In this paper the concept of force experienced by the electron density is applied to chemical reactivity. The force is based upon the gradient of a local chemical potential. Closely related concepts such as force field lines and local electron flux are defined to provide insight in chemical reactivity. The time evolution of a molecular site density is also proposed. From the divergence of the force, the nucleophilic and electrophilic behaviour of atomic sites are characterized. Finally, the relations between the force and local conceptual DFT descriptors are also given.