap of the 4p orbital, with its only electron according to the electronic configuration:

[Ar] 3d10 4s2 4p1

This mixture of covalent-metal interactions is credited with gallium’s low melting point; since, although on the one hand there may be a “sea of ​​electrons” that holds the Ga atoms tightly together in the crystal, on the other the structural units consist of Ga2 dimers, whose intermolecular interactions are weak.

Phases under high pressures

When the pressure increases from 4 to 6 GPa, the gallium crystals undergo phase transitions; from the orthorhombic it passes to the cubic centered on the body (Ga-II), and from this it finally passes to the tetragonal centered on the body (Ga-III). In the range of pressures, possibly a mixture of crystals is formed, which makes the interpretation of the structures even more difficult.

oxidation numbers

The most energetic electrons are those found in the 4s and 4p orbitals; as there are three of them, it is therefore expected that gallium can lose them when combined with elements more electronegative than it.

When this occurs, the existence of the Ga3+ cation is assumed, and its number or oxidation state is said to be +3 or Ga(III). In fact, this is the most common of all its oxidation numbers. The following compounds, for example, have gallium as +3: Ga2O3 (Ga23+O32-), Ga2Br6 (Ga23+Br6–), Li3GaN2 (Li3+Ga3+N23-) and Ga2Te3 (Ga23+Te32-).

Gallium can also be found with oxidation numbers of +1 and +2; although they are much less common than +3 (similar to aluminum). Examples of such compounds are GaCl (Ga+Cl–), Ga2O (Ga2+O2-) and GaS (Ga2+S2-).

Note that it is always assumed (correctly or not) the existence of ions with identical charge magnitudes to…

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