Oxygen is a non-metallic chemical element with the chemical symbol O, atomic number 8, and atomic mass 15.999. Under atmospheric pressure and temperature, oxygen is a colorless, odorless, and tasteless gas.
Oxygen has a 2–6 electron configuration with a valence of 2. Because of this property, oxygen does not exist in nature as single atoms; instead, pairs of oxygen atoms combine by forming a double covalent bond that results in the highly stable O2 molecule.
Oxygen’s electron configuration also endows it with high reactivity. Oxygen combines with all elements except the noble gases and with most compounds under the influence of temperature and light. A chemical reaction with oxygen is called oxidation, and the resulting compounds are called oxides. The combustion process is a special case of oxidation that features a rapid reaction rate and the release of heat and light.
Another form of elemental oxygen is ozone (O3), which is a triatomic molecule consisting of three oxygen atoms. The ozone molecule is much less stable than the diatomic O2 and is, therefore, less abundant, but it is still important to life on Earth.
In the upper atmosphere (15 to 45 km), ozone forms a thin, protective layer around the Earth that filters potentially damaging ultraviolet light from reaching the Earth’s surface. Some pollutants in the atmosphere have a detrimental effect on this ozone layer.
At the same time, however, ozone close to the Earth’s surface is toxic, with harmful effects on the respiratory systems of humans and animals. An ozone concentration of more than 125 parts per billion (ppb) is regarded as “unhealthy” by the World Health Organization.
Other properties of oxygen include:
- Boiling point: −182.97°C (at 1 atm)
- Melting point: −218.76°C (at 1 atm)
- Critical point: −118.9°C (at 1 atm)
- Density: 1.429 g/l (at 1 atm and 0°C)
- Pure oxygen is 1.1 times heavier than air
- Liquid oxygen has a strong magnetic property
- Liquid and solid oxygen have a pale blue color