  We analyze the influence of the single-particle structure on the neutron
density distribution and the neutron skin in Ca, Ni, Zr, Sn, and Pb isotopes.
The nucleon density distributions are calculated in the Hartree-Fock+BCS
approach with the SLy4 Skyrme force. A close correlation is found between the
quantum numbers of the valence neutrons and the changes in the position and the
diffuseness of the nuclear surface, which in turn affect the neutron skin
thickness. Neutrons in the valence orbitals with low principal quantum number
and high angular momentum mainly displace the position of the neutron surface
outwards, while neutrons with high principal quantum number and low angular
momentum basically increase the diffuseness of the neutron surface. The impact
of the valence shell neutrons on the tail of the neutron density distribution
is discussed.
