The simplest of all nuclei,
the deuteron.
The mass of the deuteron is
1875.63 MeV, while the combined mass of a neutron and a proton
is 1877.85 MeV. Thus the binding energy of a deuteron is only
2.22 MeV, the difference between [m_{n} + m_{p}]c^{2}
and M_{d}c^{2}.

Notice that the strong
nuclear potential energy between the n and p becomes repulsive
at short distances. This is because at short distances, the
internal state functions of n and p begin to overlap and this is
forbidden by the Pauli Principle, since both n and p are made of
quarks, and so both systems contain identical fermions (quarks)
in the same state. This is similar to the situation when
two atoms interact to form a molecule... again the potential has
a repulsive core, due to the Pauli Principle.

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