Oxygen
Element 8 · Nonmetal
Oxygen keeps you alive twice over — your cells burn food with it in respiration, and fires burn fuel with it in combustion. As O2 it makes up a fifth of the atmosphere; as part of minerals it makes up nearly half of Earth's crust by weight. Oxygen's hunger for electrons makes it a powerful oxidizer, which is why iron rusts, wood burns, and cut apples brown. Plants and cyanobacteria refill the air's oxygen through photosynthesis. Pure oxygen feeds hospital patients, steel furnaces, and rocket engines, while its high-altitude cousin ozone shields the planet from ultraviolet light.
Key facts
| Atomic number | 8 |
|---|---|
| Atomic weight | 15.999 |
| Category | Nonmetal |
| Group / Period | 16 / 2 |
| Block | p |
| Electron configuration | 1s2 2s2 2p4 |
| Common oxidation states | -2, -1, +2 |
| State at STP | Gas |
| Electronegativity | 3.44 |
| Atomic radius | 48 pm |
| Density | 0.00143 g/cm³ |
| Melting point | -218.79 °C |
| Boiling point | -182.96 °C |
Occurrence
Free oxygen is 21 percent of air by volume. Bound oxygen dominates the crust in silicate and oxide minerals, and water — nearly 90 percent oxygen by mass — covers most of the planet.
Applications
Medical breathing gas
Steelmaking furnaces
Welding and cutting torches
Water treatment
Rocket oxidizer
Common compounds
- Water (H2O)
- Carbon dioxide (CO2)
- Iron(III) oxide (Fe2O3)
- Ozone (O3)
- Hydrogen peroxide (H2O2)
Almost every oxygen molecule you breathe was released by photosynthesis — a gift from cyanobacteria, algae, and plants.
Data source: IUPAC standard atomic weights 2021