R
R-group
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A symbol used to represent an unspecified side chain in an organic compound.
racemic mixture
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A mixture of equal amounts of enantiomers.
rad
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A unit of the energy absorbed by tissue from exposure to radiation; equal to 105 J/g; short for Radiation Absorbed Dose.
radioactive
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A radioactive element gives off radiation -- alpha particles, beta particles, or gamma rays -- by the disintegration of its nucleus.
radioactivity
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The release of particles and/or energy from an unstable nucleus.
radioactivity decay series
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A sequence of spontaneous nuclear reactions ending in the formation of a stable nucleus.
radiocarbon dating
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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
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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
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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
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One of the metals appearing either in the third column of the periodic table or among the lanthanides.
rate constant
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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
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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
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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
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The mechanistic step in a reaction that by its relatively slow rate limits the overall rate of a reaction.
reactant
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A substance consumed by a chemical reaction.
reaction mechanism
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The set of elementary steps by which a reaction occurs; the individual steps correspond to molecular collisions.
redox reaction
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An oxidation-reduction reaction; the term "redox" is obtained from the first few letters of "reduction" and "oxidation."
reducing agent
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A chemical species that donates electrons in order to reduce another species. In the process the reducing agent is itself oxidized.
reducing sugar
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A sugar that reacts with copper(II) in a test for available aldehyde groups.
reduction
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That part of a chemical reaction in which a reactant gains electrons; simultaneous oxidation of a different reactant must occur.
reference solution
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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
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The ratio of the vapor pressure of water in the air to the equilibrium vapor pressure of water at the same temperature.
rem
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A unit of radiation that takes into account the biological effectiveness of the radiation; short for Roentgen Equivalent in Man.
representative element
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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
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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
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A biological polymer of nucleotides derived that is involved in protein synthesis.
RNA
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Abbreviation for ribonucleic acid; a biological polymer of nucleotides derived that is involved in protein synthesis.
roasting
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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
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A tough, elastic polymer obtained from the juices of certain tropical plants; a synthetic material having similar properties.
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