At standard temperature and pressure, oxygen is predominantly found as a gas consisting of a diatomic molecule with the chemical formula O2. O2 itself has two energetic forms: the low-energy, predominant single-bonded diradical triplet oxygen and the high-energy double-bonded molecule singlet oxygen. This native diradical quality of oxygen contributes to its destructive chemical nature.
Oxygen is a major component of air, produced by plants during photosynthesis, and is necessary for aerobic respiration in animals. The word oxygen derives from two words in Greek, oxys (acid) and geinomai (engender). The name "oxygen" was chosen because, at the time it was discovered in the late 18th century, it was believed that all acids contained oxygen. The definition of acid has since been revised to not require oxygen in the molecular structure.
Liquid O2 and solid O2 have a light blue color and both are highly paramagnetic. Liquid O2 is usually obtained by the fractional distillation of liquid air. Both liquid and solid O3 (ozone) have a deeper color of blue.
Another recently discovered allotrope of oxygen, tetraoxygen (O4), is a deep red solid that is created by pressurizing O2 to the order of 20 GPa. Its properties are being studied for use in rocket fuels and similar applications, as it is a much more powerful oxidizer than either O2 or O3.
Source: Wikipedia - en.wikipedia.org
