In addition to the symmetries described above, nuclear
scientists use a number of other approximate symmetries to
describe and predict the behavior of nuclei. Examples of these
are charge independence, the expectation that, at the nuclear
level, neutron-proton systems should behave the same as proton-
proton or neutron-neutron systems; and charge symmetry, the
expectation that the interactions between two neutrons should be
the same as that between two protons. Charge symmetry can be
demonstrated by comparing "mirror nuclei," two low-mass nuclei
that have their neutron and proton numbers interchanged and
which have very similar nuclear structure, for example 13C (Z=6,
N=7) and 13N (Z=7, N=6). A related symmetry is isospin symmetry,
which is related to interchanging the roles of neutrons and
protons in certain nuclei. These three symmetries are destroyed
when the Coulomb force becomes sufficiently strong, but
nevertheless they have proved to be useful approximations in
many areas of nuclear science.