Why does boron form a covalent bond

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Characteristics

Boron is the 5th element of the periodic table. It lies on the borderline between metal (beryllium, main group II) and non-metal (carbon, main group IV) and is therefore assigned to the semimetals like silicon (more rarely also than Metalloid, for metal-like elements): Both elements are semiconductors, i.e. Their electrical conductivity is still very low at room temperature, but increases exponentially with temperature1). Another argument in favor of the characterization of the element boron as a semimetal is that the ionization energy of boron is comparatively high2). Both boron and silicon have an unusually high melting temperature (m.p.B. approx. 2250, m.p.Si approx. 1410). Like silicon, crystalline boron has a high hardness. It exceeds that of corundum ,, and is similar to that of boron carbide ,, and is therefore almost as hard as diamond.

construction

There are six modifications of pure, crystallized boron, but the structures of only four have been largely clarified: Common characteristic basic building blocks are icosahedra (twenty surfaces), the twelve corners of which are occupied by boron atoms. In this icosahedron, each boron atom is bound to five other boron atoms, which together form a regular pentagon.

In addition to the bonds to the five neighboring atoms in the icosahedron, each boron atom also maintains bonds to the outside in order to hold the icosahedron as a whole together in the crystal. The bond relationships can only be explained by the multicenter bonds typical of electron-deficient compounds (three-center bond).

The modifications

The individual boron modifications differ mainly in the type of connection of the icosahedron (direct connection, via boron atoms or boron atom groups).

α-rhombohedral boron

The simplest structure shows red, translucent α-rhombohedral boron (discovered in 1958), in which the units form an almost cubic closest packing.

Further normal pressure modifications

The modification that is thermodynamically stable under normal pressure at all temperatures is the black-dark-gray glossy, β-rhombohedral crystallizing boron (discovered 1957). There are also the black α-tetragonal boron, which has been known since 1943, and the red β-tetragonal boron discovered in 1959. It contains 190 boron atoms per unit cell, which are divided into 8 icosahedra, 4 twin icosahedra and 10 individual boron atoms.

High pressure modification and modifications with heteroatoms

In addition to the normal pressure modifications of boron, there is also a high pressure modification as well as boron modifications with heteroatoms which, in addition to boron as the main component, also contain small stoichiometric proportions of another element (eg metals, carbon, silicon ). The transition to boron compounds (borides, boron carbide) is fluid.

The peculiarity of the boron, not like the other elements of the III. Crystallizing the main group in a metal lattice is one of a number of properties in which boron is more similar to silicon from main group IV of the periodic table than to aluminum, the element of III. Main group (oblique relationship).

Deepening: oblique boron / silicon relationship