Octet Rule

An atom other than hydrogen tends to form bonds until it is surrounded by eight valence electrons.

Open System

A system that can exchange mass and energy (usually in the form of heat) with its surroundings.

Optical Isomers

Compounds that are nonsuperimposable mirror images.

Orbital

See atomic orbital and molecular orbital.

Ore

The material of a mineral deposit in a sufficiently concentrated form to allow economical recovery of a desired metal.

Organic Chemistry

The branch of chemistry that deals with carbon compounds.

Organic Compounds

Compounds that contain carbon, usually in combination with elements such as hydrogen, oxygen, nitrogen, and sulfur.

Osmosis

The net movement of solvent molecules through a semipermeable membrane from a pure solvent or from a dilute solution to a more concentrated solution.

Osmotic Pressure

The pressure required to stop osmosis.

Overvoltage

The difference between the electrode potential and the actual voltage required to cause electrolysis.

Oxidation Number

The number of charges an atom would have in a molecule if electrons were transferred completely in the direction indicated by the difference in electronegativity.

Oxidation Reaction

The half-reaction that involves the loss of electrons.

Oxidation-Reduction Reaction

A process in which electrons are transferred from one reactant (reducing agent) to another (oxidizing agent). Also referred to as redox reaction.

Oxidizing Agent

A substance that can accept electrons from another substance or increase the oxidation numbers in another substance.

Oxoacid

An acid containing hydrogen, oxygen, and another element (the central element).

Oxoanion

An anion derived from an oxoacid.