Atomic number 8

Oxygen

O
+ phase
− phase

Shells Orbitals 2s 2p

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8 electrons in 2 shells: 2, 6.

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

Category: Nonmetal
Period: 2
Group: 16
Block: p
Standard atomic weight: 15.999(1) Da
Electron configuration: [He] 2s² 2p⁴
Phase at 25 °C: gas
Melting point: −218.79 °C
Boiling point: −182.96 °C
Density: 1.429 g/L
Electronegativity (Pauling): 3.44
Discovered: 1774, Joseph Priestley; independently Carl Wilhelm Scheele in 1771, published later

Neighbours in the table

N F

Oxygen’s eight electrons sit in two shells: two in the first, six in the second. Those six leave the atom two short of a full shell. A nonmetal makes up such a gap with enough covalent bonds to reach eight, and oxygen does it with one partner: a double bond to a second oxygen atom. So the free element in air is O₂, never a lone atom. Sulfur, one place below it in group 16, is short the same two electrons but takes them from two single bonds in rings and chains.

Even in its most stable form the O₂ molecule keeps two of its electrons unpaired. The gas shows nothing of it: no colour, no smell, no taste. At about −183 °C it condenses to a pale blue liquid. The liquid and the solid are strongly paramagnetic: pulled into a magnetic field rather than pushed out.

Oxygen’s electronegativity, the pull an atom exerts on the electrons in a bond it shares, is 3.44 on the Pauling scale. Only fluorine pulls harder, and only with fluorine does oxygen carry a positive oxidation state. It reacts directly with every element but the noble gases, the halogens and a few sluggish transition metals; rust is the everyday case. Of the elements, only fluorine can strip those electrons back out of an oxide.

Where it is found

Most of the oxygen on Earth is in the rock, not the air. PubChem’s estimate for the crust is 4.61 × 10⁵ milligrams per kilogram, or 46.1% by mass, most of it bound to silicon. Free O₂ is the small remainder: 20.94% of normal air, which is about 21% of the atmosphere by volume and about 20% of its mass.

Combined oxygen makes up about 89% of the mass of water. PubChem’s oceanic estimate, 8.57 × 10⁵ milligrams per litre, is roughly 857 g in every litre of seawater. Uncombined, it barely dissolves: 49 mL of the gas in a litre of water at standard temperature and pressure. Every fish and water plant lives on that thin share. Green plants replenish the atmosphere by photosynthesis, which turns carbon dioxide and water back into sugar and oxygen using light energy.

Only hydrogen and helium are more abundant than oxygen in the universe as a whole, and Los Alamos ranks it third in the Sun as well. Excited oxygen gives the aurora its red and yellow-green colours. The Martian atmosphere holds about 0.15%.

Why it matters

Oxygen is roughly 65% of the mass of a human body. In the lungs it binds to haemoglobin, the protein in red blood cells, forming oxyhaemoglobin; the blood carries it to the tissues and hands it over.

Inside the cell, mitochondria combine most of it with carbon and hydrogen from glucose to build ATP, the molecule that powers the cell, along with carbon dioxide and water. Those reactions end in the electron transport chain, a line of carriers passing electrons along, and oxygen accepts them at the end of it. Without that acceptor, aerobic respiration cannot finish.

Steelmaking is the largest industrial use. Streams of high-pressure oxygen burn the carbon and other impurities out of the molten iron to make steel. Enriching the air blown into steel blast furnaces is, Los Alamos says, the greatest single use of the gas.

Away from the furnace, oxygen fed to an acetylene flame gives the torch that cuts and welds metal. Chemical plants take it to make synthesis gas for ammonia and methanol, and to make ethylene oxide. Liquid oxygen is the oxidiser in rocket engines and the source of breathable gas aboard spacecraft.

