[en] In this paper we use for bound particles the concept of effective mass, which is defined as the difference between the rest mass and the binding energy of the particle. Unlike the usual mass of motion of a particle the effective mass, which includes the interaction which binds the particles, is less than the rest mass of the particle. In the case of a system of particles bound by electromagnetic interaction, e. g. the atom, using the total energy expression of the system as given by the virial theorem, we conclude that bound particles keep their identity and we give their effective masses explicitly. Then we build up the energy-momentum tensor for a system of particles in strong interaction, e. g. nucleons bound in nucleus, and we obtain the total energy expression of the system. The result leads to a different conclusion as compared to the case of electromagnetic interaction; nucleons do not entirely keep their identity in the nucleus, the expression on their effective masses reflecting collective effects. The extension of the virial theorem in quantum electrodynamics leads in the first order of the perturbation theory, which is a very good approximation to the classical expression of the bound system energy. (Author)