Racemic Mixture

An equimolar mixture of the two enantiomers.

Radiant Energy

Energy transmitted in the form of waves.

Radiation

The emission and transmission of energy through space in the form of particles and/or waves.

Radical

Any neutral fragment of a molecule containing an unpaired electron.

Radioactivity

The spontaneous breakdown of an atom by emission of particles and/or radiation.

Raoult’s Law

The partial pressure of the solvent over a solution is given by the product of the vapor pressure of the pure solvent and the mole fraction of the solvent in the solution.

Rare Earth Series

See lanthanide series.

Rate Constant

Constant of proportionality between the reaction rate and the concentrations of reactants.

Rate Law

An expression relating the rate of a reaction to the rate constant and the concentrations of the reactants.

Rate-Determining Step

The slowest step in the sequence of steps leading to the formation of products.

Reactants

The starting substances in a chemical reaction.

Reaction Mechanism

The sequence of elementary steps that leads to product formation.

Reaction Order

The sum of the powers to which all reactant concentrations appearing in the rate law are raised.

Reaction Quotient

A number equal to the ratio of product concentrations to reactant concentrations, each raised to the power of its stoichiometric coefficient at some point other than equilibrium.

Reaction Rate

The change in the concentration of reactant or product with time.

REDOX Reaction

A reaction in which there is either a transfer of electrons or a change in the oxidation numbers of the substances taking part in the reaction.

Reducing Agent

A substance that can donate electrons to another substance or decrease the oxidation numbers in another substance.

Reduction Reaction

The half-reaction that involves the gain of electrons.

Representative Elements

Elements in Groups 1A through 7A, all of which have incompletely filled s or p subshell of highest principal quantum number.

Resonance

The use of two or more Lewis structures to represent a particular molecule.

Resonance Structure

One of two or more alternative Lewis structures for a single molecule that cannot be described fully with a single Lewis structure.

Reversible Reaction

A reaction that can occur in both directions.

Ribonucleic Acid

A form of nucleic acid.

Root-Mean-Square Speed

An average molecular speed.