Boron

Boron is a tough, dark metalloid that sits right on the border between metals and non-metals. You will find it in borax washing powder, Pyrex glass, the control rods of nuclear reactors, and in some of the hardest artificial materials ever made.

  • Atomic Number55 protons, 5 electrons
  • Atomic Mass10.81 uAbout 11× heavier than hydrogen
  • State at Room TempSolidhard dark solid
  • Density2.37 g/cm³Denser than water, lighter than most metals
  • Melting / Boiling2074.8°C / 3999.8°CExceptionally high melting point for a non-metal
  • Discovered1808Davy, Gay-Lussac & Thénard, 1808

Where does boron sit in the mass scale?

Boron sits between beryllium and carbon, near the lightest end of the periodic table.

Atomic Mass Comparison
Beryllium9.0 u
Boron10.8 u
Carbon12 u
Nitrogen14 u
Iron56 u

Boron weighs 10.8 atomic mass units, heavier than beryllium but lighter than carbon. Despite its low mass, crystalline boron is almost as hard as diamond, one of the most surprising combinations of lightness and hardness on the table.

What is boron?

Boron is a metalloid, which means it has properties of both metals and non-metals. Pure boron can form as a black, shiny solid that looks metallic but conducts electricity only weakly. It is extremely hard, crystalline boron is almost as hard as diamond, and has an unusually high melting point (2076°C) for such a light element. It has three electrons in its outer shell, which it usually shares rather than gives away.

Boron gets its name from borax, the white mineral compound from which it was first extracted. Borax itself takes its name from the medieval Arabic word būraq and the Persian word būrah, both referring to the same white mineral that had been mined and traded in Asia for centuries. The symbol B comes directly from the element's name.

Fact Turkey holds more than 70% of the world's known boron reserves. The country is so dominant in boron production that it strongly influences global prices for everything from glass fibre to detergents to ceramics.

Where you find boron

On Earth

Boron is not common in the Earth's crust, it makes up only approx. 10 parts per million, but it forms useful mineral deposits in specific locations.

  • Borax and borate deposits. Turkey holds most of the world's boron reserves. Borax (sodium tetraborate) is mined from ancient dried lake beds in Anatolia, some deposits are hundreds of metres thick.
  • Kernite and colemanite. These borate minerals are found in California and the Mojave Desert in the USA, as well as in Argentina and Russia.
  • Geothermal fluids. In volcanically active regions, boron is carried to the surface in hot water and steam rising from deep in the Earth.

How we use boron

  • Pyrex and borosilicate glass. Adding boron to glass makes it expand and contract very little when heated, so it does not crack. Pyrex dishes, laboratory glassware and telescope mirror blanks are made of borosilicate glass.
  • Cleaning products. Borax is used in laundry detergents, hand soaps and household cleaners. It softens water and helps lift grease and stains.
  • Nuclear reactor control rods. Boron absorbs neutrons extremely well. Control rods made of boron or boron compounds are pushed into nuclear reactors to slow down or stop the chain reaction safely.
  • Computer chip doping. Tiny amounts of boron are added to silicon to change how it conducts electricity, a process called doping that is essential for making computer chips and solar cells.
Did you know? Boron nitride, a compound of boron and nitrogen, can be made into a form called cubic boron nitride that is almost as hard as diamond. It is used to cut and grind metals too tough for ordinary tools, and it keeps its hardness at much higher temperatures than diamond does.

How it was discovered

Boron was discovered in 1808 by three chemists working independently. In Paris, Joseph-Louis Gay-Lussac and Louis-Jacques Thénard reacted boric acid with potassium metal and obtained a brownish powder they believed was a new element. Days later in London, Humphry Davy performed a similar experiment using electricity to decompose boric acid. None of them obtained a completely pure sample, that achievement waited until 1892, when Henri Moissan reduced boron oxide with magnesium to get a purer product.

Deeper dive: boron chemistry: unusual bonding and boron nitride

Boron sits in Group 13 and has three electrons in its outer shell. Unlike most elements, boron rarely forms simple ions. Instead, it tends to share electrons in covalent bonds, forming flat triangular molecules and complex cage-like structures called boranes (boron hydrides). Some boranes were once investigated as rocket fuels because they burn with enormous energy.

The boron-nitrogen bond is almost identical in length and strength to the carbon-carbon bond, which is why boron nitride can mimic many of carbon's structures. Hexagonal boron nitride (h-BN) is slippery like graphite and used as a high-temperature lubricant. Cubic boron nitride is almost as hard as diamond but withstands higher temperatures without oxidising, useful for cutting hardened steel.

In borosilicate glass, boron atoms slot into the silicon-oxygen network of regular glass. Each boron forms bonds with three or four oxygen atoms, creating a network more resistant to thermal shock. This is why a Pyrex dish can go straight from the freezer to a hot oven without shattering, a trick ordinary glass cannot manage.

Boron is a remarkable boundary element that bridges the world of metals and non-metals. Moving one step further along the periodic table brings us to carbon, the basis of all life on Earth.