## R

 R-group racemic mixture rad radioactive radioactivity radioactivity decay series radiocarbon dating Rankine Raoult's law rare earth rate constant rate equation rate law rate-determining step reactant reaction mechanism redox reaction reducing agent reducing sugar reduction reference solution relative humidity rem representative element resonance ribonucleic acid RNA roasting rubber

## R-group

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

## racemic mixture

A mixture of equal amounts of enantiomers.

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

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

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

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

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.