Boron – facts and properties
- Symbol
- B
- Atomic number
- 5
- Atomic mass
- 10.81 u
- Period
- 2
- Group
- 13
- Category
- metalloid
- Electron configuration
- [He] 2s² 2p¹
- Electronegativity
- 2.04
- Atomic radius
- 87 pm
- Valence electrons
- 3
- State at standard conditions
- solid
- Melting point
- 2075 °C
- Boiling point
- 4000 °C
- Density
- 2.37 g/cm³
- Discovered
- 1808
Boron is a metalloid with atomic number 5, the only element in its group that is not a metal. The pure element is a hard, black solid that few people ever see, but its compounds are familiar: borax, boric acid and the borosilicate glass of laboratory beakers and oven dishes. Chemically, boron is an oddity. In simple molecules it has one electron pair too few to complete a stable octet, and this electron deficiency shapes everything it does.
Position in the periodic table
Occurrence and extraction
Boron is scarce in the solar system because stars do not make it by fusion; it forms only when cosmic rays shatter heavier nuclei. On Earth it is nevertheless concentrated in a few places, because borate minerals dissolve easily in water and accumulate when lakes evaporate in arid regions. Elemental boron does not occur in nature. The important ores are borax, kernite, colemanite and ulexite, all of them borates. Turkey holds the largest reserves, and the Mojave Desert in California hosts one of the biggest borate mines in the world. Seawater and most soils contain small amounts of boron, which plants need as a trace nutrient.
Industry produces almost exclusively boron compounds rather than the element. Borax is recovered from the ores by dissolving them in hot water and recrystallising; boric acid is made by treating borates with sulfuric acid. Impure elemental boron is obtained as a brown powder by reducing boron oxide with magnesium. Very pure boron is made by decomposing boron halides with hydrogen on a hot filament. The element was isolated in 1808 almost simultaneously by Humphry Davy in London and by Joseph Louis Gay-Lussac and Louis Jacques Thénard in Paris. Its name comes from borax, which had been imported from Tibet since the Middle Ages as a flux for soldering gold.
Properties and reactions
Boron has the electron configuration [He] 2s² 2p¹, giving it three valence electrons, and it sits in period 2 and group 13. Its electronegativity of 2.04 is close to that of hydrogen and carbon, so boron does not form simple ions but shares electrons in covalent bonds. In molecules such as boron trifluoride the boron atom is surrounded by only six electrons. These electron-deficient compounds act as Lewis acids, readily accepting an electron pair from a donor such as ammonia. The boranes, boron hydrides such as diborane, solve the shortage differently, with three-centre bonds in which one electron pair holds three atoms together.
Crystalline boron is black, extremely hard and melts only at 2075 °C, boiling at 4000 °C; its density is 2.37 g/cm³. The atoms are arranged in icosahedra of twelve boron atoms linked into a rigid framework. Boron is a semiconductor: a poor conductor at room temperature, a better one when heated. Chemically the crystalline form is very resistant and is not attacked by boiling hydrochloric or hydrofluoric acid. At high temperature it burns in oxygen to boron oxide, combines with nitrogen to form boron nitride and reacts with most metals to form borides. Boric acid is a very weak acid that works not by releasing a proton but by binding a hydroxide ion from water. Boron has two stable isotopes, boron-10 and boron-11; boron-10 is an exceptionally good absorber of neutrons.
Uses in everyday life and technology
The glass industry takes the largest share of boron. Borosilicate glass, which contains boron oxide, expands very little when heated and therefore survives being taken from the oven straight into cold water. Laboratory glassware, coffee pots, oven dishes and telescope mirrors are made from it. Glass fibres for insulation and for reinforcing plastics also contain boron. Borax and perborates were long-standing ingredients of laundry detergents and are still found in some cleaning products. Boric acid is a mild antiseptic and an insecticide used in baits against ants and cockroaches.
Engineers exploit the hardness of boron compounds. Boron carbide is one of the hardest materials known, surpassed only by diamond and cubic boron nitride, and goes into body armour, vehicle armour and abrasive powders. Cubic boron nitride grinds hardened steel, whereas hexagonal boron nitride, which has a layered structure like graphite, is a soft, white lubricant. Neodymium-iron-boron magnets are the strongest permanent magnets available and sit inside hard drives, electric motors and wind turbines. In nuclear reactors, control rods containing boron-10 absorb neutrons to regulate the chain reaction. Silicon doped with a trace of boron becomes a p-type semiconductor, the basis of transistors and solar cells. Farmers add boron to fertiliser because plants cannot build stable cell walls without it.
Molecules containing this element
- Potassium borohydride KBH4
- Boron nitride BN
- Boric acid H3BO3
Quick question
Which element has the smallest atomic radius in period 2?
FAQ
Is boron toxic?
Elemental boron and most borates have low toxicity for humans, and small amounts of boron are part of a normal diet. Boric acid and borax are nevertheless classified in the European Union as toxic to reproduction and are restricted in cosmetics and toys. For insects boric acid is lethal, which is why it is used in baits. Boranes such as diborane are poisonous gases that ignite spontaneously in air.
Why does borosilicate glass survive sudden heat?
Ordinary glass expands when heated. If one part heats faster than the rest, the uneven expansion sets up stresses and the glass cracks. Boron oxide built into the glass network greatly reduces the expansion, so even when one side is hot and the other cold the stresses stay small enough for the glass to hold together.
Why is boron called a metalloid?
Boron sits on the boundary between metals and non-metals. It is hard and lustrous like a metal, but it conducts electricity poorly and bonds covalently like a non-metal. Silicon, germanium, arsenic, antimony and tellurium share this mixed character and together with boron form the staircase of metalloids in the periodic table.