The relative excess of a neutron density introduced by Bethe is applied to classify atomic nuclei with respect to a neutron halo. Calculations are based on the relativistic mean field model and are performed for 116 spherically symmetric nuclei across the periodic system of elements and for a group of deformed nuclei studied in antiproton annihilation reaction at the LEAR facility in CERN. Basic properties of the neutron excess function are discussed.

PACS numbers: 21.10.Ma, 24.10.Jv, 21.10.Jz, 21.10.Ft