Oxygen - O
Oxygen (O) occurs in water, minerals and living organisms. Molecular oxygen (O₂) supports human respiration and many biological treatment processes; ozone (O₃) is a different form with different properties and uses.
Chemical properties · Uses and occurrence · Health effects · Environmental effects · Water and treatment
Chemical properties of oxygen
| Symbol / atomic number | O / 8 |
|---|---|
| Group / period / block | 16 / 2 / p |
| Standard atomic weight interval | [15.99903, 15.99977] |
| Electron configuration | [He] 2s2 2p4 |
| Usual elemental form | O2, a colourless gas |
| Gas density (20 °C, 1 bar; calculated) | 1.31 g L−1 |
| Melting point | −218.79 °C |
| Boiling point | −182.962 °C |
| Electronegativity (Pauling) | 3.44 |
| Non-bonded atomic radius | 0.152 nm |
| First ionisation energy | 1313.9 kJ mol−1 |
| Stable isotopes | 16O, 17O and 18O |
| Discovery | Scheele in the early 1770s; independently Priestley in 1774 |
| CAS registry number (O₂) | 7782-44-7 |
Atomic weight follows CIAAW. Intervals reflect natural isotopic variation; parentheses indicate uncertainty in the final digits. Other properties follow RSC unless noted. Atomic radii depend on their definition. Gas density is an ideal-gas estimate at 20 °C and 1 bar, calculated from ρ = pM/RT using conventional atomic weights; it is not a liquid density.
Properties, uses and occurrence
Oxygen is a reactive nonmetal. O2 accounts for about 21% of dry air and is used in steelmaking, chemical processing and healthcare. It also occurs chemically bound in water and many rocks and minerals.
Oxygen supports combustion but is not itself a fuel. Ozone is a three-atom form of oxygen and a strong oxidant used in specialist treatment systems. It must not be confused with ordinary oxygen gas. See ozone treatment.
Health effects and oxygen safety
Humans and other aerobic organisms need oxygen for respiration, but not all organisms require it. Oxygen deficiency can cause rapid unconsciousness and death. Breathing high concentrations for prolonged periods can also be harmful; medical oxygen requires appropriate clinical supervision.
Oxygen-enriched atmospheres greatly increase fire risk. Materials that burn in air can ignite more easily and burn more intensely. Oxygen equipment needs compatible materials, cleanliness and suitable operating procedures; oil and grease must be kept away from oxygen fittings. Compressed and liquid oxygen also introduce pressure and extreme-cold hazards.
Oxygen in the environment
Photosynthesis adds oxygen to air and water, while respiration and decomposition consume it. Low dissolved oxygen can stress or kill fish and other aquatic organisms. Warm water generally holds less dissolved oxygen than cold water; pressure and salinity also influence solubility.
Ozone has a separate environmental role: stratospheric ozone absorbs ultraviolet radiation, while ozone near ground level is an air pollutant. These effects should not be attributed indiscriminately to O2.
Dissolved oxygen and water treatment
Dissolved oxygen (DO) means molecular O2 dissolved in water, not the oxygen atoms chemically bound in H2O. Report DO in mg/L and, where useful, as percentage saturation. Saturation depends on temperature, atmospheric pressure and salinity, so a single concentration does not describe every water source.
Aeration or oxygen addition can support aerobic biological treatment. Nitrification also needs oxygen and consumes alkalinity. The required supply depends on the process oxygen demand and transfer efficiency; it should be assessed for the actual treatment conditions.
Adding oxygen is not equivalent to ozone disinfection. Match the gas, equipment and monitoring to the process objective. Further information: oxygen and water, oxygen applications and oxygen concentrators for ozone production.
Oxygen page last reviewed: by Lenntech.
Back to the periodic table of elements.
Sources and further reading