Boron Oxide

Boron oxide or boric anhydride is an inorganic compound with the chemical formula B 2 O 3 . Boron and oxygen elements of the p block of the periodic table, and even as the more prominent of their respective groups, the difference in electronegativity between them is not very great; Therefore, it should be expected that B 2 O 3 be covalent in nature.

2 O 3 It is prepared by dissolving borax in concentrated sulfuric acid inside a melting furnace and at a temperature of 750 ° C; thermally dehydrate boric acid, B (OH) 3 , at a temperature of about 300 °C; Or it can also be formed as a product of diborane reaction (B 2 H 6 ) with oxygen.

Boron Oxide

Boron oxide may have a semicrystalline glassy or crystalline appearance; The latter can be obtained by grinding into powder form (top image).

Although it may not seem so at first sight, it is considered to be B 2 O 3 one of the most complex inorganic oxides; Not only from a structural point of view, but also because of the variable properties that glasses and ceramics acquire by adding them to their matrix.

Structure of Boron Oxide

BO Unit 3

2 O 3 is a covalent solid, so in principle there are no B ions in its structure 3+ nor O 2- , but B-O links. Boron, according to valence bond theory (VTE), can form only three covalent bonds; In this case, three B-O links. Consequently, the expected geometry should be triangular, bo 3 .

Bo molecule 3 It is deficient in electrons, especially oxygen atoms; However, many of them can negotiate with each other to supply the said shortfall. So, the triangles BO 3 they are connected by sharing an oxygen bridge, and they are distributed in space as a triangular row network along their planes.

crystal structure

The upper image shows an example of the above lines with triangular units BO 3 . If you look carefully, all the faces of the planes are not pointed towards the reader, but the other way. The inclination of these faces may be responsible for the way B 2 O 3 is defined at a given temperature and pressure.

When these networks have a long-range structural pattern, it is a crystalline solid, which can be constructed from its unit cell. This is where B 2 O 3 is called. It has two crystalline polymorphs: α and ine.

α-B 2 O 3 occurs at ambient pressure (1 atm), and is said to be kinetically unstable; In fact, this is one of the reasons why boron oxide is probably a difficult compound to crystallize.

The other polymorph, β-B 2 O 3 , is obtained at high pressures in the range of GPa; Therefore, its density should be greater than α-B 2 O 3 .

vitro structure

Network BO 3 Naturally they adopt amorphous structures; These are, that there is a lack of a pattern that describes the molecules or ions in the solid. By synthesizing B 2 O 3 its major form is amorphous and not crystalline; Precisely speaking: it is a solid substance rather than a crystalline solid.

Then it is said that B 2 O 3  when it has a BO network it is vitro or amorphous 3 they are dirty. Not only that, but they also change the way they come together. Instead of being arranged in a triangular geometry, they are joined at the end to form what researchers call a boroxol ring (top image).

Note the clear distinction between the triangular and hexagonal units. The triangular ones denote B 2 O 3 crystalline, and the hexagonal ones B 2 O 3 vitreous. Another way to refer to this amorphous phase is boron glass, or by a formula: G-B 2 O 3 (the ‘G’ comes from the word glassy, ​​in English).

Thus, the G–B network is composed of 2 O 3 O boroxol rings and not BO units . However, G-B 2 O 3 can crystallize to α-B 2 O 3 , which will be linked to each other from ring to trigonal, and will also define the degree of crystallization achieved.


physical appearance

It is a colorless and glassy solid. In its crystalline form it is white in colour.

molecular mass

69.6182 g/mol.


a little bitter


-Crystalline: 2.46 g/mL.

-Vitrus: 1.80 g / ml.

Melting point

It does not have a perfectly defined melting point, as it varies depending on whether it is crystalline or vitreous. The purely crystalline form melts at 450 °C; However, the shape of the glass melts in temperatures ranging from 300 to 700 ºC.

boiling point

Then, the reported values ​​do not match this value. Apparently liquid boron oxide (melted from its crystal or its glass) boils at 1860ºC.


It must be kept dry, as it absorbs moisture to convert it into boric acid, B(OH) 3 .


Boron oxide can be named in other ways, such as:

-Diboro trioxide (systematic nomenclature).

-Boron oxide (III) (stock nomenclature).

-boric oxide (conventional nomenclature).


Some of the uses of Boron Oxide are as follows:

Synthesis of Boron Trihalides

From B 2 O 3 boron trihalides can be synthesized, BX 3 (X = F, Cl and Br). These compounds are Lewis acids, and with them it is possible to introduce boron atoms to some molecules to obtain other derivatives with new properties.


A solid mixture with boric acid, B 2 O 3 -B (OH) 3 , represents a formula that is used as a household insecticide.

Solvent of metal oxides: manufacture of glasses, ceramics and boron alloys

Liquid boron oxide is able to dissolve metal oxides. From this resulting mixture, once cooled, solids are obtained by boron and metals.

Depending on the amount of Bi 2 O 3 used, as well as the technique, and the type of metal oxide, you can get a rich variety of glasses (borosilicates), ceramics (nitrides and boron carbides), and alloys. (only metal is used).

In general, glass or ceramics tend to have greater strength and strength, as well as greater durability. In the case of glasses, they are being used for optical lenses and telescopes, and for electronic devices.

binding machine

In the construction of steel melting furnaces, refractory bricks with a magnesium base are used. In them, boron oxide is used as a binder, which helps them to bind tightly.