accurate from 4.3 billion years (the age of the Earth)
to about 100,000 years before the present.
Radiocarbon Dating
SeeRADIOCARBON DATING.
Thermoluminescence Dating
A technique used for dating ceramics, bricks, sediment
layers, burnt flint, lava, and even cave structures like
stalactites and stalagmites, based on the fact that some
materials, when heated, give off a flash of light. The
intensity of the light is used to date the specimen and is
proportional to the quantity of radiation to which it
has been exposed and the time span since it was
heated. The technique is similar to electron spin reso-
nance (ESR). Good for dates between 10,000 and
230,000 years.
radionuclide A radioactive NUCLIDE. The term
nuclide implies an atom of specified atomic number
and mass number. In the study of biochemical pro-
cesses, RADIOACTIVE ISOTOPEs are used for labeling
compounds that subsequently are used to investigate
various aspects of the reactivity or METABOLISMof pro-
teins, carbohydrates, and lipids, or as sources of radia-
tion in IMAGING. The fate of the radionuclide in
reactive products or metabolites is determined by fol-
lowing (counting) the emitted radiation. Prominent
among the radionuclides used in biochemical research
are :^3 H,^14 C,^32 P,^35 Ca, 99mT c,^125 I, and^131 I.
radius ratio The radius of the positive ion, r+,
divided by the radius of the negative ion, r–, in an ionic
compound.
radon (Rn) A radioactive element with atomic num-
ber 86. It has several ISOTOPEs, the most important of
which has atomic weight 222 (radon 222). Radon is a
colorless, tasteless, odorless, naturally occurring inert
gas derived from the natural radioactive decay of three
radioactive isotopes: uranium 238 (Rn-222), uranium
235 (Rn-219), and thorium 232 (Rn-220). The chemi-
cally inert gas enters homes through soil, water, and
building materials and is the second leading cause of
lung cancer.
Raman spectroscopy A spectroscopic method of
analysis that utilizes the Raman effect to extract infor-
mation about the vibrational structure of a molecule.
The Raman effect is a collision between an incident
photon and a molecule, resulting in the vibrational or
rotational energy of the molecule being changed, and
the energy of the scattered photon differing from the
energy of the incident photon.Raoult’s law Physical law relating the change in
VAPOR PRESSUREof a liquid to the amount of solute dis-
solved in it. The vapor pressure of a solvent in an ideal
solution decreases as its mole fraction decreases.rate coefficient SeeORDER OF REACTION; KINETIC
EQUIVALENCE.rate constantk (SI unit: s–1 [dm^3 mol–1]n–1) See
ORDER OF REACTION.rate-controlling step(rate-determining step, rate-lim-
iting step) A rate-controlling (rate-determining or
rate-limiting) step in a reaction occurring by a compos-
ite reaction sequence is an ELEMENTARY REACTION, the
rate constant for which exerts a strong effect—stronger
than that of any other rate constant—on the overall
rate. It is recommended that the expressions rate-
controlling, RATE-DETERMINING, and rate-limiting be
regarded as synonymous, but some special meanings
sometimes given to the last two expressions are consid-
ered under a separate heading.
A rate-controlling step can be formally defined on
the basis of a control function (or control factor), CF,
identified for an elementary reaction having a rate con-
stant kiby
CF = (∂ln v/∂ln ki)Kj,kj
where vis the overall rate of reaction. In performing
the partial differentiation, all equilibrium constants Kj
and all rate constants except kiare held constant. The
elementary reaction having the largest control factor
exerts the strongest influence on the rate ν, and a step
having a CF much larger than any other step may be
said to be rate-controlling.230 radionuclide