Properties

Properties of oxygen 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	8
Standard atomic weight	15.999(1) Da	Da	IUPAC/CIAAW 2024
Melting point	−218.79 °C	°C	Los Alamos: −218.79 °C
Boiling point	−182.96 °C	°C	Los Alamos: −182.96 °C
Density	1.429 g/L	g/L
Electronegativity (Pauling)	3.44
Electron configuration	1s² 2s² 2p⁴
Phase at 25 °C	gas

The standard atomic weight of this element is an interval in nature, from 15.99903 to 15.99977 Da; the value shown is IUPAC's conventional abridged value.

Safety

Oxygen does not burn; it makes other materials burn faster. EU rules therefore give the gas the signal word Danger and one hazard statement, H270, may cause or intensify fire. The hazard belongs to the compressed and refrigerated product that industry and hospitals handle, not to ordinary air; the cold liquid also causes frostbite. The PubChem Laboratory Chemical Safety Summary for oxygen is the authoritative sheet.

Questions

How is oxygen separated from air?

Air is cooled and compressed until it turns liquid, then allowed to warm again. Nitrogen boils at a lower temperature than oxygen, so it comes off first and leaves the oxygen behind as a liquid, which does not boil until about −183 °C. Air separation plants make about 99% of commercial oxygen and electrolysis of water the remaining 1%. A second route pushes compressed air through zeolite beds that hold the nitrogen back.

What is the difference between oxygen and ozone?

They are two forms of the same element: dioxygen has two atoms, ozone three. Ozone forms when an electric discharge passes through dry oxygen, which is part of the sharp smell around sparking equipment, and naturally where sunlight strikes oxygen in the stratosphere, 10 to 50 kilometres up. It bleaches oils, waxes and fabrics, and disinfects water in place of chlorine.

Why do hospitals give people oxygen?

Oxygen-enriched air is standard practice when a patient cannot take in enough of it, in shock or pneumonia for example, and the World Health Organization lists oxygen as an essential medicine for acute or chronic respiratory failure. Hospital supply is 99% pure medical grade gas, piped from insulated liquid tanks; in the United States the cylinders are colour-coded green. Home concentrators deliver 30% to 95%.

Who discovered oxygen, Priestley or Scheele?

Both did, independently, and the credit turns on printing dates. PubChem's history for the element puts Carl Wilhelm Scheele's preparation of the gas in 1771, with his Chemical Treatise on Air and Fire delayed until 1777. Joseph Priestley made it in 1774 and published first, so he is usually credited. Both heated mercuric oxide; Antoine Lavoisier supplied the name.

Compounds containing oxygen

CO Carbon monoxide 2 atoms, 1 triple bond.
ZnO Zinc oxide 2 atoms in one formula unit of an ionic lattice.
CO₂ Carbon dioxide 3 atoms, 2 double bonds, linear.
N₂O Nitrous oxide 3 atoms, 2 bonds.
O₃ Ozone 3 atoms, 2 bonds, bent.
KOH Potassium hydroxide 3 atoms in one formula unit of an ionic lattice.
NaOH Sodium hydroxide 3 atoms in one formula unit of an ionic lattice.
SO₂ Sulfur dioxide 3 atoms, 2 double bonds.

