Triplet Oxygen is the ground state of the O₂ molecule. ( (#2) ) The electron configuration of the molecule has two unpaired electrons occupying two degenerate molecular orbitals. (An orbital is a concept from quantum mechanics that models an electron as a wave-like particle that has a spacial distribution about an atom or molecule.) These orbitals are classified as antibonding (weakening the bond order from three to two), so the diatomic Oxygen bond is weaker than the diatomic Nitrogen triple bond in which all bonding molecular orbitals are filled, but some antibonding orbitals are not.

In normal triplet form, O³4_{5³ molecules are paramagnetic—they form a magnet in the presence of a magnetic field—because of the spin magnetic moments of the unpaired electrons in the molecule, and the negative exchange energy between neighboring O³67³ molecules. Liquid oxygen is attracted to a magnet to a sufficient extent that, in laboratory demonstrations, a bridge of liquid oxygen may be supported against its own weight between the poles of a powerful magnet. ( (#3) ) (Oxygen's paramagnetism can be used analytically in paramagnetic oxygen gas analysers that determine the purity of gaseous Oxygen. ( (#4) ))}

Singlet Oxygen. a name given to several higher-energy species of molecular O³8_{9³ in which all the electron spins are paired, is much more reactive towards common organic molecules. In nature, singlet oxygen is commonly formed from water during photosynthesis, using the energy of sunlight. (Krieger-Liszkay 2005, 337-46) It is also produced in the troposphere by the photolysis of ozone by light of short wavelength, (Harrison 1990) and by the immune system as a source of active oxygen. (Wentworth 2002) Carotenoids in photosynthetic organisms (and possibly also in animals) play a major role in absorbing energy from singlet oxygen and converting it to the unexcited ground state before it can cause harm to tissues. (Hirayama 1994, 149-150)}

See also:

• Characteristics of oxygen
• oxygen