Boron

Boron is a metalloid with symbol B and atomic number 5. It is a black solid, resistant to most chemicals at room temperature.

Properties

Chemical

Boron's chemical properties are closer to silicon than to aluminium, the element next in group. Crystalline boron is chemically inert and resistant to attack by hot hydrohalic acids, such as hydrochloric acid and hydrofluoric acid. Boron does not react with air at standard conditions, but it burns at high temperatures to form boron trioxide. Its halides are Lewis acids due to an unfilled valence shell.

Boron-containing compounds can be tested for in solution using curcumin. The normally yellow curcumin reacts with boron, turning deep red. This test will not work in basic conditions, however, as curcumin is normally red in strongly basic solutions.

Physical

Chemically, boron is a metalloid. It is a brown-black solid at standard conditions. It has a very high melting point, 2076 °C and boils at 3927 °C. It's density varies between 2.35 and 2.52 g/cm³.

Boron exists as two main allotrope forms:

• Amorphous boron: brown powder, similar in aspect with cocoa powder. This one in turn consists of two forms: powder (brown-black) and glassy (opaque black).
• Crystalline boron: black, extremely hard (about 9.5 on the Mohs scale). It is a poor electrical conductor at room temperature. Crystalline boron also consists of several varieties: α-rhombohedral, α-tetragonal, β-rhombohedral, β-tetragonal. γ-orthorhombic, cubic, high-pressure superconducting and borospherene. Among these, β-rhombohedral is the most thermodynamically stable allotrope.

Availability

Both amorphous and crystalline boron can be bought online. Metallium sells both types, sealed in glass ampoules.

Preparation

Elemental boron can be extracted from boric acid, by dehydrating it to boron trioxide and reducing the oxide with magnesium. The resulting mass, more of a slag actually is treated with hydrochloric acid to remove the impurities. This part should be done slowly, as the residual magnesium diboride reacts with HCl to release borane and diborane that are pyrophoric. Once the reaction has stopped, filter the black boron powder and dry it.

Projects

• Make boranes and organoboron compounds
• Boron halides

Handling

Safety

Elemental boron, boron oxide, boric acid, borates, and certain organoboron compounds are non-toxic to humans and animals, but are toxic for insects. Boron halides are corrosive.

Storage

No special storage is required.

Disposal

Boron poses little toxicity to the environment and can be dumped in the trash.