The atomic nucleus is the center of an atom, around which orbits a cloud of electrons. The nucleus accounts for almost all of an atom's mass. It consists of protons and neutrons, collectively known as nucleons, which are bound bound together by the strong force. The number of protons in a nucleus is called its atomic number, while the number of nucleons is the mass number .Because protons are positively charged and neutrons have no charge, a nucleus has an overall positive charge; this is cancelled out, however, by the negatively charged electrons that orbit the nucleus. Under the right conditions, nuclei may undergo nuclear fission or fusion, with the release of large amounts of energy.
Following the discovery (1896) of radioactivity by Henri Becquerel, in 1911 Ernest Rutherford proposed the existence of the nucleus after identifying alpha and beta particles. For more, see Rutherford's experiment and atomic model.