34 Se 78.971

Selenium

Reactive nonmetal Selenium solid

[Ar] 3d¹⁰ 4s² 4p⁴ · 2 · 8 · 18 · 6

← As Arsenic Br Bromine →

A nonmetal whose conductivity depends on how much light falls on it — the property behind the first photocells and the whole xerographic process. Selenium is also an essential trace element with an unusually narrow safety window.

3D model

Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.

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Atomic properties

Atomic number: 34
Atomic mass: 78.971 а.е.м.
Electron configuration: [Ar] 3d¹⁰ 4s² 4p⁴
Electrons per shell: 2 · 8 · 18 · 6
Block: p-block
Group: Chalcogens
Period: 4
Electronegativity (Pauling): 2.55
Electronegativity (Allen): 2.42
Atomic radius: 103 pm
Covalent radius: 120 pm
Van der Waals radius: 190 pm
Ionisation energy 1: 941 kJ/mol
Ionisation energy 2: 2 045 kJ/mol
Ionisation energy 3: 2 973.7 kJ/mol
Ionisation energy 4: 4 144 kJ/mol
Ionisation energy 5: 6 590 kJ/mol
Electron affinity: 195 kJ/mol
Common oxidation states: -2, +4, +6
Oxidation states: -2, -1, +1, +2, +3, +4, +5, +6

Physical properties

State at 25 °C: solid
Density: 4.81 g/cm³
Melting point: 494 K · 220.9 °C
Boiling point: 958 K · 684.9 °C
Speed of sound: 3 350 m/s

Thermal properties

Heat of fusion: 6.69 kJ/mol
Heat of vaporisation: 95.48 kJ/mol
Specific heat capacity: 0.321 J/(g·K)
Thermal conductivity: 0.519 W/(m·K)

Mechanical properties

Young's modulus: 10 GPa
Shear modulus: 3.7 GPa
Bulk modulus: 8.3 GPa
Poisson ratio: 0.33
Mohs hardness: 2
Brinell hardness: 736 MPa

Electrical and magnetic properties

Electrical resistivity: 1·10⁶ nΩ·m
Magnetic ordering: diamagnetic
Curie point: no data
Néel point: no data
Superconducting point: no data

Crystal structure

Crystal structure: hexagonal close-packed (hcp)
Lattice constants: a = 436.6 pm · c = 495.4 pm

Abundance

In the crust: 0.05 mg/kg
In the ocean: 0.0002 mg/L
In the universe: 0.03 mg/kg
In the human body: 0.05 mg/kg

Isotopes

Isotope	Abundance	Half-life	Decay mode
⁷⁴Se	0.89 %	stable	—
⁷⁶Se	9.37 %	stable	—
⁷⁷Se	7.63 %	stable	—
⁷⁸Se	23.77 %	stable	—
⁸⁰Se	49.61 %	stable	—
⁸²Se	8.73 %	stable	—

Selenium-79, half-life 327 000 years, is a long-lived fission product that matters in assessing radioactive waste repositories.

Discovery

Year of discovery: 1817
Discovered by: Jöns Jacob Berzelius
Where: Sweden
Origin of the name: from Greek selene (Moon), to pair with tellurium (earth)

History

Berzelius examined a red deposit in the lead chambers of a sulfuric acid works in 1817, expecting tellurium, and found a new element. He named it after the Moon, to pair with tellurium's Earth.

Where it occurs

0.05 mg/kg of the crust — one of the rarest nonmetals. It has virtually no ores of its own and follows sulfur into sulfides.

How it is produced

From the anode slimes of electrolytic copper refining, where selenium collects along with tellurium and the precious metals.

Role in living things

Selenium is in selenocysteine, the twenty-first amino acid, encoded by a translation mechanism of its own. Selenoproteins shield cells from oxidative stress and handle thyroid hormone metabolism.

Safety

The gap between necessary and toxic is unusually narrow: the daily requirement is about 55 µg and 400 µg is already the upper safe limit. Chronic poisoning announces itself with garlic breath and hair loss.

Uses

Decolourising glass: selenium cancels the green cast from iron impurities
Red glass and ruby filters
Photoreceptor drums in laser printers and copiers
Dietary and feed supplements in selenium-poor regions
Alloying steel to improve machinability

Curiosities

Selenium and tellurium are named after the Moon and the Earth respectively.
Keshan disease is a cardiomyopathy caused by selenium-poor soils in parts of China.
The world's first photocell, built in 1883, was selenium.
A single Brazil nut can hold more than a day's selenium requirement.