The chemical potential equalization principle is used to define the fukui function of the kth atom in a molecule A with N-A electrons, f(Ak) q(Ak)(N-A) - q(Ak)(N-A-1), for electrophilic attack, and f(Ak)(+) = q(Ak)(N-A + 1) - q(Ak)(N-A), for nucleophilic attack, the softness of an atom in a molecule, s(Ak)(+/-) = s(A)(+/-)f(Ak)(+/-), where s(A) is the global softness, and the hardness of an atom in a molecule n(Ak)(+/-) = 1/s(Ak)(+/-) (q(Ak) is the charge of the kth atom in the molecule). With these definitions it is shown that, in general, the reactive site of a molecule is located at the atom with the largest value of the fukui function (the softest atom), however, the interaction between two chemical species will not necessarily occur through their softest atoms, but rather through those whose fukui functions are approximately equal.