Boron with Nonmetallic Elements

Boron has a great affinity for oxygen and occurs in nature only in boric acid or borates. Borates are composed from clusters of flat trigonal BO3 and tetrahedral BO4 groups. The structural chemistry of borates is as rich and complicated as those of silicates, borides, or boranes. Boron oxide is an essential part of borosilicate glasses such as Pyrex. Boron halides are volatile molecular compounds. They are Lewis acids and react violently with water. The subhalides consist of boron chains or clusters that have terminally bound halogen atoms. They are substitution derivatives of the lower boranes. 

Boron phosphide (BP), a very hard refractory semiconductor, has a zinc blende structure as most III-V (13-15) semiconductors, such as GaAs and InP. It has a bandgap of 2eV and a slightly larger interatomic distance than SiC or AIN. 

The borides of this type can be considered as sort of inverse molecular lattices. In molecular compounds the bonding between the molecules is weaker than between the atoms in the molecule. An external force will strain the intermolecular distances more than the molecular configuration. In these borides, on the other hand, the bonds between the icosahedra are stronger than the ones with them, which explains their remarkable combination of physical properties. Their lattice stiffness and high sound propagation rate are inconsistent with their very low heat conductivity. 

---

When hydrogen forms molecular compounds with nonmetallic elements, with the exception of boron, it always forms a single covalent bond. However, catenation can cause the molecules to be quite complex. Catenation involves the formation of chains of elements like in hydrogen peroxide H-O-O-H. This occurs in many organic molecules because cairbon has the greatest tendency to catenate. 

Nonmetallic elements include hydrogen and the upper right-hand portion of the p block (see Topic B2. Fig. 7). Covalent bonding is characteristic of the elements, and of the compounds they form with other nonmetals. The bonding possibilities depend on the electron configurations of the atoms (see Topics A4 and Cl). Hydrogen (Topic F2) is unique and normally can form only one covalent bond. Boron (Topic F3) is also unusual as compounds such as BF3 have an incomplete octet. Electron... 

Boron is a hard metalloid with pronounced nonmetallic properties. Aluminum is a light, strong, amphoteric, reactive metallic element with a surface that becomes passivated when exposed to air. 

Boron, a metalloid with largely nonmetallic properties, has acidic oxides. Aluminum, its metallic neighbor, has amphoteric oxides (like its diagonal neighbor in Group 2, beryllium). The oxides of both elements are important in their own right, as sources of the elements, and as the starting point for the manufacture of other compounds. 

Finally, the structural modifications of elemental boron exhibit complex extended lattices of cages in the solid state, whereas those of metals possess much simpler close-packed atomic lattices. These differences are a direct reflection of atomic properties and result in the respective nonmetallic and metallic behavior. However, boron combines with most other elements including metals. There are a wide range of metal borides known with stoichiometric as well as nonstoi-chiometric atomic ratios. The amazingly varied interpenetration of the two characteristic structural motifs and the subtly balanced competition between the two modes of solid state bonding found in the metal borides constitutes further justification of our theme. This is discussed in some detail in Section II,C. 

Boron is the only group 3A element that can be considered nonmetallic and thus is our final element in this chapter. The element has an extended network structure with a melting point (2300 °C) that is intermediate between the melting points of carbon (3550 °C) and sihcon (1410°C). The electron configuration of boron is [He]2s 2pK... 

---