The highly different probability of ionization of each shell of a given element by X-ray photons is discussed. The chemical shift of ionization energies I indicates differing Hartree potential. The multiple I of an inner shell in systems having positive spin quantum number S are studied in 11 high-spin nickel (ll) compounds. The choice of scotch tape as internal standard and the Madelung potential in almost ionic compounds are treated, as well as the relation between optical electronegativities and I of valence electrons.