X-rays are scattered by the electron cloud of the atom and hence the scattering amplitude of X-rays increases with the atomic number, formula_11, 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, formula_12 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.