Two or more species
of the same element differing in their mass
numbers because of differing numbers of
neutrons in their nuclei. The nuclei must
have the same number of protons (an element
is characterized by its proton number).
Isotopes of the same element have
very similar properties because they have
the same electron configuration, but differ
slightly in their physical properties. An unstable
isotope is termed a radioactive isotope
or radioisotope
Isotopes of elements are useful in chemistry
for studies of the mechanisms of
chemical reactions. A standard technique is
to label one of the atoms in a molecule by
using an isotope of the element. It is then
possible to trace the way in which this
atom behaves throughout the course of the
reaction. For example, in the esterification
reaction:
ROH + R′COOH≒H2O + R′COOR
it is possible to find which bonds are broken
by using a labeled oxygen atom. If the
reaction is performed using 18O in the alcohol
it is found that this nuclide appears
in the ester, showing that the C–OH bond
of the acid is broken in the reaction. In labeling,
radioisotopes are detected by counters;
stable isotopes can also be used, and
detected by a mass spectrum.
Isotopes are also used in kinetic studies.
For example, if the bond between two
atoms X–Y is broken in the rate-determining
step, and Y is replaced by a heavier isotope
of the element, Y*, then the reaction
rate will be slightly lower with the Y* present.
This kinetic isotope effect is particularly
noticeable with hydrogen and
deuterium compounds, because of the
large relative difference in mass.
応用例（製薬）
Atoms of the same element can have different numbers of neutrons; the different possible versions of each
element are called isotopes.
