  Various estimates of the even-odd effect of the mass shell of atomic nuclei
are considered. Based on the experimental mass values of the Ca, Sn, and Pb
isotopes, the dependence of the energy gap on the neutrons number is traced and
the relationship of this characteristic to the properties of external neutron
subshells is shown. In nuclei with closed proton shells, effects directly
related to neutron pairing and effects of nucleon shells are discussed.
