- Atomic form factor
In

physics , the**atomic form factor**, or atomic scattering factor, is a measure of the scattering intensity of a wave by an isolated atom. The atomic form factor depends on the type of scattering, typically X-ray, electron or neutron. For crystals, atomic form factors are used to calculate thestructure factor of aunit cell .**X-ray form factor**X-rays are scattered by the electron cloud of the atom and hence the scattering power 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 ashydrogen andhelium , and there is very little contrast between elements adjacent to each other in theperiodic table . The x-ray form factor is defined as theFourier transform of theelectron charge density .**Electron form factor**Electron form factors can be defined as the

Fourier transform of the potential distribution of the atom. [*cite book |last=Cowley |first=John M. |authorlink=John M. Cowley |title=Diffraction Physics |year=1981 |publisher=North-Holland Physics Publishing |isbn=0-444-86121-1 |pages=p. 78*] The electron form factors are normally calculated from X-ray form factors using theMott-Bethe formula . [*cite book |last=De Graef |first=Marc |title=Introduction to Conventional Transmission Electron Microscopy |year=2003 |publisher=Cambridge University Press |isbn=0-521-62995-0 |pages=p. 113*] This formula takes into account both elastic electron-cloud scattering and elastic nuclear scattering.**Neutron form factor**Neutrons are scattered by the nucleus of the atom but due to their finite

magnetic moment they will also interact with the electron clouds of magnetic ions. Neutron form factors are usually described by the neutron scattering length, "b". The neutron scattering length may only be determined experimentally since the theory of nuclear forces is not adequate to calculate or predict "b" from other properties of the nucleus. [*Squires, "Introduction to the theory of thermal neutron scattering", Dover Publications (1996) ISBN 048669447X*] Neutron scattering lengths vary erractically between neighbouring elements in theperiodic table and even betweenisotopes of the same element. Hence isotopic substitution inneutron diffraction may be used to distinguish between individual atomic sites in a sample. Since the measured intensity of the diffraction patterns are related to the magnitude of the neutron scattering lengths, differences between subsequent diffraction patterns of compositionally identical samples containing different isoptopes may be taken to yield the individual atomic contributions.**References**

*Wikimedia Foundation.
2010.*