When calculating the structure factor as a result of x-ray scattering we have that the form factor is proportional to the atomic number of the atom in question. However often the atoms (or more appropiately ions) in these crystals are held together by ionic bonds, i.e. one atom has (Z+1) electrons and one atom (Z'-1) electrons, hence when we calculate the structure factor should we use the atomic number of the atom or the no. of electrons in the ion?
In case my question isn't clear an example might help- BaF2 is ionically bonded, FCC with a basis. When calculating the structure factor do we use f(Ba)=56 and f(F)=9 as the form factors or alternatively f(Ba)=54 and f(F)=10?