Boron breaks the ranks

Boron is the chemical element with the symbol B and the atomic number 5 in the table of the elements. It is neither metal (iron, copper, gold …) nor nonmetal (oxygen, carbon, phosphorus …). Boron is a so-called metalloid such as silicon or germanium and it is not surprising that the physical and chemical properties of boron are sometimes typical for metals and sometimes typical for nonmetals.

Boron belongs to the group 13 in the table of the elements together with aluminum, gallium, indium and thallium. All elements in this group have 3 valence electrons. Elements with less than four valence electrons normally crystallize to metal lattices except boron, which does not form free ions with the charge +3. Boron atoms have 3 valence electrons and 4 valence orbitals. They prefer covalent bonds due to the large ionization energy and electronegativity. The unique but complicated lattice structures of the boron modifications (crystal forms) follow from this electron deficit of the boron atoms. Boron never exists in pure form in nature due to the large affinity to oxygen. The instable pure boron has 4 allotrope modifications besides the amorphous form. That is, boron can take on 4 different crystal forms in the same state of aggregation. Another example of such a behavior are the 2 allotrope modifications graphite and diamond of the element carbon.

No other element exhibits a similar flexibility in its modifications. Boron atoms have coordination numbers ranging from 4 to 9 with highly varying bond gaps. The large coordination numbers require the formation of multiple-center bonds, since the boron atoms have 3 valence electrons, only. These multiple-center bonds yield boron modifications with extremely large hardness. The thermodynamic stable b-rhombohedral boron has a Mohs hardness of 9.3 and is the second-hardest material after diamond with a Mohs hardness of 10. Other materials based on boron with large hardness are the black shiny boron carbides such as B₁₃C₂ or the cubic boron nitride cBN. In contrast, the hexagonal boron nitride hBN has a crystal form that is similar to that of graphite and has little hardness but very good lubrication properties. This is why hBN is often used as high-temperature lubricant.

The outstanding properties of boron can be improved even further with nanotechnology. For example, the boron diamond powder (MCDP) produced by NNT is used for friction reduction despite and because of the extremely large hardness (the base material is boron carbide). This is due to the tininess of the crystals, which acts as rolling bearing balls without abrasion of the treated surface.

Please look into the subsequent categories of this website for further information about the element boron. Alternatively, you may simply search the internet or you visit the links Wikipedia or ChemicalelementsInfo.