All 117 compounds containing oxygen

H₂O Water 3 atoms, 2 single bonds, bent.
CH₂O Formaldehyde 4 atoms, 3 bonds.
H₂O₂ Hydrogen peroxide 4 atoms, 3 single bonds.
HNO₂ Nitrous acid 4 atoms, 3 bonds.
SO₃ Sulfur trioxide 4 atoms, 3 double bonds.
Al₂O₃ Aluminium oxide 5 atoms in one formula unit of an ionic lattice.
CaCO₃ Calcium carbonate 5 atoms in one formula unit of an ionic lattice.
Ca(OH)₂ Calcium hydroxide 5 atoms in one formula unit of an ionic lattice.
HCOOH Formic acid 5 atoms, 4 bonds.
Fe₂O₃ Iron(III) oxide 5 atoms in one formula unit of an ionic lattice.
Mg(OH)₂ Magnesium hydroxide 5 atoms in one formula unit of an ionic lattice.
HNO₃ Nitric acid 5 atoms, 4 bonds.
KNO₃ Potassium nitrate 5 atoms in one formula unit of an ionic lattice.
AgNO₃ Silver nitrate 5 atoms in one formula unit of an ionic lattice.
BaSO₄ Barium sulfate 6 atoms in one formula unit of an ionic lattice.
CaSO₄ Calcium sulfate 6 atoms in one formula unit of an ionic lattice.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CuSO₄ Copper(II) sulfate 6 atoms in one formula unit of an ionic lattice.
MgSO₄ Magnesium sulfate 6 atoms in one formula unit of an ionic lattice.
CH₃OH Methanol 6 atoms, 5 single bonds.
KMnO₄ Potassium permanganate 6 atoms in one formula unit of an ionic lattice.
NaHCO₃ Sodium bicarbonate 6 atoms in one formula unit of an ionic lattice.
Na₂CO₃ Sodium carbonate 6 atoms in one formula unit of an ionic lattice.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
H₂SO₄ Sulfuric acid 7 atoms, 6 bonds, tetrahedral.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
H₃PO₄ Phosphoric acid 8 atoms, 7 bonds.
CH₃COONa Sodium acetate 8 atoms in one formula unit of an ionic lattice.
CO(NH₂)₂ Urea 8 atoms, 7 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₂H₅NO₂ Glycine 10 atoms, 9 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₄H₄N₂O₂ Uracil 12 atoms, 12 bonds.
C₃H₇NO₂ Alanine 13 atoms, 12 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₄H₅N₃O Cytosine 13 atoms, 13 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
C₃H₇NO₂S Cysteine 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₅H₆N₂O₂ Thymine 15 atoms, 15 bonds.
C₅H₅N₅O Guanine 16 atoms, 17 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₅H₄N₄O₃ Uric acid 16 atoms, 17 bonds.
C₃H₈NO₅P Glyphosate 18 atoms, 17 bonds.
C₅H₉NO₄ Glutamic acid 19 atoms, 18 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₅H₈NO₄Na Monosodium glutamate 19 atoms in one formula unit of an ionic lattice.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₈H₉NO₂ Acetaminophen 20 atoms, 20 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₈H₁₁NO₂ Dopamine 22 atoms, 22 bonds.
C₆H₁₃NO₂ Isoleucine 22 atoms, 21 bonds.
C₆H₁₃NO₂ Leucine 22 atoms, 21 bonds.
C₉H₁₁NO₂ Phenylalanine 23 atoms, 23 bonds.
C₈H₁₀N₄O₂ Caffeine 24 atoms, 25 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₉H₁₁NO₃ Tyrosine 24 atoms, 24 bonds.
C₁₀H₁₂N₂O Serotonin 25 atoms, 26 bonds.
C₉H₁₃NO₃ Epinephrine 26 atoms, 26 bonds.
C₁₁H₁₂N₂O₂ Tryptophan 27 atoms, 28 bonds.
C₁₂H₁₇NO DEET 31 atoms, 31 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₃H₁₆N₂O₂ Melatonin 33 atoms, 34 bonds.
C₁₅H₁₁I₄NO₄ Thyroxine 35 atoms, 36 bonds.
C₁₃H₂₁NO₃ Albuterol 38 atoms, 38 bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₄H₁₈N₂O₅ Aspartame 39 atoms, 39 bonds.
C₁₄H₂₂N₂O Lidocaine 39 atoms, 39 bonds.
C₁₇H₂₁NO Diphenhydramine 40 atoms, 41 bonds.
C₁₇H₁₈F₃NO Fluoxetine 40 atoms, 41 bonds.
C₁₆H₁₈N₂O₄S Penicillin G 41 atoms, 43 bonds.
C₁₆H₁₉N₃O₅S Amoxicillin 44 atoms, 46 bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₀H₁₆N₅O₁₃P₃ Adenosine triphosphate 47 atoms, 49 bonds.
C₂₀H₂₄N₂O₂ Quinine 48 atoms, 51 bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.
C₃₃H₃₅FN₂O₅ Atorvastatin 76 atoms, 79 bonds.