4.1. CHEMICAL IDENTITY

Boron appears in Group 13 (IIIA) of the periodic table and is the only nonmetal of this group (Jansen 2003). Table 4-1 lists common synonyms, trade names, and other pertinent information to identify boron and selected compounds.

4.2. PHYSICAL AND CHEMICAL PROPERTIES

Boron is a nonmetal and is typically found in nature bound to oxygen. It is never found as the free element (Cotton et al. 1999). Boron has two stable isotopes, ¹⁰B and ¹¹B, which are naturally present at 19.10–20.31 and 79.69–80.90%, respectively (Jansen 2003). There are ten other known non-radioactive isotopes of boron (Lide 2008). Elemental boron exists in several allotropic forms (Jansen 2003). In addition to being an amphorous powder, boron has four crystalline forms: α-rhombohedral, β-rhombohedral, α-tetragonal, and β-tetragonal (Jansen 2003).

The chemical properties of boron are more similar to carbon and silicon than elements of its own group, although boron is more electron deficient. Boron has a high affinity for oxygen-forming borates, and reacts with water at temperatures above 100 °C to form boric acid and other boron compounds. The electron deficiency of boron does not allow conventional two-electron bonds, but rather multicenter bonds that are exclusively covalent (Jansen 2003; Schubert and Brotherton 2006). Boron hydrides consequently have structures quite different from hydrocarbons (Jansen 2003). The multi-center nature of boron allows the formation of a diverse array of covalent networks, rings, cages, and clusters. Boron forms strong covalent bonds with electronegative elements such as fluorine and oxygen (Schubert and Brotherton 2006).

Table 4-2 lists important physical and chemical properties of boron and selected compounds.

Table 4-1

Chemical Identity of Boron and Selected Boron Compounds.

Table 4-2

Physical and Chemical Properties of Boron and Selected Boron Compounds.