|Atomic mass||16.0 amu|
|Date of discovery||1774|
|Name of discoverer||Joseph Priestly|
|Name origin||From the Greek words oxus (acid) and gennan (generate)|
|Obtained from||From liquid air|
Oxygen is a chemical element. Its atomic number, derived from number of protons and electrons it contains, is 8. Oxygen's atomic mass is slightly under 16, since it also has 8 neutrons (a slight amount of mass is "lost" in the energy contained in the subatomic bonds).
Under normal conditions it is a colorless, odorless gas consisting of two oxygen atoms which has the chemical formula O2. This means it is diatomic. Approximately 20% of the atmosphere is made up of oxygen. Oxygen also forms a triatomic molecule called ozone (O3), which although unstable under normal conditions, is a very important ingredient in the upper atmosphere.
Oxygen is highly reactive, and in many other elements and compounds undergo exothermic reactions with it (they burn or rust). If it were not for its continual replenishment as a waste product of plant photosynthesis, the atmospheric oxygen would disappear, forming compounds with other chemicals at the earth's surface.
Many animals depend upon oxygen as a highly efficient ingredient for breaking down food for energy, due to its reactivity. Those that do not are called "anaerobic", meaning "without air", and include many important bacteria.