In multiple-electron atoms the effective charge of nucleus for a given electron, is reduced by the presence of other electrons (including those from more external shells, against the shell theorem): Z_eff = Z - s where the screening constant s depends on Z and the concerning orbital. It is usually calculated by semi-empirical so called Slater's rules: https://en.wikipedia.org/wiki/Slater's_rules I have tried to find some experimental values. On page 286 of 1936 English translation of Arnold Sommerfeld's "Atomic structure and spectral lines" there is a clear figure (on the left below) with dots suggesting experimental values (but I couldn't find it being explicitly written). Wikipedia article cites 1967 "Atomic Screening Constants from SCF Functions. II. Atoms with 37 to 86 Electrons" by Clementi, Raimondi, Reinhardt ( http://scitation.aip.org/content/aip/journal/jcp/47/4/10.1063/1.1712084 ) which contains Hartee-Fock calculations of screening constants (figure on the right) - unfortunately it doesn't seem to refer to any experiment (?) https://dl.dropboxusercontent.com/u/12405967/sceen.jpg [Broken] These two figures have some essential differences (including order!) - could anybody refer to some better experimental results?