Here is the diffraction pattern for a simple rectangular array of atoms. The intensity falls off sharply moving out from the center of the pattern. Note that we have chosen larger atoms here than in preset 6! As the atom size becomes comparable with the x-ray wave length, there can be partial destructive interference of waves scattered from electrons in different parts of the atom. The envelope defining the relative intensities of the different peaks for a monatomic crystal is called the atomic form factor.
Use the rA slider to change the size of the atoms and establish the connection between the atom size and the rate of fall off of the intensities in the diffraction pattern.