8 O 15.999

Oxygen

Reactive nonmetal Oxygenium gas

[He] 2s² 2p⁴ · 2 · 6

← N Nitrogen F Fluorine →

The most abundant element in the Earth's crust and in the human body. Oxygen is so reactive that it barely existed free for the planet's first two billion years — cyanobacteria put it into the atmosphere.

3D model

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

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Atomic properties

Atomic number: 8
Atomic mass: 15.999 а.е.м.
Electron configuration: [He] 2s² 2p⁴
Electrons per shell: 2 · 6
Block: p-block
Group: Chalcogens
Period: 2
Electronegativity (Pauling): 3.44
Electronegativity (Allen): 3.61
Atomic radius: 48 pm
Covalent radius: 66 pm
Van der Waals radius: 152 pm
Ionisation energy 1: 1 313.9 kJ/mol
Ionisation energy 2: 3 388.3 kJ/mol
Ionisation energy 3: 5 300.5 kJ/mol
Ionisation energy 4: 7 469.2 kJ/mol
Ionisation energy 5: 10 990 kJ/mol
Electron affinity: 141 kJ/mol
Common oxidation states: -2
Oxidation states: -2, -1, +1, +2

Physical properties

State at 25 °C: gas
Density: 0.00143 g/cm³
Melting point: 54.36 K · -218.8 °C
Boiling point: 90.19 K · -183 °C
Speed of sound: 330 m/s

Thermal properties

Heat of fusion: 0.444 kJ/mol
Heat of vaporisation: 6.82 kJ/mol
Specific heat capacity: 0.918 J/(g·K)
Thermal conductivity: 0.02658 W/(m·K)

Mechanical properties

Young's modulus: no data
Shear modulus: no data
Bulk modulus: no data
Poisson ratio: no data
Mohs hardness: no data
Brinell hardness: no data

Electrical and magnetic properties

Electrical resistivity: no data
Magnetic ordering: paramagnetic
Curie point: no data
Néel point: no data
Superconducting point: no data

Crystal structure

Crystal structure: monoclinic
Lattice constants: a = 540.3 pm · b = 342.9 pm · c = 508.6 pm

Abundance

In the crust: 461 000 mg/kg
In the ocean: 857 000 mg/L
In the universe: 1·10⁷ mg/kg
In the human body: 610 000 mg/kg

Isotopes

Isotope	Abundance	Half-life	Decay mode
¹⁶O	99.757 %	stable	—
¹⁷O	0.038 %	stable	—
¹⁸O	0.205 %	stable	—

The oxygen-18 to oxygen-16 ratio in ice cores and shells is the principal palaeothermometer: it depends on the temperature at which ocean water evaporated.

Discovery

Year of discovery: 1771
Discovered by: Carl Wilhelm Scheele
Where: Sweden
Origin of the name: from Greek oxys (acid) and genes (forming): Lavoisier thought it was in every acid

History

Carl Scheele made oxygen around 1771 but published after Priestley, who did the same in 1774. Lavoisier gave it its name and a theory of combustion, wrongly concluding that oxygen was in every acid.

Where it occurs

46 % of the crust by mass and 86 % of the ocean. The atmosphere is 21 % oxygen by volume, and all of it is biological in origin.

How it is produced

Cryogenic distillation of air at scale, and zeolite adsorption in medical concentrators. In the laboratory, by decomposing hydrogen peroxide or potassium chlorate.

Role in living things

About 65 % of body mass. Cellular respiration uses oxygen as the final electron acceptor, which yields roughly fifteen times more energy per glucose molecule than fermentation.

Safety

Pressurised pure oxygen turns ordinary materials into fire hazards — steel burns in it. Breathing pure oxygen for long damages the lungs, and under pressure it causes seizures.

Uses

Oxygen blowing in blast furnaces and basic oxygen steelmaking — the largest use by far
Medical oxygen for respiratory failure
Oxy-acetylene welding and metal cutting
Rocket oxidiser in liquid form
Wastewater treatment and pulp bleaching

Curiosities

Liquid oxygen is pale blue and sticks to a magnet — the O₂ molecule is paramagnetic.
Ozone, O₃, is the allotrope of oxygen that shields the surface from ultraviolet.
The Great Oxidation Event around 2.4 billion years ago was the largest extinction in history: free oxygen was poison to the anaerobic biosphere of the time.
Roughly every second atom in the crust is an oxygen atom.
Oxygen is forged in stars by helium and carbon burning, and is the third most abundant element in the universe.