Boron

Boron is the chemical element with atomic number 5 and the symbol B. It is a hard, high-melting semimetal in group 13 of the periodic table, and in its pure form it is a dark powder or a dark crystalline solid. Boron is rarely seen on its own: people meet it mostly as borax, boric acid and borosilicate glass, and plants cannot grow without it.

History of boron

Compounds of boron were used long before the element was known. For centuries the main source of borax was crystallized deposits at Lake Yamdok Cho in Tibet, and goldsmiths used it as a flux, a substance that helps metals melt and join. The name of the element comes from the Arabic word buraq, a name for borax.

In 1808 the French chemists Louis-Joseph Gay-Lussac and Louis-Jacques Thénard, working in Paris, and the English chemist Humphry Davy in London, independently extracted boron by heating a boron compound with potassium metal. In fact, none of them had made the pure element, which is very hard to obtain. A purer sample was produced by Henri Moissan in 1892, and later work by E. Weintraub in the United States, who sparked a mixture of boron trichloride vapor and hydrogen, gave truly pure boron with properties very different from those first reported.

Properties of boron

Boron has a standard atomic weight of about 10.81, expressed by IUPAC as an interval from 10.806 to 10.821 because the isotope mix varies slightly between natural sources. Its electron configuration is [He] 2s2 2p1, so it has three outer electrons. It is a solid with a density of about 2.34 grams per cubic centimeter. It melts at about 2,077 degrees Celsius and boils at about 4,000 degrees Celsius.

PubChem classifies boron as a semimetal, an element whose properties fall between those of metals and nonmetals. Its oxide is mildly acidic, and in compounds it most often has an oxidation state of plus three. Because it forms strong bonds with oxygen, boron is nearly always found combined with oxygen as borates.

Measured atomic data from PubChem describe a small atom, with a covalent radius of about 84 picometers and a van der Waals radius of 192 picometers. Its first ionization energy is 8.298 electronvolts, which is relatively high for a group 13 element. Together these values help explain why boron tends to share electrons in covalent bonds, rather than lose them as a typical metal does.

Occurrence and production of boron

Boron does not occur free in nature. It appears as orthoboric acid, also called boric acid, in some volcanic spring waters, and as borates in minerals such as borax and colemanite. Extensive borax deposits are found in Turkey, and the most important single source of boron is the mineral rasorite, also called kernite, found in the Mojave Desert in California. The crust holds roughly 10 milligrams of boron per kilogram.

Industrial producers make boric acid and borates from these ores. High-purity boron is prepared by reducing boron trichloride or boron tribromide with hydrogen on electrically heated filaments. Impure, or amorphous, boron can be made by heating boron trioxide with magnesium powder, and this crude form is what most chemical uses require.

Seawater also carries boron, at roughly 4.4 milligrams per liter according to PubChem, much more than the trace levels of many other elements. It is present there as dissolved borate rather than as a free element, and the oceans are one reason boron cycles through the environment as water moves between rock, soil, plants and sea.

Uses of boron

Amorphous boron serves as an igniter for rocket fuel and in pyrotechnic flares, where it gives the flame a distinctive green color. The most commercially important compounds are sodium borate, boric acid and boron oxide. Sodium borate pentahydrate is used in large quantities in the making of fiberglass insulation and in sodium perborate bleach, while boric acid is used for textile fiberglass and as a flame retardant in cellulose insulation.

Boric acid and borax also appear in eye drops, mild antiseptics, washing powders and tile glazes. Boron oxide is a main ingredient of borosilicate glass, sold under names such as Pyrex, which resists heat and sudden temperature changes. Fiberglass textiles and insulation are made from related glass, and sodium octaborate acts as a flame retardant. Boron filaments are valued in the aerospace industry because they are strong and light.

Nuclear technology depends on one isotope. Boron-10 absorbs neutrons very effectively, so it is used in the control rods that regulate nuclear reactors, in radiation shielding and in instruments that detect neutrons.

Isotopes of boron

Boron has two stable isotopes, boron-10 and boron-11. IUPAC data give boron-11 an abundance of about 80.1 percent and boron-10 about 19.9 percent, with a small uncertainty reflecting natural variation. Boron-10 is the useful neutron absorber. When it captures a neutron it breaks apart into high-energy charged particles, lithium-7 and helium-4, that deposit their energy over a very short distance.

That property has been explored in a cancer approach called boron neutron capture therapy, in which a boron-containing compound is taken up by tumor cells and the tumor is then exposed to a beam of neutrons. The charged particles are meant to destroy the cancer cells while sparing neighboring tissue. It remains a specialized research and clinical topic and is mentioned here only to explain the physics.

Health and environmental effects of boron

Boron is essential for the cell walls of plants, so it is a necessary plant nutrient. The Royal Society of Chemistry notes that boron is not considered poisonous to animals, but that higher doses can upset the body’s metabolism. People take in about 2 milligrams of boron each day from food, adding up to around 60 grams in a lifetime. Soils can hold too little or too much boron for crops, and both conditions can affect plant growth. This entry is background only and not health advice.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue