  In the framework of a recently developed alpha-cluster model a nucleus is
represented as a core (alpha-cluster liquid drop with dissolved excess neutron
pairs in it) and a nuclear molecule on its surface. From analysis of
experimental nuclear binding energies one can find the number of alpha-clusters
in the molecule and calculate the nuclear charge radii. It was shown that for
isotopes of one Z with growing A the number of alpha-clusters in the molecule
decreases to three, which corresponds to the nucleus 12C for even Z and 15N for
odd Z, and the specific density of the core binding energy \rho grows and
reaches its saturation value. In this paper it is shown that the value
\rho=2.55 MeV/fm^3 explains the particular number of excess neutrons in stable
nuclei.
