Boron is a chemical element with an atomic number of five and the atomic symbol B. Notable properties of boron include its effectiveness at neutron capture and the consequent effectiveness of one of its isotopes as a radiation shield; its extreme hardness, tensile strength, and hardness of several boron compounds; and the existence of several boron allotropes and polymorphs. It is a metalloid with a standard atomic weight of 10.811. It is solid at room temperature, with a melting point of 3769°F (2349°K) and a boiling point of 7101°F (4200°K) at atmospheric pressure.
The physical properties of boron depend on its allotrope. Allotropes are different configurations of the same element, with the atoms of the element bonded together in different ways. The main allotropes of boron are called crystalline boron and amorphous boron. Amorphous boron, which is made of icosahedral boron crystals bonded together at random with no larger overall structure, takes the form of a brown powder.
Crystalline boron is black and extremely hard. It is diamagentic; in the presence of a magnetic field, it produces a magnetic field of its own that results in a repellent effect. Crystalline boron can be arranged in four different major crystalline structures, called polymorphs. Starting at pressures of around 23,206,000 pounds-force per square inch (about 160 gigapascals), boron's properties shift and it becomes a superconductor.
Boron forms primarily covalent chemical bonds and can form stable molecular networks. It is a member of the aluminum family, but the properties of boron are actually closer to silicon than aluminum. Boron is most commonly found in nature in the compound sodium tetraborate decahydrate, also known as borax. Boron carbide and cubic boron nitride are among the hardest materials known. Boron is essential to the biochemistry of plant life, and ultratrace amounts are used in animals as well.
Boron has 13 known isotopes, of which two, 10B and 11B, are stable. About 80 percent of all naturally occurring boron is11B, with 10B composing the remainder. 10B is highly effective at capturing thermal neutrons and so is effective as a radiation shield. The other nine known isotopes are short-lived, with half-lives of milliseconds or even less.
The properties of boron give the element and its compounds a number of uses. Boron's strength makes it valuable in the aerospace industry. Boron carbide and cubic boron nitride are useful as industrial abrasives due to their extreme hardness, and boron carbide is also incorporated into modern bulletproof vests and armored vehicles. Semiconductors made from substances such as silicon, silicon carbide, and germanium are doped with boron. Boron's 10B isotope is used in the control rods and emergency shutdown systems of nuclear reactors shielding and is being experimented with for use in a form of radiotherapy called boron neutron capture therapy to treat cancers of the head, neck, and brain.