The scattering factors for X-rays are used. The atom is shown schematically as a disk.
Later examples show the actual electron density distribution.
Now lets calculate the corresponding diffraction pattern. X-rays are used in this example. The left image shows a one dimensional cut throught the center of reciprocal space. You can see that the intensity drops off continuously with increasing scattering vector. This is caused by the finite size of the electron density distribution at the atom.