  In previous work it was shown that the radius of nucleus R is determined by
the alpha-cluster structure and can be estimated on the number of
alpha-clusters disregarding to the number of excess neutrons. A hypothesis also
was made that the radius R_m of a beta-stable isotope, which is actually
measured at electron scattering experiments, is determined by the volume
occupied by the matter of the core plus the volume occupied by the peripheral
alpha-clusters. In this paper it is shown that the condition R_m = R restricts
the number of excess neutrons filling the core to provide the beta-stability.
The number of peripheral clusters can vary from 1 to 5 and the value of R for
heavy nuclei almost do not change, whereas the number of excess neutrons should
change with the number of peripheral clusters to get the value of R_m close to
R. It can explain the path of the beta-stability and its width. The radii R_m
of the stable isotopes with 12 =< Z =< 83 and the alpha-decay isotopes with 84
=< Z =< 116 that are stable to beta-decay have been calculated.
