Average mass of an atom in unified atomic mass units (1 u is 1/12 of the mass of a carbon-12 atom). Accounts for the natural mix of the element’s isotopes.
Number of the vertical column, 1 to 18 in IUPAC numbering. Elements in a group share the same outer-shell layout, so their properties are alike.
Number of the horizontal row. Usually equal to the number of electron shells in the atom; palladium is the exception, with four shells in period 5.
The subshell (s, p, d or f) that receives the last electrons in the usual filling order; it divides the table into four blocks. Real atoms sometimes differ: lanthanum starts the f-block but has no f electrons.
[He] 2s² 2p¹
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
2, 3
Number of electrons in each shell, from the nucleus outwards.
2,075 °C (2,348 K)
Temperature at which the solid turns liquid at standard pressure. Helium, carbon and arsenic do not melt at normal pressure: helium needs about 25 atm, carbon and arsenic turn straight into vapour.
4,000 °C (4,273 K)
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
2.37 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
2.04 Pauling
How strongly an atom pulls electrons towards itself in a chemical bond. On the Pauling scale it runs from 0.79 for caesium to 3.98 for fluorine.
8.298 eV
Energy needed to remove the most loosely bound electron from an atom. The lower it is, the more easily the element gives up an electron, which is typical of metals.
0.28 eV
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
84 pm
Half the length of a single covalent bond between two atoms of the element. It shows the size of an atom inside a molecule or crystal: 76 pm for carbon, 132 pm for iron.
+3
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Discovered
1808
Discovered by
Humphry Davy, Joseph Louis Gay-Lussac, and Louis Jacques Thénard
Boron (B) is a chemical element with atomic number 5. It sits in group 13 and period 2 of the periodic table, in the p-block. It is a metalloid. At room temperature it is a solid. Its standard atomic weight is 10.81.
Origin of the name
The element was named after borax, the mineral it was obtained from; the word goes back to Arabic būraq and Persian būrah. Humphry Davy shaped the English name boron after carbon, an element that boron resembles chemically.
Where is boron found?
Boron is rare: stars do not make it, and it forms in space when cosmic rays split heavier nuclei. On Earth it never occurs free, only as borates. Water has gathered these into rich deposits in dry lake beds; the main minerals are borax (tincal), kernite, colemanite and ulexite. Turkey and the United States are the largest producers.
What is boron used for?
Most boron goes into glass: glass fibre for insulation, and borosilicate glass for heat-resistant cookware and laboratory flasks. Sodium perborate is a laundry bleach, and borax a cleaning agent. Boron-10 absorbs neutrons and is used in the control rods of nuclear reactors. Boron also gives fireworks and flares a green flame, and traces of it in silicon make semiconductors.
Interesting facts
Plants need a little boron to build their cell walls, but even slightly too much in the soil damages their leaves.
Borates are about as poisonous to people as table salt.
Boric acid used to be collected from the steaming hot springs of Larderello in Tuscany.