General:
| Name: Boron | ||||
| Type: Metalloid | ||||
Density @ 293 K: 2.34 g/cm3
| ||||
Discovery of Boron Boron compounds such as borax (sodium tetraborate, Na2B4O7·10H2O) have been known and used by ancient cultures for thousands of years. Borax's name comes from the Arabic buraq, meaning "white." Boron was first partially isolated in 1808 by French chemists Joseph L. Gay-Lussac and L. J. Thénard and independently by Sir Humphry\ Davy in London.Gay-Lussac & Thénard reacted boric acid with magnesium or sodium to yield boron,a gray solid. (1) They believed it shared characteristics with sulfur and phosphorus and named it bore. (2) Davy first tried to produce boron by electrolysis of boric acid, but was not satisfied with the results.He enjoyed greater success reacting boric acid with potassium in a hydrogen atmosphere. The result was a powdery substance. Davy commented the substance was, "of the darkest shades of olive. It is opake, very friable, and its powder does not scratch glass." After carrying out a number of chemical reactions to verify the uniqueness of the substance, Davy wrote, "there is strong reason to consider the boracic basis as metallic in nature, and I venture to propose for it the name of boracium." (2) Neither party had, in fact, produced pure boron. Their samples were only about 60% pure.In 1909 William Weintraub was able to produce 99% pure boron, by reducing boron halides with hydrogen. Almost a century later, in 2004, Jiuhua Chen and Vladimir L. Solozhenko produced a new form of boron, but were uncertain of its structure. In 2009,a team led by Artem Oganov was able to demonstrate the new form of boron contains two structures, B12 icosohedra and B2 pairs. (3) Gamma-boron, as it has been called, is almost as hard as diamond and more heat-resistant than diamond. Talking about boron's part metal, part non-metal properties, Oganov said, "Boron is a truly schizophrenic element. It's an element of complete frustration. It doesn't know what it wants to do. The outcome is something horribly complicated." (4) | ||||
States
| State (s, l, g): solid | |
| Melting point: 2348 K (2075 oC) Boiling point: 4000 K (3727 oC) |
Energies
| Heat of atomization: 563 kJ mol-1 |
| Heat of vaporization: 480 kJ mol-1 |
| 2nd ionization energy: 2427.1 kJ mol-1 |
| Electron affinity: 26.7 kJ mol-1 |
| Specific heat capacity: 1.02 J g-1 K-1 | ||
| Heat of fusion: 50.2 kJ mol-1 | ||
| 1st ionization energy: 800.6 kJ mol-1 | ||
| 3rd ionization energy: 3659.7 kJ mol-1 |
Oxidation & Electrons
| Shells: 2,3 | |
| Minimum oxidation number: 0 | |
| Min. common oxidation no.: 0 | |
| Electronegativity (Pauling Scale): 2.04 |
Color: black
| ||||
| Harmful effects: Elemental boron is not known to be toxic. Characteristics: Boron is a metalloid, intermediate between metals and non-metals. It exists in many polymorphs (different crystal lattice structures), some more metallic than others. Metallic boron is extremely hard and has a very high melting point. Boron does not generally make ionic bonds, it forms stable covalent bonds. Boron can transmit portions of infrared light. Boron is a poor room temperature conductor of electricity but its conductivity improves markedly at higher temperatures. Uses: Boron is used to dope silicon and germanium semiconductors, modifying their electrical properties. Boron oxide (B2O3) is used in glassmaking and ceramics. Borax (Na2B4O7.10H2O) is used in making fiberglass, as a cleansing fluid, a water softener, insecticide, herbicide and disinfectant. Boric acid (H3BO3) is used as a mild antiseptic and as a flame retardant. Boron Nitride's hardness is second only to diamond, but it has better thermal and chemical stability, hence boron nitride ceramics are used in high-temperature equipment. Boron nitride nanotubes can have a similar structure to carbon nanotubes. BN nanotubes are more thermally and chemically stable than carbon nanotubes and, unlike carbon nanotubes, boron nitride nanotubes are electrical insulators. Boron carbide (B4C) is used in tank armor and bullet proof vests. |
Appearance & Characteristics
| Structure: rhombohedral; B12 is icosahedral. | |
| Hardness: 9.3 mohs | |
Reactions
| Reaction with air: mild, w/ht ⇒ B2O3 | |||
Reaction with 15 M HNO3: none
|
Compounds
| Oxide(s): B2O3 | Chloride(s): BCl3 and many BxCly |
| Hydride(s): B2H6 and many BxHy |
Radius
| Atomic radius: 85 pm | Ionic radius (1+ ion): pm |
| Ionic radius (2+ ion): pm | Ionic radius (3+ ion): 41 pm |
| Ionic radius (2- ion): pm | Ionic radius (1- ion): pm |
Conductivity
| Thermal conductivity: 27.4 W m-1 K-1 | Electrical conductivity: 5.0 x10-4 S cm-1 |
Abundance & Isotopes
| Abundance earth's crust: 10 parts per milllion by weight, 1 part per million by moles | |
| Abundance solar system: 2 parts per billion by weight, 0.2 parts per billion by moles | |
| Cost, pure: $1114 per 100g | |
| Cost, bulk: $500 per 100g | |
| Source: Boron compunds are usually is found in sediments and sedimentary rock formations. The chief sources of boron are Na2B4O6(OH)2.3H2O - known as rasorite or kernite; borax ore (known as tincal); and with calcium in colemanite (CaB3O4(OH)4.H2O). Boron also occurs as orthoboric acid in some volcanic spring waters. | |
| Isotopes: 11 whose half-lives are known, with mass numbers 7 to 17. Of these, two are stable: 10B and 11B. 10B is used in nuclear reactors as a neutron-capturing substance. | |
