  The isoscalar and isovector particle densities and the surface tension
coefficients at the average binding energy are used to derive analytical
expressions of the neutron skin thickness and the isovector stiffness of sharp
edged proton-neutron asymmetric nuclei. For most Skyrme forces these quantities
are significantly larger than the well known ones. Using the analytical
isovector surface energy constants in the framework of the hydrodynamical and
the Fermi-liquid droplet models the mean energies and the sum rules of the
isovector giant dipole resonances are in fair agreement with the experimental
data.
