Allotropes of oxygen

There are 4 known allotropes of oxygen:

• dioxygen, O₂ - colorless
• ozone, O₃ - blue
• tetraoxygen, O₄ - red
• metallic oxygen - obtained at very high pressures[1]

Additional recommended knowledge

Weighing the right way

What is the Sensitivity of my Balance?

Guide to balance cleaning: 8 simple steps

The common allotrope of elemental oxygen on Earth, O₂, is known as dioxygen. Elemental oxygen is most commonly encountered in this form, as about 21% (by volume) of the Earth's atmosphere.

  Ozone (O₃), the less common triatomic allotrope of oxygen, is a poisonous gas with a distinct, sharp odor. It is thermodynamically unstable toward the more common dioxygen form. It is formed continuously in the upper atmosphere of the Earth by short-wave ultraviolet (UV) radiation, and also functions as a shield against UV radiation reaching the ground (see ozone layer). Ozone is also formed by electrostatic discharge in the presence of molecular oxygen. The immune system produces ozone as an antimicrobial (see below). Liquid and solid O₃ have a deeper blue color than ordinary oxygen and they are unstable and explosive. Traces of it can be detected as a sharp, chlorine-like smell coming from electric motors, laser printers and photocopiers.

A newly 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.^[1][2] When tetraoxygen is subjected to a pressure of 96 GPa, it becomes metallic, similarly to hydrogen, and becomes more similar to the heavier chalcogens, such as tellurium and polonium, both of which show significant metallic character.