The common allotrope of elemental oxygen on Earth is called dioxygen, O
2. It is the form that is a major part of the Earth's atmosphere (see Occurrence). O2 has a bond length of 121 pm and a bond energy of 498 kJ·mol−1, which is smaller than the energy of other double bonds or pairs of single bonds in the biosphere and responsible for the exothermic reaction of O2 with any organic molecule. Due to its energy content, O2 is used by complex forms of life, such as animals, in cellular respiration (see Biological role). Other aspects of O
2 are covered in the remainder of this article.