  Recently an alpha-cluster model based on the pn-pair interactions with using
the isospin invariance of nuclear force has been proposed. According to the
model the excess neutron pairs fill out the free space in the core determined
by the difference in the charge and matter radii of the alpha-clusters. Then
the number of excess neutrons in beta-stable nuclei depends on the number of
the core alpha-clusters. In such a representation the specific density of
binding energy of core is the only parameter to fit the experimental binding
energies of beta-stable nuclei and it turned out to be a constant value equal
to 2.57 MeV/fm^3. Knowing the value allows one to estimate the size of a
nucleus from its experimental binding energy.
