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, −1, 0, +2 |
| Phase at STP | Gas |
| Category | Reactive Nonmetal |
| Period / Group | 2 / 16 |
| CAS Number | 7782-44-7 |
Electron Configuration
[He] 2s2 2p4
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 4 | 8 |
| Total | 8 | 8 | ||
Abundance & Occurrence
Oxygen is present in Earth's crust at approximately 461000 ppm by mass and at approximately 10000 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Oxidiser — fire and explosion: Oxygen itself does not burn, but vigorously accelerates combustion of almost all materials; even materials that are difficult to ignite in air — including some metals — burn fiercely in enriched oxygen atmospheres.
- Oxygen-enriched atmospheres: Concentrations above ~23% in air dramatically increase fire risk; clothing and hair become highly flammable, and ignition sources that would normally pose little risk become extremely dangerous.
- High-pressure oxygen: High-pressure oxygen in contact with hydrocarbons, greases, or oils can ignite spontaneously; all oxygen equipment must be scrupulously clean, grease-free, and rated for oxygen service.
- Liquid oxygen (LOX): Liquid oxygen at −183 °C causes cryogenic burns on contact; porous or organic materials (asphalt, wood, clothing) soaked in LOX become shock-sensitive explosives.
- Oxygen toxicity: Breathing 100% oxygen at elevated pressure (e.g. in diving or hyperbaric medicine) causes pulmonary and central nervous system oxygen toxicity; use time and pressure limits strictly.
Oxygen in the Real World
Real-World Uses
- Medical and respiratory therapy — Compressed oxygen is essential in hospitals for mechanical ventilators, surgical anaesthesia, hyperbaric oxygen therapy, and as a supplement for patients with breathing disorders.
- Steel manufacturing — Basic oxygen steelmaking (BOS) blows pure oxygen through molten pig iron to rapidly oxidise carbon and impurities, producing steel more efficiently than older open-hearth furnaces.
- Rocket oxidiser — Liquid oxygen (LOX) is the oxidiser in most large rocket engines, including the Space Shuttle main engines and SpaceX Falcon 9, reacting with liquid hydrogen or kerosene to generate thrust.
- Water treatment — Ozone (O₃) and oxygen-based compounds are used to disinfect drinking water, remove organic contaminants, and treat industrial wastewater without leaving chlorine residues.
- Welding and cutting — Oxy-acetylene and oxy-fuel torches use oxygen to sustain an intense flame capable of welding, cutting, and brazing metals including steel and cast iron.
- Chemical synthesis — Oxygen is a feedstock in the industrial production of ethylene oxide, acetic acid, vinyl chloride, and many other high-volume chemicals through catalytic partial oxidation processes.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is oxygen used for?
Oxygen is essential in medicine (for respiratory therapy, anaesthesia, and hyperbaric treatment), in steel manufacturing (basic oxygen steelmaking), in rocket propulsion (liquid oxygen as the oxidiser), in water treatment (ozone and dissolved oxygen), in welding (oxy-acetylene torches), and as a feedstock in many chemical syntheses including the production of ethylene oxide, acetic acid, and other bulk chemicals.
Can you breathe pure oxygen?
Breathing pure oxygen is safe for short periods (minutes to hours) at atmospheric pressure, as used during surgery and in aviation emergencies. However, prolonged breathing of 100% oxygen leads to oxygen toxicity: pulmonary damage, chest pain, and fluid in the lungs within 12–24 hours. At elevated pressure (as in deep-sea diving), oxygen becomes toxic to the central nervous system within minutes, causing convulsions. Normal air (21% oxygen) is the safe long-term breathing mixture.
How is oxygen produced commercially?
Most industrial oxygen is produced by the cryogenic fractional distillation of liquefied air. Air is cooled to below −183°C, whereupon oxygen (boiling point −183°C) and nitrogen (boiling point −196°C) condense and can be separated in a distillation column. Smaller volumes of oxygen are produced on-site by pressure-swing adsorption (PSA), which uses a molecular sieve (zeolite) to selectively adsorb nitrogen from air.
Is ozone the same as oxygen?
No. Ordinary oxygen is the diatomic molecule O₂ (two oxygen atoms bonded together). Ozone is the triatomic allotrope O₃ (three oxygen atoms), which is a pale blue, pungent gas. Ozone in the stratosphere (15–35 km altitude) absorbs UV-B and UV-C radiation, protecting life on Earth. Near ground level, ozone is an air pollutant and respiratory irritant produced by photochemical reactions between sunlight, nitrogen oxides, and volatile organic compounds from vehicle emissions and industry.