There are several known allotropes of oxygen. The most familiar is molecular oxygen (), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (). Others are: Atomic oxygen (), a free radical. Singlet oxygen (), one of two metastable states of molecular oxygen. Tetraoxygen (), another metastable form. Solid oxygen, existing in six variously colored phases, of which one is and another one metallic. Atomic oxygen Atomic oxygen, denoted O(3P) or O(3P), is very reactive, as the single atoms of oxygen tend to quickly bond with nearby molecules. On Earth's surface, it exists naturally for a very short time. In outer space, the presence of ample ultraviolet radiation results in a low Earth orbit atmosphere in which 96% of the oxygen occurs in atomic form. Atomic oxygen has been detected on Mars by Mariner, Viking, and the SOFIA observatory. Dioxygen The common allotrope of elemental oxygen on Earth, , is generally known as oxygen, but may be called dioxygen, diatomic oxygen, molecular oxygen, dioxidene or oxygen gas to distinguish it from the element itself and from the triatomic allotrope ozone, . As a major component (about 21% by volume) of Earth's atmosphere, elemental oxygen is most commonly encountered in the diatomic form. Aerobic organisms use atmospheric dioxygen as the terminal oxidant in cellular respiration in order to obtain chemical energy. The ground state of dioxygen is known as triplet oxygen, , because it has two unpaired electrons. The first excited state, singlet oxygen, , has no unpaired electrons and is metastable. The doublet state requires an odd number of electrons, and so cannot occur in dioxygen without gaining or losing electrons, such as in the superoxide ion () or the dioxygenyl ion (). The ground state of has a bond length of 121 pm and a bond energy of 498 kJ/mol. It is a colourless gas with a boiling point of . It can be condensed from air by cooling with liquid nitrogen, which has a boiling point of . Liquid oxygen is pale blue in colour, and is quite markedly paramagnetic due to the unpaired electrons; liquid oxygen contained in a flask suspended by a string is attracted to a magnet. Singlet oxygen Singlet oxygen is the common name used for the two metastable states of molecular oxygen () with higher energy than the ground state triplet