Periodic table › Oxygen
1. Identity and classification
Colorless, odorless gas (O₂); pale sky-blue paramagnetic liquid when condensed below 90.2 K
- Name and symbol
- Oxygen (Oxygen) · O
- Atomic number
- 8
- Relative atomic weight / mass number
- 15.999 ± 0.001
- Group · period · block
- 16 · 2 · p
- Category
- Reactive Nonmetal
- State near room temperature
- Gas
- Stable isotopes
- At least one stable isotope
- CAS number
- 7782-44-7
Square brackets [A] indicate a representative isotope mass number, not an average atomic weight. Neutron counts differ between isotopes.
2. Atomic structure and electron configuration
- Protons · electrons in a neutral atom
- 8 · 8
- Neutrons in reference isotope
- 8 (16O)
- Full electron configuration
1s² 2s² 2p⁴- Abbreviated configuration
[He]2s² 2p⁴- Electrons per shell
- 2, 6
- Listed oxidation states
- -2
- Electronegativity
- 3.44 (Pauling)
- First ionization energy
- 1313.937 kJ/mol · 13.618 eV
- Electron affinity
- 140.965 kJ/mol
- Covalent radius
- 66 pm
- van der Waals radius
- 152 pm
Configurations of superheavy elements may include theoretical predictions. Different definitions of atomic radius are not interchangeable.
3. Physical and thermal properties
- Density
- 0.001429 g/cm³
- Melting point
- 54.36 K · -218.79 °C · -361.822 °F
- Boiling point
- 90.2 K · -182.95 °C · -297.31 °F
- Crystal structure
- Simple Cubic
- Specific heat
- 0.918 J/(g·K)
- Thermal conductivity
- 0.02658 W/(m·K)
- Enthalpy of fusion
- 0.444 kJ/mol
- Enthalpy of vaporization
- 6.82 kJ/mol
- Magnetic ordering
- Paramagnetic
- Mohs hardness
- Not established
- Speed of sound
- 330 m/s
Density and structure depend on temperature, pressure and allotrope. 25°C is a room-temperature reference, not STP. Unestablished values are not zero.
4. Chemical behavior and compounds
O₂ is an oxidizing diatomic molecule; ozone O₃ is a distinct allotrope.
Selected compounds: H₂O (Water) · SiO₂ (Silica) · Fe₂O₃ (Iron(III) oxide) · CO₂ (Carbon dioxide) · Al₂O₃ (Alumina)
Reaction reference
Oxygen supports combustion and oxidizes many substances under suitable conditions.
Oxygen dissolves in water; it does not simply react with water at room temperature.
5. Isotopes
| Isotope | Status | Natural abundance | Half-life |
|---|---|---|---|
| 16O | Stable | 99.757% | Stable |
| 17O | Stable | 0.03835% | Stable |
| 18O | Stable | 0.2045% | Stable |
Half-life units: s seconds, m minutes, h hours, d days, Y years. A dash means no natural-abundance value is given, not zero. Listed records are selected ground states; metastable states are distinct.
6. Discovery and occurrence
- Discovery
- 1774 CE
- Scientists associated with discovery
- Carl Wilhelm Scheele & Joseph Priestley
- Location
- Sweden / England
- Origin
- Natural
- Listed crustal abundance
- 461000 ppm
Minerals and naming reference
Silicates (quartz SiO₂, feldspars), oxides (hematite Fe₂O₃), water (H₂O); 20.95% of atmosphere
From Greek 'oxys' (acid/sharp) + 'genes' (forming), named by Antoine Lavoisier
7. Applications
Cellular respiration, water purification (ozone O₃), combustion
Detailed applications by field
- everyday
- Cellular respiration, water purification (ozone O₃), combustion
- industry
- Basic oxygen steelmaking (converting pig iron to steel), oxy-acetylene welding and cutting, ethylene oxide production
- electronics
- Thermal oxidation of silicon wafers to form insulating SiO₂ gate layers
- medicine
- Supplemental oxygen therapy, hyperbaric oxygen chambers, life-support ventilators
- energy
- Oxy-fuel combustion for carbon capture, coal gasification
- aerospace
- Liquid oxygen (LOX) oxidizer in orbital rockets, astronaut EVA life-support systems
- agriculture
- Dissolved oxygen aeration in aquaculture and wastewater treatment
- science
- Paleoclimate reconstruction via δ¹⁸O ratios in polar ice cores and marine foraminifera
8. Biological role and safety
Hazard depends on the isotope, chemical form, dose and exposure route.
Elemental substances, ions, compounds and radioisotopes can have different health effects. This is an educational reference; laboratory work requires the material safety data sheet and trained supervision.
Biological role reference
Terminal electron acceptor in aerobic cellular respiration within mitochondria, enabling high-efficiency ATP energy generation.