AbstractIn an attempt to treat independently the contribution of the inner and outer electrons of the different atoms to the total x-ray scattering the selected-electron shell (SES) method was developped, representing the atomic scattering factor by a three- or four-term Gaussian polynomial and associating the Gaussian constants with the electron shells of the atom.For most atoms however and contrary to the hypothesis no simple relation can be made.Therefore, the atom is considered as a non-rigid body, built up from two distinguishable terms: the core and the outer electrons. The core is assumed to form a closed, spherically symmetric shell, making an overall description of its scattering power by a one-term Gaussian function, within some limits in reciprocal space, quite possible. The