A chemical gradient is a concentration gradient that shows a slow or gradual change in molecular density. It is a concept in mathematics that is used to measure the change of a scalar function over a set distance.
Active particles almost always move from areas with high concentrations to areas with low concentrations. When a high concentration of particles is separated and moves away from an area with high concentration, a chemical gradient is created. Chemical gradients are not to be confused with electrical gradients. Electrical gradients are only formed when there is a separation of two molecules with opposite charges.