<P> Atomic oxygen, denoted O (P), O (3P) or O ((3) P), is very reactive, as the single atoms of oxygen tend to quickly bond with nearby molecules . On Earth's surface, it does not exist naturally for very long, but in outer space, the presence of plenty of ultraviolet radiation results in a low Earth orbit atmosphere in which 96% of the oxygen occurs in atomic form . </P> <P> Atomic oxygen has been detected at planet Mars by Mariner, Viking, and the Sofia observatory . </P> <P> The common allotrope of elemental oxygen on Earth, O 2, is generally known as oxygen, but may be called dioxygen, diatomic oxygen, or molecular oxygen to distinguish it from the element itself . Elemental oxygen is most commonly encountered in this form, as about 21% (by volume) of Earth's atmosphere . 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 . </P> <P> O 2 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 − 183 ° C (90 K; − 297 ° F). It can be condensed from air by cooling with liquid nitrogen, which has a boiling point of − 196 ° C (77 K; − 321 ° F). Liquid oxygen is pale blue in colour, and is quite markedly paramagnetic--liquid oxygen contained in a flask suspended by a string is attracted to a magnet . </P>

Which of the following along with o2 is an allotrope of oxygen