BORON: HISTORY, PROPERTIES, STRUCTURE, USES - CHEMISTRY - 2025

The boron is a nonmetallic element that leads the group 13 of the periodic table and represented by the chemical symbol B. Its atomic number is 5, and the only non - metallic element of the 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.

Boron sample with a purity around 99%. Source: Alajhasha

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 mined 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 ¹¹ B, which constitutes 80.1% of boron by weight, and ¹⁰ B, 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 boron concentration, as well as a source of poisoning with the element.

History

Background

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 making 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 hot spring stream near Florence.

Element discovery

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 (B ₂ O ₃) 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 an oxidation product of boron.

Identification and isolation

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.

Properties

Physical description

Crystalline solid or amorphous black-brown powder.

Molar mass

10.821 g / mol.

Melting point

2076 ° C.

Boiling point

3927 ° C.

Density

-Liquid: 2.08 g / cm ³.

-Crystalline and amorphous at 20 ºC: 2.34 g / cm ³.

Heat of fusion

50.2 kJ / mol.

Heat of vaporization

508 kJ / mol.

Molar caloric capacity

11.087 J / (mol K)

Ionization energy

-First level: 800.6 kJ / mol.

-Second level: 2,427 kJ / mol.

-Third level: 3,659.7 kJ / mol.

Electronegativity

2.04 on the Pauling scale.

Atomic radio

90 pm (empirical).

Atomic volume

4.16 cm ³ / mol.

Thermal conductivity

27.4 W / mK

Electrical resistivity

~ 10 ⁶ Ω.m (at 20ºC).

Boron at high temperatures is a good electrical conductor, but at room temperature it becomes almost an insulator.

Hardness

~ 9.5 on the Mohs scale.

Reactivity

Boron is not affected by hydrochloric acid at boiling temperature. However, it is converted by hot nitric acid to boric acid (H ₃ BO ₃). Boron chemically behaves like a nonmetal.

Reacts with all halogens to give highly reactive trihalides. These have the general formula BX ₃, where X represents halogen.

It combines with various elements to produce borides. Some of them are among the hardest substances; for example, boron nitride (BN). Boron combines with oxygen to form boron trioxide.

Structure and electron configuration of boron

Links and structural units in boron

Geometries of common structural units for boron. Source: Materialscientist

Before addressing the structures of boron (crystalline or amorphous) it is essential to keep in mind how its atoms can be linked. The BB 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.

In boron a special type of covalent bond is observed: 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 (top image).

From left to right we have: octahedron (a, B ₆), cuboctahedron (b, B ₁₂), and isocashedron (c, B _{12 as} well). All of these units share one characteristic: they are electron poor. Therefore, they tend to link covalently with each other; and the result is an amazing bonding party.

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).

Α-rhombohedral boron

Crystal structure of the α-rhombohedral boron allotrope. Source: Materialscientist at English Wikipedia

There may be other polyhedral boron units, as well as one composed of only two atoms, B ₂; 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 ₁₂ units intertwine to define the rhombohedral crystal of Boron-α.

If one 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.

Β-rhombohedral boron

Crystal structure of the allotrope boron β-rhombohedral. Source: Materialscientist at English Wikipedia

The allotrope β-rhombohedral boron, as its name already indicates, possesses 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, the icosahedral units can be seen in a discrete and fused way (in the center). There are also B ₁₀ units and lone boron atoms that act as a bridge for the mentioned units. Of all, this is the most stable boron allotrope.

Boron-γ rock salt

Boron-γ crystal structure. Source: Materialscientist at English Wikipedia

In this boron allotrope the B ₂ and B ₁₂ units coordinate. B ₂ is so electronically deficient that it actually removes electrons from B ₁₂ and there is therefore an ionic character within this solid. That is, they are not only covalently bound, but there is electrostatic attraction of sorts.

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.

Cubic and amorphous

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 ₁₂ units is random and messy. It occurs as a fine powder or glassy solid of dark and opaque brown colors.

Borophenes

Structure of the simplest of the borophenes, B36. Source: Materialscientist

And finally there is the most novel and bizarre allotrope of boron: borophenes (top image). It consists of a monolayer of boron atoms; extremely thin and analogous to graphene. Note that it conserves the famous triangles, characteristic of the electronic deficiency suffered by its atoms.

In addition to the borophenes, of which B ₃₆ 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, B ₄₀; but instead of having smooth edges, they are rough and jagged:

Borosphere unit, B40. Source: Materialscientist

Electronic configuration

The electron configuration of boron is:

2s ² 2p ¹

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.

Obtaining

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.

Applications

In the industry

Elemental boron has long been used to harden steel. In an alloy with iron that contains 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 degassing agent 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 (B ₂ O ₃) is mixed with silica to make heat resistant glass (borosilicate glass), used in cookware and certain laboratory equipment.

Boron carbide (B ₄ C) is an extremely hard substance that is used as an abrasive and reinforcing agent in composite materials. Aluminum boride (AlB ₁₂) 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 magnets formed are used in the manufacture of microphones, magnetic switches, headphones, and particle accelerators.

In medicine

The ability of the boron-10 (¹⁰ B) isotope 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 ¹⁰ B in the form of compounds is accumulated in the cancerous tumor. Subsequently, the tumor area is irradiated with neutrons. These interact with ¹⁰ B, 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.

Biological action

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 could 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.

Risks and caution

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 about boron toxicity are scarce and in many cases, toxicity occurs at very high doses, higher than those exposed to the general population.

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.

References

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