Abstract

Boron, B, is unique in that it is the only nonmetal in Group 13 (IIIA) of the Periodic Table. Boron, at wt 10.81, at no. 5, has more similarity to carbon and silicon than to the other elements in Group 13. There are two stable boron isotopes, ¹⁰B and ¹¹B, which are naturally present at 19.10–20.31% and 79.69–80.90%, respectively. There is a very low cosmic abundance of boron. Boron is the 51st most common element present in the earth's crust at a concentration of three grams per metric ton. Elemental boron has a diverse and complex chemistry, influenced by its having a high ionization energy; by its small size; and by virtue of the electronegativities of boron (2.0), carbon (2.5), and hydrogen (2.1) all being very similar, resulting in extensive and unusual covalent chemistry. Boron has an expected valence of three. There is no formation of univalent compounds. Boron has a high affinity for oxygen-forming borates, polyborates, borosilicates, peroxoborates, etc. Boron reacts with water at temperatures >100°C to form boric acid and other boron compounds. In addition to being an amorphous powder, boron has four crystalline forms: -rhombohedral, -rhombohedral, -tetragonal, and -tetragonal. Boron is an extremely hard refractory solid having a hardness of 9.3 on Mohs' scale and a very low (1.5 × 10^–6 –1 cm^–1 room temperature electrical conductivity so that boron is classified as a metalloid or semiconductor. These values are for the -rhombohedral form. The high ionization energies and small size prevent boron from adopting metallic bonding to compensate for its electron deficiency and that of other hypoelectronic elements. A high purity (>99%) boron comes from the direct thermal decomposition of boron hydrides such as diborane, B₂H₆. The Moissan process, the reduction of boric oxide with magnesium, is the most widely used commercial process for producing boron. Elemental boron is used in very diverse industries ranging from metallurgy to electronics. Other areas of application include ceramics, propulsion, pyrotechnics, and nuclear chemistry. Boron is nontoxic. It is a nutrient essential to plant growth. Workplace hygienic practices include avoiding the breathing of boron dust.