The boron It is a non-metallic element that heads group 13 of the periodic table and is represented by the chemical symbol B. Its atomic number is 5, and it is the only non-metallic element in its group; although some chemists consider it a metalloid.
It appears as a blackish brown powder, and is found in a ratio of 10 ppm in relation to the earth's crust. Therefore it is not one of the most abundant elements.
It is found as part of several minerals such as borax or sodium borate, this being the most common boron mineral. There are also kurnite, another form of sodium borate; colemanite or calcium borate; and ulexite, sodium and calcium borate.
Borates are extracted in the United States, Tibet, China and Chile with a world production of approximately two million tons per year.
This element has thirteen isotopes, the most abundant being elevenB, which constitutes 80.1% of the boron by weight, and 10B, which forms the remaining 19.9%.
Boron is an essential trace element for plants, intervening in the synthesis of some vital plant proteins and contributes to the absorption of water. In mammals it appears to be necessary for bone health.
Although boron was discovered in 1808 by the English chemist Sir Humphry Davy, and the French chemists Jacques Thérnard and Joseph Gay-Lussac, since the beginning of our era in China borax was used in the manufacture of enamel ceramics.
Boron and its compounds have many uses and applications, ranging from its use in the preservation of food, especially margarine and fish, to its use in the treatment of cancerous tumors of the brain, bladder, prostate and other organs.
Boron is poorly soluble in water, but its compounds are. This could be a mechanism of concentration of boron, as well as a source of poisoning with the element..
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Since ancient times, man has used boron compounds in various activities. Borax, a mineral known as tincal, was used in China in 300 AD. in the production of enamel ceramics.
The Persian alchemist Rhazes (865-925) made the first mention of boron compounds. Rhazes classified the minerals into six classes, one of which was the boracios that included boron.
Agricola, around 1600, reported the use of borax as a flux in metallurgy. In 1777, the presence of boric acid was recognized in a thermal water stream near Florence.
Humphry Davy, by electrolysis of a borax solution, observed the accumulation of a black precipitate on one of the electrodes. He also heated boron oxide (BtwoOR3) with potassium, producing a blackish brown powder that was the known form of boron.
Gay-Lussac and Thénard reduced boric acid at high temperatures in the presence of iron to produce boron. They also showed the reverse process, that is, where boric acid is a product of the oxidation of boron.
Jöns Jakob Berzelius (1827) succeeded in identifying boron as a new element. In 1892, the French chemist Henri Moissan managed to produce boron with 98% purity. Although, it is pointed out that boron was produced in pure form by the American chemist Ezekiel Weintraub, in the year 1909.
Crystalline solid or amorphous black-brown powder.
10.821 g / mol.
2076 ºC.
3927 ºC.
-Liquid: 2.08 g / cm3.
-Crystalline and amorphous at 20 ºC: 2.34 g / cm3.
50.2 kJ / mol.
508 kJ / mol.
11.087 J / (mol K)
-First level: 800.6 kJ / mol.
-Second level: 2,427 kJ / mol.
-Third level: 3,659.7 kJ / mol.
2.04 on the Pauling scale.
90 pm (empirical).
4.16 cm3/ mol.
27.4 W / m.K
~ 106 Ω.m (at 20ºC).
Boron at high temperatures is a good electrical conductor, but at room temperature it becomes almost an insulator.
~ 9.5 on the Mohs scale.
Boron is not affected by hydrochloric acid at boiling point. However, it is converted by hot nitric acid to boric acid (H3BO3). Boron chemically behaves like a nonmetal.
Reacts with all halogens to give highly reactive trihalides. These have the general formula BX3, where X represents halogen.
It combines with various elements to originate borides. Some of them are among the hardest substances; for example, boron nitride (BN). Boron combines with oxygen to form boron trioxide.
Before addressing the structures of boron (crystalline or amorphous) it is essential to keep in mind how its atoms can be linked. The B-B bond is essentially covalent; Not only that, but because boron atoms naturally present electronic deficiency, they will try to supply it in their bonds in one way or another..
A special type of covalent bond is observed in boron: the one with three centers and two electrons, 3c2e. Here three boron atoms share two electrons, and define a triangle, one of the many faces found in their structural polyhedra (upper image).
From left to right we have: octahedron (a, B6), cuboctahedron (b, B12), and the isocashedron (c, B12 also). All of these units share one characteristic: they are electron poor. Therefore, they tend to bond covalently with each other; and the result is a party of amazing links.
In each triangle of these polyhedra the 3c2e bond is present. Otherwise it could not be explained how boron, capable of forming only three covalent bonds according to the Valencia Bond Theory, can have up to five bonds in these polyhedral units..
Boron structures then consist of an arrangement and repetition of these units that end up defining a crystal (or an amorphous solid).
There may be other polyhedral boron units, as well as one composed of only two atoms, Btwo; a boron “line” that must be bound to other atoms due to its high electronic deficiency.
The icosahedron is by far the preferred unit of boron; the one that suits you best. In the image above, for example, you can see how these B units12 intertwine to define the rhombohedral crystal of Boron-α.
If you wanted to isolate one of these icosahedra it would be a complicated task, since its electronic deficiency forces them to define a crystal where each one contributes the electrons that the other neighbors need..
