(e.g., the benzene radical cation C 6 H 6 +); a negatively
charged radical is called a “radical anion” (e.g., the
benzene radical anion C 6 H 6 – or the benzophenone radi-
cal anion Ph 2 C–OO–). Commonly, but not necessarily,
the odd electron and the charge are associated with the
same atom.
Unless the positions of unpaired spin and charge
can be associated with specific atoms, superscript dot
and charge designations should be placed in the order
.+ or .– suggested by the name “radical ion” (e.g.,
C 3 H 6 .+).
radical pair (geminate pair) The term is used to
identify two RADICALs in close proximity in solution,
within a solvent CAGE. They may be formed simultane-
ously by some unimolecular process, e.g., peroxide
decomposition, or they may have come together by DIF-
FUSION. While the radicals are together, correlation of
the unpaired electron spins of the two species cannot
be ignored: this correlation is responsible for the CIDNP
phenomenon.
See also GEMINATE RECOMBINATION; MOLECU-
LARITY.
radioactive isotope Atoms with the same number of
protons but different numbers of neutrons are called
isotopes. There are radioactive and nonradioactive iso-
topes, and some elements have both, such as carbon.
Each radioactive isotope has its own unique HALF-LIFE,
which is the time it takes for half of the parent radioac-
tive element to decay to a daughter product. Some
examples of radioactive elements, their stable daugh-
ters, and half-lives are: potassium 40–argon 40 (1.25
billion years); rubidium 87–strontium 87 (48.8 billion
years), thorium 232–lead 208 (14 billion years); ura-
nium 235–lead 207 (704 million years); uranium
238–lead 206 (4.47 billion years); carbon 14–nitrogen
14 (5,730 years).
See alsoELEMENT.
radioactive tracer (radioactive label) A small
amount of RADIOACTIVE ISOTOPEused to replace a non-
radioactive isotope of the element in a compound
whose path is to be traced or monitored by detection of
RADIOACTIVITY.
radioactivity The spontaneous disintegration of
atomic nuclei, which liberates particles and/or energy
(e.g., alpha or beta particles, neutrons, and gamma
rays).radiocarbon dating A dating method used to deter-
mine the age of samples containing carbon, particularly
useful to archaeologists. The method measures the dis-
integration of the^14 C atom, which is produced in the
atmosphere by cosmic ray bombardment and has a
HALF-LIFEof 5,570 years, making it useful for dating
samples in the range of 0–40,000 years. The carbon-14
method was developed by the American physicist
WILLARDFRANKLIBBYin 1947.radiolysis The cleavage of one or several bonds
resulting from exposure to high-energy RADIATION. The
term is also often used loosely to specify the method of
irradiation (pulse radiolysis) used in any radiochemical
reaction, not necessarily one involving bond cleavage.radiometric dating The use of RADIOACTIVE
ISOTOPEs and their HALF-LIVESto give absolute dates to
rock formations, artifacts, and fossils. Radioactive ele-
ments tend to accumulate in human-made artifacts,
igneous rocks, and the continental crust, and so they
are not very useful for sedimentary rocks, although in
some cases when certain elements are found, it is possi-
ble to date them using this technique. Other radiomet-
ric dating techniques used are:Electron Dating Spin Resonance
Electrons become trapped in the crystal lattice of min-
erals from adjacent radioactive material and alter the
magnetic field of the mineral at a known rate. This
technique is used for dating bone and shell, since it
does not destroy the material, such as carbonates (cal-
cium) in limestone, coral, egg shells, and teeth, by
exposing it to different magnetic fields.Fission Track Dating
This technique is used for dating glassy material like
obsidian or any artifacts that contain uranium-bearing
material such as natural or human-made glass, ceram-
ics, or stones that were used in hearths for food prepa-228 radical pair