BORON 119
4. CHEMICAL AND PHYSICAL INFORMATION
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,
10
B and
11
B, 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.