relative molecular mass (molecu-
lar weight)Symbol Mr. The ratio of
the average mass per molecule of the
naturally occurring form of an el-
ement or compound to 1/12 of the
mass of a carbon–12 atom. It is equal
to the sum of the relative atomic
masses of all the atoms that com-
prise a molecule.relative permittivity See permit-
tivity.
relativistic quantum mechanics
Quantum mechanics that is in accord
with special relativity theory. The
main equation of relativistic quan-
tum mechanics is the *Dirac equa-
tion. It is necessary to use relativistic
quantum mechanics to describe the
electronic properties of heavy atoms
and all the *Üne structure of atomic
spectra. The colour of solid gold and
mercury existing as a liquid are both
due to relativistic effects in quantum
mechanics.
relativistic quantum theory See
quantum theory.relaxation The return to the equi-
librium state of a system after it has
experienced a sudden change due to
an external inÛuence. The time for
relaxation to take place is called the
relaxation time. An example is the
average time that a system remains
in the higher energy state before it
falls to the lower energy state in nu-
clear magnetic resonance (NMR).
In general, any process involving
decay is assumed to have exponential
decay, with the relaxation time being
the time it takes for the variable to
fall from its initial value to 1/e of its
initial value. Another example of re-
laxation time is the time needed for
a gas to return to the Maxwell distri-
bution of velocities, after it has been
suddenly disturbed from that state.
release agent A substance that is
applied to surfaces to prevent themfrom sticking together. Release
agents are used in many industrial
processes, including the manufacture
of food, glass, paper, and plastics.
They include polyethene, polyte-
traÛuoroethene (PTFE), polyvinyl al-
cohol (PVA), silicones, and waxes, as
well as stearates and other glyc-
erides. They are also known as abher-
entsor parting agents.rem See radiation units.
renaturation The reconstruction
of a protein or nucleic acid that has
been denatured such that the mol-
ecule resumes its original function.
Some proteins can be renatured by
reversing the conditions (of tempera-
ture, pH, etc.) that brought about de-
naturation.renewable energy sources
Sources of energy that do not use up
the earth’sÜnite mineral resources.
Nonrenewable energy sources are
*fossil fuels andÜssion fuels. Various
renewable energy sources are being
used or investigated, including geo-
thermal energy, hydroelectric power,
nuclear fusion, solar energy, tides,
wind power, and wave power.rennin An enzyme secreted by cells
lining the stomach in mammals that
is responsible for clotting milk. It
acts on a soluble milk protein (ca-
seinogen), which it converts to the
insoluble form casein. This ensures
that milk remains in the stomach
long enough to be acted on by pro-
tein-digesting enzymes.
Reppe processes A set of related
industrial reactions of acetylene to
produce vinyl compounds, such as
HC≡CH + ROH →H 2 C=CHR
HC≡CH + RCN →H 2 C=CR(CN)
They take place at high pressure
using metal acelylide catalysts. The
processes are named after the Ger-
man chemist Walter Reppe (1892–relative molecular mass 458r