Atomic number 5

Boron

B
+ phase
− phase

Shells Orbitals 2s 2p

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5 electrons in 2 shells: 2, 3.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Metalloid
Period: 2
Group: 13
Block: p
Standard atomic weight: 10.81(2) Da
Electron configuration: [He] 2s² 2p¹
Phase at 25 °C: solid
Melting point: 2076 °C
Boiling point: 3927 °C
Density: 2.37 g/cm³
Electronegativity (Pauling): 2.04
Discovered: 1808, Gay-Lussac and Thénard, and independently Humphry Davy

Neighbours in the table

Be C

Boron has four orbitals in its outer shell and only three electrons to fill them. Its five electrons sit two in the first shell and three in the second, a helium core then 2s² 2p¹. An s subshell holds at most two electrons, so the fifth goes into a 2p orbital. Ionisation energy is the energy needed to remove the outermost electron. Boron’s, 8.298 eV, is below beryllium’s despite the extra proton, because a p electron leaves more easily than an s electron of the same shell.

Most stable boron compounds put the element in the +3 oxidation state, the charge it would carry if every bond were ionic. Its binary compounds tend to be covalent. Boron trifluoride, BF₃, is best drawn with three single bonds, leaving boron six electrons instead of eight. The vacant p orbital lets flat, three-bonded boron compounds act as Lewis acids, accepting another molecule’s lone pair. Yet the B–F bonds measure slightly shorter than single bonds, a hint of some double-bond character.

In the solid, boron atoms gather into icosahedra: twelve-cornered shapes with twenty triangular faces, every bond inside one about 176 pm long. The forms of boron differ in how these clusters are arranged and joined. Los Alamos describes a brown powder and a silvery crystalline form; OpenStax calls pure crystalline boron transparent.

Where it is found

Boron never occurs free, and in nature it is always combined with oxygen. The PubChem record gives 10 mg per kilogram of crust and 4.44 mg per litre of seawater. In volcanic districts it occurs as boric acid, B(OH)₃, in some spring waters. In dry lake beds it lies as borates, salts of boron and oxygen, such as the borax of Searles Lake in California.

Four minerals, colemanite, kernite, tincal and ulexite, make up 90 per cent of the borates that industry uses. The Mojave Desert, the Alpide belt along the southern edge of Eurasia and the Andes hold the largest economically viable deposits, the US Geological Survey says. Turkey refined an estimated 3 million tonnes of borates in 2024 and holds reserves of 950 million tonnes, against 48 million in the United States.

Three Californian companies mined borates in 2024, from open pits and from brines. Two routes lead on to the element. OpenStax reduces boric oxide with magnesium to a brown amorphous powder, 95 to 98.5 per cent pure, then refines it through boron trichloride heated with hydrogen. Jefferson Lab describes borax heated with carbon.

Why it matters

The US Geological Survey gives elemental boron limited commercial uses; the value lies in its compounds, which are priced by their boric oxide content. Borates serve more than 300 applications, and ceramics, detergents, fertilisers and glass take over three-quarters of world consumption. Boric oxide is needed for heat-resistant borosilicate glass. Los Alamos ranks sodium borate pentahydrate, Na₂B₄O₇·5H₂O, first by dollar sales, for insulation fibreglass and perborate bleach, while OpenStax names borax the most important borate commercially.

Boron-10, about a fifth of natural boron, captures slow neutrons readily. Its thermal absorption cross section, a measure of how likely a nucleus is to catch a neutron, is large. That suits it to reactor control rods, radiation shields and neutron counters, and it is being studied as a replacement for helium-3 in detectors. In medicine, a boron-10 nucleus that takes in a neutron gives lithium-7 and an alpha particle. The alpha particle carries away a relatively large kinetic energy. IUPAC, chemistry’s international standards body, notes that these particles are useful in treating malignant tumours.

Boron filaments, strong and light, go into advanced aerospace structures. Los Alamos says boron nitride can be made into a material that rivals diamond in hardness, and adds that it insulates against electric current yet conducts heat like a metal.

Properties

Properties of boron Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	5
Standard atomic weight	10.81(2) Da	Da	IUPAC/CIAAW 2024
Melting point	2076 °C	°C	Los Alamos: 2076 °C
Boiling point	3927 °C	°C	Los Alamos: 3927 °C
Density	2.37 g/cm³	g/cm³
Electronegativity (Pauling)	2.04
Electron configuration	1s² 2s² 2p¹
Phase at 25 °C	solid

The standard atomic weight of this element is an interval in nature, from 10.806 to 10.821 Da; the value shown is IUPAC's conventional abridged value.

Safety

Los Alamos does not regard elemental boron as toxic, and 52.2% of the European Chemicals Agency reports gathered on PubChem find that it meets no hazard criterion. The classified reports carry the signal word Warning and the statement “harmful if swallowed”; a small set adds eye and respiratory irritation and harm if inhaled. Haz-Map describes the fine powder as a dust explosion hazard. The PubChem Laboratory Chemical Safety Summary has the details.

Questions

Is boron a metal or a nonmetal?

Neither: boron is a metalloid, with properties between the two. Los Alamos notes that it conducts electricity poorly at room temperature but well when hot. Its band gap, the energy an electron needs to become free to carry current, is 1.50 to 1.56 eV, larger than that of silicon or germanium. OpenStax traces the semiconducting behaviour of metalloids to electrons held more tightly by their nuclei than in a metal.

Who discovered boron?

Boron was first obtained as an element in 1808. Jefferson Lab, quoted in PubChem, credits the French chemists Joseph-Louis Gay-Lussac and Louis-Jacques Thénard and, working independently, Humphry Davy in England. Each route combined boric acid with potassium. CIAAW, the IUPAC body that sets atomic weights, derives the name from the Arabic buraq, meaning white.

Why is the atomic weight of boron given as a range?

Natural boron is a mix of two stable isotopes, which Los Alamos puts at 19.78 per cent boron-10 and 80.22 per cent boron-11, while CIAAW, the IUPAC commission on atomic weights, gives boron-10 as 18.9 to 20.4 per cent of natural samples. Natural processes sort the isotopes slightly; in water, the ratio depends on where the water came from and the region it flowed through. The 10.81 Da on this page is the conventional value.

What is borax used for?

Borax, Na₂B₄O₇·10H₂O, is a hydrated sodium borate. Jefferson Lab lists it in laundry products, as a mild antiseptic and in a material called Oobleck. Jefferson Lab ranks borax third among boron compounds, after sodium borate pentahydrate and boric acid. The US Geological Survey adds that sodium percarbonate can take the place of borates in detergents and needs lower temperatures to break down in water, which it calls an environmental consideration.

Appears in these articles

An acid gives a proton and a base takes it: Arrhenius, Brønsted–Lowry and Lewis Hydrogen chloride gives its proton to water effectively completely; acetic acid gives only a small fraction.