A set of eight electrons in the valence shell of an atom.

olefin

A hydrocarbon containing one or more double bonds between carbon atoms. Also called alkene.

optical isomer

One of two or more compounds that have the same chemical composition and that have the same atoms joined to each other, but that oriented in space differently.

optically active

Describing a substance having the ability to rotate the plane of polarized light passed through a solution of the substance.

orbital

A mathematically defined region of electron density around one or more atoms.

ore

Minerals from which metals are extracted.

organic

In chemistry, organic refers to a species containing carbon. Certain small ions and compounds containing carbon (such as carbon dioxide) are usually not considered to be organic, but rather are classed as inorganic.

ortho

Describing the relationship between two groups attached to carbon atoms on a benzene-like ring that have no carbon atoms between them.

osmosis

The passage of solvent through a semipermeable membrane from a region of higher concentration of solute to a region of lower concentration of solute.

osmotic pressure

The pressure required to prevent solvent from passing through a semipermeable membrane from a region of higher concentration of solute to a region of lower concentration of solute.

oxidation

That part of a chemical reaction in which a reactant loses electrons; simultaneous reduction of a different reactant must occur.

oxidation number

A formally defined charge that an atom in a compound or ion would have if the compound or ion existed entirely of monatomic ions.

oxidation state

A formally defined charge that an atom in a compound or ion would have if the compound or ion existed entirely of monatomic ions.

oxidation-reduction reaction

A reaction in which one reactant is oxidized (loses electrons) and one reactant is reduced (gains electrons), also called "redox reaction."

oxidizing agent

A chemical species that accepts electrons in order to oxidize another species. In the process the oxidizing agent is itself reduced.

oxoacid

An acid containing hydrogen, oxygen, and another element. Also called oxyacid.

oxoanion

An anion containing oxygen and another element. Also called oxyanion.

oxyacid

An acid containing hydrogen, oxygen, and another element. Also called oxoacid.

oxyanion

An anion containing oxygen and another element. Also called oxoanion.