The contribution of the shape factor sigma(r) to the electronegativity is established. This shape factor, measuring the shape of the electron density rho(r), is defined as sigma(r) = rho(r)/N and integrates to unity : integral sigma(r) dr = 1. A new response function is thereby introduced, giving the change of the energy of an atomic or molecular system with respect to the shape factor sigma(r). Finally, a physical interpretation is given for this response function.