Boron carbon allotropes

The structural analogy with the carbon allotropes can be extended to the properties of the two borrm-nitride allotropes. For instance, hexagonal boron nitride (h-BN) is soft and lubricious like grsqihite, and cubic boron nitride (c-BN), like diamond, is extremely hard. 

The a tetragonal form, firstly reported as a boron allotrope, has been reformulated as B50C2 or B50N2, depending on the preparative conditions it never forms in absence of carbon or nitrogen. 

Boron s chemistry is so different from that of the other elements in this group that it deserves separate discussion. Chemically, boron is a nonmetal in its tendency to form covalent bonds, it shares more similarities with carbon and silicon than with aluminum and the other Group 13 elements. Like carbon, boron forms many hydrides like silicon, it forms oxygen-containing minerals with complex structures (borates). Compounds of boron have been used since ancient times in the preparation of glazes and borosilicate glasses, but the element itself has proven extremely difficult to purify. The pure element has a wide diversity of allotropes (different forms of the pure element), many of which are based on the icosahedral Bj2 unit. 

Boron (B), the second hardest element, is the only allotropic element in Group 13. It is second only to carbon (C) in its ability to form element-element bonded networks. Thus, in addition to amorphous boron, several different allotropes of boron are known, of which three are well characterized. These are red crystalline a-rhombohedral boron, black crystalline /3-rhombohedral boron (the most thermodynamically stable allotrope), and black crystalline /3-tetragonal boron. All are polymeric and are based on various modes of condensation of the Bj2 icosahedron (Figure 2). 

The propensity of boron to form polyhedral structures is reflected also in the structures of elemental boron and boron-rich metal borides. In hydrocarbon chemistry, benzene is characterized by its extra stability the thermodynamically most stable allotrope of carbon, namely, graphite is formed by the condensation of benzene units. This beautiful relationship between compounds and allotropes exists in boron chemistry as well, where the stable allotropes of elemental boron and many of the boron-rich metal borides are made up of icosahedral subunits. 

There is a weak electronic interaction between the ic-systems of adjacent sheets and that, with van der Waals forces operating, holds the sheets together. The delocalization of the ic electrons within each sheet provides the stability of the structure, which is 190 J moh more stable than the other principal allotrope, diamond. In diamond, each carbon atom exerts its valency of four in a tetrahedral manner, and the carbon-carbon distance is 154 pm, Le. that expected for a single covalent bond. The enthalpy of atomization of carbon is significantly larger than that of boron, reflecting the effect of the extra valence electron. 

Covalent-network solids consist of atoms held together in large networks or chains by covalent bonds. Because covalent bonds are much stronger than intermolecular forces, these solids are much harder and have higher melting points titan molecular solids. Diamond and graphite, two allotropes of carbon, are covalent-network solids. Other examples include quartz, Si02 silicon carbide, SiC and boron nitride, BN. 

Boron is the thirteenth element in the periodic table and it is in the same group as aluminum. However, it is more chemically similar to silicon. The physical and chemical properties of boron are shown in Table 6.1 [3-7]. Boron is a lustrous gray-black colored and the second hardest element, with the diamond allotrope of carbon being the hardest. In the crystal, the / -rhombohedral form is thermodynamically stable, and the amorphous boron becomes the /I-rhombohedral form by heating at 1200 C. At 1500 C or more, it becomes the a-rhombohedral [4]. 

One allotropic form of an element X is a colorless crystalline solid. The reaction of X with an excess amount of oxygen produces a colorless gas. This gas dissolves in water to yield an acidic solution. Choose one of the following elements that matches X (a) sulfur, (b) phosphorus, fc) carbon, (d) boron,... 

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