Ozone in the troposphere is a secondary chemical pollutant and a radiatively active gas. Its abundance reflects photochemical production, chemical loss, dry deposition and transport. The photodissociation of nitrogen dioxide alone is part of a null ozone cycle unless peroxy radicals regenerate nitrogen dioxide without consuming ozone.

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.

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

Atomic oxygen is oxygen present as individual atoms rather than the molecules ${\mathrm{O}}_2$ or ${\mathrm{O}}_3$. Its open-shell electronic structure makes many forms of atomic oxygen highly reactive.

Photodissociation and electrical excitation can generate it in gases. In the upper atmosphere, atomic oxygen participates in reaction networks and can interact strongly with exposed spacecraft materials. Its chemical behavior cannot be inferred by treating it as ordinary molecular oxygen at a lower concentration.

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

An oxocarbenium ion at the anomeric carbon of a cyclic glycosyl fragment. Positive charge is delocalized between that carbon and the ring oxygen. A counterion can remain nearby and influence which face a nucleophile approaches.

PbO, with a formal Pb(II) state and retained 6s2 pair. Its layered structures and asymmetric local environments contrast with a simple molecular monoxide model.

PbO2, with a formal Pb(IV) state. A common polymorph has a rutile-type extended structure with six-coordinate lead.

The amount of ATP formed per oxygen atom reduced during coupled respiration. Oxygen molecules contain two atoms, so the ATP:${\mathrm{O}}_2$ ratio is twice the conventional phosphorylation-to-oxidation ratio for the same measurements.

Proton return across a coupling membrane that is not coupled to productive ATP synthase turnover. Its absolute rate, fraction of total proton return and flux control coefficient are different quantities.

Conversion of an alkene to an epoxide. Concerted oxygen transfer from a peracid preserves the relative stereochemical information of the alkene.

Oxidation of an alcohol functional group. Primary and secondary alcohols have different accessible oxidation products, and overoxidation must be controlled.

Reaction of zirconium with steam to form zirconium oxide and hydrogen. Its rate increases strongly with temperature; oxide formation and oxygen uptake alter heat transfer and mechanical behavior.

An accumulated small solute that helps balance osmotic pressure with relatively limited interference with cellular macromolecules.

Coupled oscillators influence one another's motion through shared forces or other interactions. Two pendulums connected by a light spring provide a visible example: moving either bob changes the force on the other. For equal masses and lengths, the symmetric motion leaves the connecting spring unstretched, while the antisymmetric motion stretches it and has a higher frequency.

For a conservative linear system, the equations of motion take the form $M\ddot{\mathbf{q}}+K\mathbf{q}=0$, where $M$ is the mass matrix and $K$ the elastic stiffness matrix. A normal mode has a fixed displacement pattern $\mathbf{a}$ satisfying $K\mathbf{a}={\omega }^{2}M\mathbf{a}$. Expanding the initial motion in normal modes separates the evolution. Displacing just one pendulum generally excites several normal modes, producing beats and visible transfer of energy. A driving force excites only the modes onto which its spatial pattern has a nonzero projection.