Effect of valence nucleons on RMS charge radii and surface thickness

The nucleonic promiscuity factor P=N p N n /(N p +N n ), where N p (N n ) is the number of valence protons (neutrons) or holes, is shown to be a useful and physically meaningful parameter in the description of RMS charge radii. The introduction of P in the radius formula R approximately=R c +0.07P/R c (R c ; droplet model 'closed-shell radius') reduces the chi 2 /n' value by a factor of three. The empirically found mass number dependence of the P-dependent contribution to radii is in agreement with the qualitative theoretical expectation for the average strength of the isoscalar p-n interaction. A significant correlation between surface diffusity and promiscuity as well as between diffusity and radius is pointed out.