Allotropes are different forms of the same chemical element that exhibit different physical and chemical properties. Different bonding arrangement between the 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 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. It is the most abundant element in the Earth's crust and makes up about one fifth of the Earth's atmosphere.
Dioxygen (O2) is the most common form of elemental oxygen. It 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. It 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 in cellular respiration.
Another common allotrope is ozone (O3). It is thermodynamically unstable and highly reactive. 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 dioxygen 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.
Although this allotrope was suspected to exist since the early 1900's, it was identified in 2001. It was known as oxozone. The team which discovered it, 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.
Apart from these known allotropes, it also exists as metastable states, that have higher energy than the ground state.