Atoms are ([#References|references]) elementary bricks of matter. \index{atom} Atoms themselves are compound by a set of electrons in interaction with a nucleus. This system can be described by a planetary model (see fig figatome), but it should be rigorously described by using the quantum mechanics formalism (see section secatomemq). The nucleus is itself a N body problem: it is compound by nucleons, themselves compound by quarks.

Planetary model of the atom: electrons revolve round the nucleus. This description is now replaced by the probabilistic description in the frame of quantum mechanics.