The allotrope boron β-rhombohedral, as its name already indicates, has rhombohedral crystals like boron-α; however it differs in its structural units. It looks like an alien ship made of boron atoms.
If you look carefully, you can see the icosahedral units in a discrete and fused way (in the center). There are also B units10 and solitary boron atoms that act as a bridge for the mentioned units. Of all, this is the most stable boron allotrope.
In this boron allotrope the B units are coordinatedtwo and B12. The Btwo is so electronically deficient that it actually removes electrons from B12 and there is, therefore, an ionic character within this solid. That is, they are not only covalently bound, but there is electrostatic attraction of type [Btwo-] [B12+].
Boron-γ crystallizes into a rock-salt-like structure, the same as for NaCl. It is obtained by subjecting other boron allotropes to high pressures (20 GPa) and temperatures (1800 ° C), to later remain stable under normal conditions. Its stability actually competes with that of β-rhombohedral boron.
Other boron allotropes consist of aggregates of B atoms as if they were joined by a metallic bond, or as if they were ionic crystals; that is, it is a cubic boron.
Also, and no less important, is amorphous boron, whose arrangement of B units12 it is random and messy. It occurs as a fine or glassy solid powder of dull dark and brown colors.
And finally there is the newest and most bizarre allotrope of boron: borophenes (top image). It consists of a monolayer of boron atoms; extremely thin and analogous to graphene. Note that it preserves the famous triangles, characteristic of the electronic deficiency suffered by its atoms.
In addition to the borophenes, of which the B36 is the simplest and smallest, there are also the boron clusters. The borosphere (image below) consists of a ball-like spherical cage of forty boron atoms, B40; but instead of having smooth edges, they are abrupt and jagged:
The electron configuration of boron is:
[He] 2stwo 2 P1
It therefore has three valence electrons. It takes five more to complete its valence octet, and it can barely form three covalent bonds; it would need a fourth dative link to complete its octet. Boron can lose its three electrons to acquire an oxidation state of +3.
Boron is isolated by reducing boric acid with magnesium or aluminum; method similar to that used by Gay-Lussac and Thénard. It has the difficulty of contaminating boron with the borides of these metals.
A high purity sample can be obtained by gas phase reduction of boron trichloride, or tribromide, with hydrogen on electrically heated filaments of tantalum..
A high purity boron is prepared by high temperature decomposition of diborane, followed by purification by zone fusion or Czocharalski processes..
Elemental boron has been used in the hardening of steel. In an alloy with iron containing 0.001 to 0.005% boron. It is also used in the non-ferrous industry, usually as a deoxidizer.
In addition, boron is used as a degasser in high-conductance copper and copper-based alloys. In the semiconductor industry, small amounts of boron are carefully added as a doping agent for silicon and germanium..
Boron oxide (BtwoOR3) is mixed with silica to make a heat resistant glass (borosilicate glass), used in kitchen utensils and certain laboratory equipment.
Boron carbide (B4C) is an extremely hard substance that is used as an abrasive and reinforcing agent in composite materials. Aluminum boride (AlB12) is used as a substitute for diamond dust for grinding and polishing.
Boron is used in alloys, for example rare earth magnets, by alloying iron and neodymium. The formed magnets are used in the manufacture of microphones, magnetic switches, headphones and particle accelerators.
The capacity of the boron-10 isotope (10B) to trap neutrons, emitting α-type radiation has been used for the treatment of brain tumors in a technique known as Boron Neutron Capture Therapy (BNCT).
The 10Compound B is accumulated in the cancerous tumor. Subsequently, the tumor area is irradiated with neutrons. These interact with 10B, which causes the emission of α particles. These particles have a high relative biological effect and due to their large size they have little range..
Therefore, the destructive action of the α particles remains confined in the tumor cells, carrying out their destruction. BNCT is also used in the treatment of cancerous tumors of the neck, liver, bladder and prostate..
A small amount of boron, in the form of boric acid or borate, is necessary for the growth of many plants. A boron deficiency manifests itself in misshapen plant growth; the "brown heart" of vegetables; and the "dry rot" of sugar beets.
Boron may be needed in small amounts to maintain bone health. There are studies that indicate that the lack of boron would be involved in the generation of arthritis. It would also intervene in brain functions such as memory and hand-eye coordination.
Some experts point out that 1.5 to 3 mg of boron should be included in the daily diet.
Boron, boron oxide, boric acid, and borates are considered non-toxic. The LD50 for animals is 6 g of boron / kg of body weight, while substances with an LD50 greater than 2 g / kg of body weight are considered non-toxic..
On the other hand, the consumption of more than 0.5 mg / day of boron for 50 days causes minor digestive problems, suggestive of toxicity. Some reports indicate that an excess in the intake of boron can affect the functioning of the stomach, liver, kidneys and brain.
Also, short-term irritant effects on the nasopharynx, upper respiratory tract, and eyes have been reported from boron exposure..
Reports of boron toxicity are scarce and in many cases, toxicity occurs at very high doses, higher than those to which the general population is exposed..
The recommendation is to monitor the boron content of foods, especially vegetables and fruits. Government health agencies must ensure that the boron concentration of the water does not exceed the permitted limits..
Workers exposed to boron-containing dust should wear respiratory protective masks, gloves, and special boots..
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