Oxygen ($\mathrm{O}$)

Joseph Priestley's historical name for the unusually combustion-supporting air now associated with oxygen. The name expressed a phlogiston theory interpretation, rather than the modern account of chemical composition.

An isotope of oxygen with 8 protons and 9 neutrons. Its nuclear spin concerns the coupled nuclear quantum state, not the electronic configuration.

An electronically excited atomic oxygen state produced in part by ozone photodissociation. Reaction with water can yield two hydroxyl radicals, while collisional quenching returns the atom to its triplet ground term without the same radical production. Its short-lived steady balance is distinct from the lifetime of ozone or water.

An electronically excited atomic-oxygen state that can be quenched collisionally or react with species such as water. Quenching back to the ground state and radical production compete, so the photochemical source rate need not equal the radical source rate.

Absence of available oxygen in a stated environment. It can change decay and mineralization pathways but does not stop all microbial activity.

Singlet oxygen is an electronically excited form of molecular oxygen with reactivity distinct from ground-state oxygen. Energy transfer from triplet chlorophyll can generate it during photosynthetic excitation stress.

The diatomic form of oxygen. Modern terrestrial abundance is maintained largely by oxygenic photosynthesis, balanced by chemical and biological sinks.