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2, the major part of the Earth's atmospheric oxygen (see Occurrence). O 2 has a bond length of 121 pm and a bond energy of 498 kJ/mol. [42] O 2 is used by complex forms of life, such as animals, in cellular respiration. Other aspects of O 2 are covered in the remainder of this article. Trioxygen (O
The existence of a very long C–C bond length of up to 290 pm is claimed in a dimer of two tetracyanoethylene dianions, although this concerns a 2-electron-4-center bond. [4] [5] This type of bonding has also been observed in neutral phenalenyl dimers. The bond lengths of these so-called "pancake bonds" [6] are up to 305 pm.
The ground state of O 2 has a bond length of 121 pm and a bond energy of 498 kJ/mol. [3] It is a colourless gas with a boiling point of −183 °C (90 K; −297 °F). [4] It can be condensed from air by cooling with liquid nitrogen, which has a boiling point of −196 °C (77 K; −321 °F).
All diatomic molecules are linear and characterized by a single parameter which is the bond length or distance between the two atoms. Diatomic nitrogen has a triple bond, diatomic oxygen has a double bond, and diatomic hydrogen, fluorine, chlorine, iodine, and bromine all have single bonds. [6]
Molecular geometries can be specified in terms of 'bond lengths', 'bond angles' and 'torsional angles'. The bond length is defined to be the average distance between the nuclei of two atoms bonded together in any given molecule. A bond angle is the angle formed between three atoms across at least two bonds.
With 133 pm, the ethylene C=C bond length is shorter than the C−C length in ethane with 154 pm. The double bond is also stronger, 636 kJ mol −1 versus 368 kJ mol −1 but not twice as much as the pi-bond is weaker than the sigma bond due to less effective pi-overlap.
However, the alternative structure •O–O• is also inadequate, since it implies single bond character, while the experimentally determined bond length of 121 pm [6] is much shorter than the single bond in hydrogen peroxide (HO–OH) which has a length of 147.5 pm. [7] This indicates that triplet oxygen has a higher bond order.
The bond order between carbon and oxygen in carbon dioxide O=C=O is also 2. In phosgene O=CCl 2, the bond order between carbon and oxygen is 2, and between carbon and chlorine is 1. In some molecules, bond orders can be 4 (quadruple bond), 5 (quintuple bond) or even 6 (sextuple bond).