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.
[Ar] 4s² 3d¹⁰ 4p⁴
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, 8, 18, 6
Number of electrons in each shell, from the nucleus outwards.
220.5 °C (493.65 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.
685 °C (958 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).
4.809 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
2.55 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.
9.752 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.
2.021 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.
120 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.
+6, +4, +2, -2
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Selenium (Se) is a chemical element with atomic number 34. It sits in group 16 and period 4 of the periodic table, in the p-block. It is a nonmetal. At room temperature it is a solid. Its standard atomic weight is 78.971.
Origin of the name
In 1817 Jöns Jacob Berzelius examined a sludge from a sulfuric acid works in Sweden. At first he took it for tellurium, the element named after the Earth, then realised it was a new element and named it after the Moon, the Greek Selēnē, as tellurium’s companion.
Where is selenium found?
Selenium hides in sulfide ores, where it takes the place of some of the sulfur; minerals of its own are rare. It is obtained as a by-product, mostly when copper is refined.
What is selenium used for?
Selenium is used in glassmaking: it removes the greenish tint from glass and colours glass ruby red. It conducts electricity much better in light than in the dark, so it went into photocells and light meters, and for decades it coated the drums of photocopiers.
Interesting facts
Selenium is a curious double-edged element: people need a tiny amount of it every day, but larger doses are poisonous.
Brazil nuts are the richest source of selenium in food.