Atomic structure. Computer model of the structure of an atom, the basic unit of matter. Electrons (dark blue balls) follow orbits (light blue) round the central nucleus. The electrons carry negative electric charge. It is the movement of electrons and their electric charge that explains chemical reactions. Different chemical elements have atoms that have a different number of electrons. Atoms are electrically neutral because the electrons' negative charge is balanced by the positive charge of protons, particles found in the nucleus. The nucleus also contains neutral particles called neutrons. Nuclear energy comes from the nucleus. This simple model is based on that of Niels Bohr.