Atomic number 34

Selenium

Se
+ phase
− phase

Shells Orbitals 3d 4s 4p

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34 electrons in 4 shells: 2, 8, 18, 6.

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

Category: Nonmetal
Period: 4
Group: 16
Block: p
Standard atomic weight: 78.971(8) Da
Electron configuration: [Ar] 3d¹⁰ 4s² 4p⁴
Phase at 25 °C: solid
Melting point: 221 °C
Boiling point: 685 °C
Density: 4.809 g/cm³
Electronegativity (Pauling): 2.55
Discovered: 1817, Jöns Jacob Berzelius

Neighbours in the table

As Br

Selenium first turned up as a contaminant. In Jefferson Lab’s account in PubChem’s record, the Swedish chemist Jöns Jacob Berzelius was analysing an impurity that fouled the sulfuric acid made at a Swedish factory in 1817. He took the material for tellurium at first, then realised it was an unknown element.

In selenium the four shells hold 2, 8, 18 and 6 electrons, 34 in all, which the shorthand writes as [Ar] 3d¹⁰ 4s² 4p⁴. OpenStax counts a completely filled d subshell as core, so the six electrons in 4s and 4p are the valence electrons, the ones used in bonding. A group 16 atom needs only two more to complete an octet. Selenium takes them as the selenide ion, Se²⁻, or through two single bonds, which OpenStax says sulfur, selenium and tellurium link into rings and chains.

Los Alamos recognises three main allotropes, forms of one element built differently. Amorphous selenium, with no regular crystal pattern, is red as a powder and black as a glass, and monoclinic crystals are deep red. The most stable variety, hexagonal selenium, is metallic grey. Below its melting point of 221 °C it is a semiconductor: a small gap separates its filled band of electron energies from an empty one.

Where it is found

PubChem’s record puts selenium at 0.05 milligrams per kilogram of crustal rock and 0.0002 milligrams per litre of seawater. Rich minerals exist, among them clausthalite, PbSe, and eucairite, CuAgSe. NIOSH notes that most sulfide ores carry it as an impurity, and PubChem’s hazard summary finds it in most rocks and soils, usually combined with sulfide or metal minerals. The Geological Survey adds that coal generally holds significant quantities. Los Alamos notes that some soils hold enough to cause serious effects in animals feeding on plants grown there, such as locoweed.

Jefferson Lab calls selenium’s own minerals too rare to be a major source, so the element is recovered mainly as a byproduct. It comes out of electrolytic copper refining, collecting in the residues left by the copper anodes. Los Alamos notes that flue dust from processing copper sulfide ores was the older source. The Geological Survey says supply therefore rises and falls with copper. China produced nearly 50 per cent of the world’s refined selenium in 2024, when refinery output outside the United States reached about 3,700 tonnes.

Why it matters

Light changes how selenium conducts. Jefferson Lab’s text says the brighter the light, the better the element carries a current, which made it useful in devices that respond to light intensity. Los Alamos separates two effects: photovoltaic action, turning light straight into electricity, and photoconductive action, where resistance falls as illumination grows. It lists xerography, the copying of documents, among the uses, and the Geological Survey notes that scrap from older photocopiers went abroad for recovery. Selenium rectifiers turned alternating current into direct current; silicon now replaces them at low and medium voltages.

Those electronic uses are now a minority. PubChem’s hazard summary says most processed selenium goes to electronics, but the Survey’s estimate for 2024 gives electronics and photovoltaics 10 per cent of world consumption. Metallurgy took 40 per cent, including electrolytic manganese production, where selenium raises yields. Agriculture and animal health took 20 per cent, as did glass manufacturing. Thin-film solar cells of copper indium gallium diselenide, CIGS, and antidandruff shampoos use it as well.

The body needs a little of it. OpenStax’s Anatomy and Physiology names brewer’s yeast, wheat germ, liver, fish and whole grains as dietary sources, and the Survey lists selenium as a supplement for people and livestock. A shortfall shows up as muscle pain, in the same OpenStax table.

Properties

Properties of selenium 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	34
Standard atomic weight	78.971(8) Da	Da	IUPAC/CIAAW 2024
Melting point	221 °C	°C	Los Alamos: 221 °C
Boiling point	685 °C	°C	Los Alamos: 685 °C
Density	4.809 g/cm³	g/cm³
Electronegativity (Pauling)	2.55
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴
Phase at 25 °C	solid

Safety

Elemental selenium is toxic, and all five classifications PubChem gathers use the signal word Danger. Company reports to the European Chemicals Agency call it toxic if swallowed or inhaled, and say it may damage organs through repeated exposure and harm aquatic life long-term. NIOSH lists the eyes, skin, respiratory system, liver, kidneys, blood and spleen as target organs for people exposed at work. PubChem’s Laboratory Chemical Safety Summary and the NIOSH Pocket Guide card are the sheets to consult.

Questions

Why is selenium named after the Moon?

The name comes from Selene, the Greek goddess of the Moon. The CIAAW text in PubChem's element record explains the choice by the company selenium keeps: it is found chemically with tellurium, whose name honours Tellus, the Roman goddess of the Earth. Berzelius had first mistaken his new substance for tellurium. Los Alamos gives the shorter derivation, straight from the Greek word for moon.

Is selenium good or bad for you?

Both, depending on the amount. OpenStax's Anatomy and Physiology lists it as a trace mineral with roles in antioxidant defence, thyroid function and immunity. An EPA summary in PubChem's safety data calls it toxic at high concentrations, and selenosis, selenium poisoning, has been documented in animals and people living where soil and groundwater carry heavy selenium contamination. The element itself is classified toxic if swallowed or inhaled.

How many isotopes does selenium have?

Natural selenium is a mixture of six, with mass numbers 74, 76, 77, 78, 80 and 82; selenium-80 makes up 49.80 per cent, by the CIAAW data in PubChem's record. Los Alamos and Jefferson Lab call all six stable. The IAEA mass-evaluation data in the same record give selenium-82 a half-life of 87.6 × 10¹⁸ years by double beta decay. Selenium-75, half-life 120 days, is used for radiographing welds.

Why is selenium added to glass?

It works in two directions. The US Geological Survey says glassmakers add it to cancel the green tint that iron impurities give container glass, and glass manufacturing took about 20 per cent of world consumption in 2024. Los Alamos lists the opposite job as well, making ruby-coloured glass and enamels, and the Survey counts red pigments among its applications. Photographic toner, stainless steel and rubber vulcanising chemicals draw on it too.