Allotropes of Oxygen

Allotropes are different forms of the same chemical element that exhibit different physical and chemical properties. Different bonding arrangement between atoms of an element result in a new molecular configuration, with new physical as well as chemical properties. Examples of elements that have allotropes include carbon, oxygen, phosphorus and sulfur.

Oxygen Allotropes

Oxygen is a chemical element represented by the symbol 'O' and has the atomic number 8. It is the first element in Group VI A of the modern periodic table. Oxygen is the most abundant element in the earth's crust and makes up about one fifth of the earth's atmosphere.

Dioxygen
Dioxygen (O₂), also known as diatomic oxygen, is the most common form of elemental oxygen. Dioxygen is a colorless gas at room temperature and forms about 21% of the earth's atmosphere. It exists as a diradical and is the only allotrope that has unpaired electrons. Diatomic oxygen has a bond length of 121 pm and a bond energy of 498 kJ/mol. Most of the complex forms of life use this form of oxygen in cellular respiration.

Ozone
Another common allotrope of oxygen is triatomic oxygen or ozone (O₃). It is thermodynamically unstable and is a highly reactive form of oxygen. Ozone was discovered in 1840, by Christian Friedrich Schonbein. It exists as a pale blue gas at normal temperature and pressure conditions. However, it is converted to a dark blue liquid, below -112ºC and to a dark blue solid below -193ºC. Ozone is produced in the upper atmosphere, when diatomic oxygen combines with atomic oxygen, that is made by splitting of O₂ by ultraviolet radiation. As ozone strongly absorbs in the ultraviolet radiation, it functions as a protective shield for the biosphere against the mutagenic and harmful effects of the UV radiation. Ozone is also formed as a by-product of automobile exhaust, however, it is considered as a pollutant when present near the earth's surface.

Tetraoxygen
Although this allotrope of oxygen was suspected to exist since the early 1900's, it was identified in 2001. It was known as oxozone and was thought to be in one of the phases of solid oxygen. The team who discovered tetraoxygen believes that it consists of two dumbbell shaped O₂ molecules, loosely held together by induced dipole dispersion forces. It exists as a deep red solid that is created by pressurizing O₂ to the order of 20 Gpa. It has stronger oxidizing properties than dioxygen as well as ozone and hence, it is being studied for use in rocket fuels and other such applications.

These were the different allotropes of oxygen. Apart from these known allotropes, oxygen also exists as metastable states, that have higher energy than the ground state oxygen.