In materials science, dispersion is the fraction of atoms of a material exposed to the surface. In general, D = N S / N, where D is the dispersion, N S is the number of surface atoms and N T is the total number of atoms of the material. It is an important concept in heterogeneous catalysis, since only atoms exposed to the surface can affect catalytic surface reactions. Dispersion increases with decreasing crystallite size and approaches unity at a crystallite diameter of about 0.1 nm.