Both NaCl and KCI crystallize with the fcc structure. However, the X-ray powder diffraction pattern of NaCl corresponds to the fcc structure whereas, that of KCI corresponds to simple cubic structure. This is because

Since K⁺ and Cl^- are isoelectric, some of the peaks are missing entirely. The scattering intensities for X-rays are directly related to the number of electrons in the atom. Light atoms scatter X-rays weakly, while heavy atoms scatter X-rays more effectively.