O2

The common allotrope of elemental oxygen on Earth, O2, is generally known as oxygen, but may be called dioxygen or molecular oxygen to distinguish it fr

om 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.

O2 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). 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.