Oxygen
Atomic Data
| Atomic Number | 8 |
| Symbol | O |
| Atomic Weight | 16.0 u |
| Density (STP) | 1.429 g/L |
| Melting Point | −218.79 °C (54.36 K) |
| Boiling Point | −182.95 °C (90.2 K) |
| Electronegativity | 3.44 (Pauling) |
| Electron Config. | 1s² 2s² 2p⁴ |
| Oxidation States | +2, 0, −1, −2 |
| Phase at STP | Gas |
| Category | Reactive nonmetal |
| Period / Group | 2 / 16 |
| CAS Number | 7782-44-7 |
Electron Configuration
1s2 2s2 2p4
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 4 | 8 |
| Total | 6 | 8 | ||
Isotopes of Oxygen
Oxygen has three stable isotopes: oxygen-16 (99.757%), oxygen-18 (0.205%), and oxygen-17 (0.038%). The ratio of oxygen-18 to oxygen-16 in ice cores and marine sediments is a key palaeoclimate proxy, used to reconstruct ancient ocean temperatures and ice volumes.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Oxygen-16 | ¹⁶O | 8 | 8 | 99.757% | Stable |
| Oxygen-17 | ¹⁷O | 8 | 9 | 0.038% | Stable |
| Oxygen-18 | ¹⁸O | 8 | 10 | 0.205% | Stable |
Abundance & Occurrence
Oxygen is by far the most abundant element in Earth's crust at approximately 46.1% by mass, present in silicate and oxide minerals that make up the majority of rocks and soils. In Earth's atmosphere it constitutes 20.95% by volume as O2, produced and maintained by photosynthetic organisms. In the human body, oxygen makes up about 65% of total mass, found in water, proteins, carbohydrates, and every biological molecule.
Earth's Crust By Mass (%)
Human Body Composition By Mass (%)
Discovery & History
Safety & Handling
- Oxygen-enriched atmospheres: Concentrations above ~23.5% O2 significantly increase the flammability and explosiveness of materials that would be safe in normal air. Clothing and hair ignite far more readily. Oxygen-enriched environments require all ignition sources to be eliminated.
- High-pressure oxygen and organics: Compressed oxygen in contact with oils, greases, or organic materials can cause spontaneous ignition or violent explosion. All equipment used with high-pressure oxygen must be rigorously cleaned and rated for oxygen service — standard lubricants are forbidden.
- Liquid oxygen (LOX) cryogenic hazards: Liquid oxygen at −183°C causes severe cryogenic burns on contact with skin or eyes. Materials that are normally non-flammable (such as asphalt) can ignite when saturated with LOX.
- Ozone toxicity: Ground-level ozone (O3) at concentrations above 0.1 ppm causes respiratory irritation, chest pain, and reduced lung function. Chronic ozone exposure is linked to asthma, cardiovascular disease, and premature death.
- Oxygen toxicity in diving: Breathing oxygen at high partial pressure (above ~1.6 bar) causes central nervous system oxygen toxicity, leading to seizures — a serious risk for divers breathing pure or enriched oxygen mixtures at depth.
- Handling pure oxygen: Store oxygen cylinders securely upright, away from heat, flames, and flammable materials. Never use oxygen as a substitute for compressed air in pneumatic tools.
Oxygen in the Real World
Real-World Uses
- Steel and iron smelting — The basic oxygen process (BOP) accounts for about 70% of global steel production. Pure oxygen blown at supersonic speed into a converter of molten pig iron rapidly oxidises carbon, silicon, and phosphorus to produce clean steel. This has replaced the slower open-hearth process, reducing steelmaking time from hours to about 20 minutes per heat.
- Medical respiratory therapy — Medical-grade oxygen is supplied to patients with respiratory conditions including COPD, pneumonia, COVID-19, and anaesthesia. Oxygen concentrators separate O2 from air using molecular sieves, providing a continuous supply without high-pressure cylinders. Hyperbaric oxygen therapy treats decompression sickness and promotes wound healing.
- Rocket oxidiser — Liquid oxygen (LOX) is the most widely used rocket oxidiser. Combined with liquid hydrogen (LH2) it produces the highest specific impulse of any practical non-nuclear propellant. LOX is used in the Saturn V, Space Shuttle Main Engines, SpaceX Merlin and Raptor engines, and many other launch vehicles.
- Water treatment — Ozone (O3) generated from oxygen is used to disinfect drinking water and treat wastewater, killing bacteria and breaking down organic contaminants more effectively than chlorine without leaving halogenated byproducts. Dissolved oxygen monitoring is critical in environmental water quality assessment.
- Chemical synthesis — Oxygen is a feedstock in the production of ethylene oxide, propylene oxide, acetic acid, formaldehyde, nitric acid, and many other industrial chemicals. Partial oxidation reactions using oxygen underpin the production of plastics, resins, solvents, and pharmaceuticals.
- Welding and cutting — Oxy-fuel welding and cutting uses oxygen combined with acetylene, propylene, or hydrogen to achieve flame temperatures up to 3,500°C — sufficient to cut and weld steel. Oxygen lances are used to cut through thick steel and reinforced concrete in demolition and rescue operations.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is oxygen used for industrially?
The largest industrial use of oxygen is in steelmaking, where it is blown into molten pig iron in basic oxygen furnaces to remove excess carbon and impurities. Oxygen is also used in medical respiratory therapy, as a rocket oxidiser (combined with liquid hydrogen), in water treatment (ozone generation), welding and cutting, and as a feedstock in the chemical industry for producing ethylene oxide, methanol, and other key intermediates.
What is the difference between oxygen and ozone?
Oxygen (O2) is the stable diatomic form that makes up 21% of the atmosphere and is essential to respiration. Ozone (O3) is a triatomic allotrope formed when UV radiation or electrical discharges split O2 molecules, allowing single oxygen atoms to recombine with O2. Stratospheric ozone (the ozone layer) absorbs harmful UV-B and UV-C radiation, protecting life on Earth. At ground level, ozone is a pollutant and respiratory irritant.
Who discovered oxygen?
Oxygen was discovered independently by Carl Wilhelm Scheele in Sweden around 1772 (though his results were published later) and by Joseph Priestley in England in 1774. Priestley is generally credited with first publicising the discovery, and Antoine Lavoisier named the element oxygène in 1777, recognising its role in combustion and respiration and overturning the phlogiston theory.
Why is oxygen so electronegative?
Electronegativity measures how strongly an atom attracts shared electrons. Oxygen's high electronegativity (3.44 on the Pauling scale, second only to fluorine) stems from its small atomic radius and six valence electrons. Its nucleus exerts a strong pull on the two electrons it needs to complete its octet, making oxygen a powerful electron acceptor in covalent and ionic bonds. This drives the polarity of water, the chemistry of hydrogen bonding, and oxygen's reactivity with nearly all other elements.
How is oxygen produced commercially?
Industrial oxygen is produced almost exclusively by the cryogenic distillation of air (the Linde process). Air is first compressed and cooled below its liquefaction temperature, then separated by fractional distillation: nitrogen (boiling point −196°C) and argon (−186°C) are removed first, leaving liquid oxygen (boiling point −183°C). Pressure-swing adsorption (PSA) and membrane separation are used for smaller on-site oxygen generation at lower purity.