X-ray Form Factor
X-rays are scattered by the electron cloud of the atom and hence the scattering amplitude of X-rays increases with the atomic number, of the atoms in a sample. As a result X-rays are not very sensitive to light atoms, such as hydrogen and helium, and there is very little contrast between elements adjacent to each other in the periodic table. For X-ray scattering, in the above equation is the electron charge density about the nucleus, and the form factor the Fourier transform of this quantity. The assumption of a spherical distribution is usually good enough for X-ray crystallography.
In general the X-ray form factor is complex but the imaginary components only become large near an absorption edge. Anomalous X-ray scattering makes use of the variation of the form factor close to an absorption edge to vary the scattering power of specific atoms in the sample by changing the energy of the incident x-rays hence enabling the extraction of more detailed structural information.
Read more about this topic: Atomic Form Factor
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