A symbol used to represent an unspecified side chain in an organic compound.

racemic mixture

A mixture of equal amounts of enantiomers.

rad

A unit of the energy absorbed by tissue from exposure to radiation; equal to 10⁵ J/g; short for Radiation Absorbed Dose.

radioactive

A radioactive element gives off radiation -- alpha particles, beta particles, or gamma rays -- by the disintegration of its nucleus.

radioactivity

The release of particles and/or energy from an unstable nucleus.

radioactivity decay series

A sequence of spontaneous nuclear reactions ending in the formation of a stable nucleus.

radiocarbon dating

The process of measuring the amount of carbon-14 remaining in an organic material and from the measurement calculating the age of the material.

Rankine

The temperature scale that uses as its zero point the absolute zero of the temperature scale and that has degrees the same size as the Fahrenheit scale.

Raoult's law

The statement that the partial pressure of a solute in equilibrium with a solution is equal to the mole fraction of the solute times the vapor pressure of the pure solute.

rare earth

One of the metals appearing either in the third column of the periodic table or among the lanthanides.

rate constant

In the differential rate equation, the proportionality constant between the rate and the products of concentrations that determine how the rate varies with concentration.

rate equation

An equation that describes the rate of a reaction as a function of the concentrations of reactants (and products), also called rate law.

rate law

An equation which describes the rate of a reaction as a function of the concentrations of reactants (and products), also called differential rate equation.

rate-determining step

The mechanistic step in a reaction that by its relatively slow rate limits the overall rate of a reaction.

reactant

A substance consumed by a chemical reaction.

reaction mechanism

The set of elementary steps by which a reaction occurs; the individual steps correspond to molecular collisions.

redox reaction

An oxidation-reduction reaction; the term "redox" is obtained from the first few letters of "reduction" and "oxidation."

reducing agent

A chemical species that donates electrons in order to reduce another species. In the process the reducing agent is itself oxidized.

reducing sugar

A sugar that reacts with copper(II) in a test for available aldehyde groups.

reduction

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

reference solution

A reference solution used in spectroscopy is a solution containing all components except the absorbing species of interest. For example, if one wanted to measure the concentration of iodine in the solvent heptane, one would use a reference solution which contained only pure heptane. Iodine is the species which absorbs light and should NOT be present in the reference solution.

relative humidity

The ratio of the vapor pressure of water in the air to the equilibrium vapor pressure of water at the same temperature.

rem

A unit of radiation that takes into account the biological effectiveness of the radiation; short for Roentgen Equivalent in Man.

representative element

One of the elements in the area of the periodic table over which the s and p atomic orbitals are being filled; also called main-group element.

resonance

The existence of two or more descriptions of the covalent bonding in an ion or molecule; the bonding is considered to be an average superposition of the individual descriptions.

ribonucleic acid

A biological polymer of nucleotides derived that is involved in protein synthesis.

RNA

Abbreviation for ribonucleic acid; a biological polymer of nucleotides derived that is involved in protein synthesis.

roasting

Heating in the presence of air or oxygen; most commonly used in converting natural metal sulfide ores to oxides as first step in recovery of the metal from its ore.

rubber

A tough, elastic polymer obtained from the juices of certain tropical plants; a synthetic material having similar properties.