Boron has three valence electrons and cannot transfer electrons to form an ionic bond. It is essentially covalent and often described as “electron deficient”. Boranes are compounds of boron and hydrogen that have led to a revision of chemical bonding theories. Elemental boron is difficult to prepare in pure form, but metal-boron alloys and compounds have been produced for their hardness and high melting points. An example is aluminum magnesium boride (BAM), which has the lowest known coefficient of friction and is used as a low-friction coating for machine parts.
The atomic structure of boron, element number 5 on the periodic table, shows a complete inner shell of two electrons, with three electrons in the outermost shell, giving the atom three valence electrons available for bonding. In this respect it resembles aluminum, the next element in the boron group; however, unlike aluminum, it cannot transfer electrons to other atoms to form an ionic bond with a B3+ ion, as the electrons are too bound to the nucleus. Boron generally does not accept electrons to form a negative ion, so it does not normally form ionic compounds: the chemistry of boron is essentially covalent. The electron configuration and resulting bonding behavior also determine the crystalline structure of boron in its various elemental forms.
Boron compounds can often be described as “electron deficient”, as there are fewer electrons involved in bonding than are required for normal covalent bonds. In a single covalent bond, two electrons are shared between atoms and in most molecules the elements follow the octet rule. The structures of boron compounds such as boron trifluoride (BF3) and boron trichloride (BCl3), however, show that the element has only six, not eight, electrons in its valence shell, making them exceptions to the octet rule.
An unusual bond is also found in the structure of boron compounds known as boranes: investigation of these compounds has led to a revision of chemical bonding theories. Boranes are compounds of boron and hydrogen, the simplest of which is the trihydride, BH3. Again, this compound contains a boron atom which is two electrons short of an octet. Diborane (B2H6) is unusual in that each of the two hydrogen atoms in the compound shares its electron with two boron atoms—this arrangement is known as a two-electron three-center bond. Today, more than 50 different boranes are known, and the complexity of their chemistry rivals that of hydrocarbons.
Elemental boron does not occur naturally on Earth and is difficult to prepare in pure form, as usual methods – such as oxide reduction – leave impurities that are difficult to remove. Although the element was first prepared in impure form in 1808, it was not until 1909 that it was produced in sufficient purity to study its crystal structure. The basic unit for the crystal structure of boron is a B12 icosahedron, with — at each of the 12 vertices — a boron atom bonded to five other atoms. The interesting feature of this structure is that the boron atoms form semibonds by sharing one electron instead of the usual two electrons in a covalent bond. This gives the boron atoms an effective valence of 6, with an extra bond available at each of the vertices to allow them to bond to adjacent units.
Icosahedra do not pack tightly and leave voids in the crystal structure that can be filled by boron atoms or other elements. A number of useful metal-boron alloys and compounds of boron with B12 icosahedra in combination with other elements have been produced. These materials are known for their hardness and high melting points. An example is aluminum magnesium boride (BAM), with the chemical formula AlMgB14. This material has the distinction of having the lowest known coefficient of friction – in other words, it is extremely slippery – and is used as a tough, low-friction coating for machine parts.
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