Nucleus contains protons (charged particles) and neutrons (neutral particles); the nucleons. The number of protons and neutrons determines what kind of element the atom is. Most stable isotopes have an equal amount of protons and neutrons: heavier nuclei tend to have more neutrons than protons if I recall correctly. Nuclei don’t have to be stable, they can decay into e.g. lighter, daughter, nuclei, which occurs most often for heavier nuclei. The nucleus contains most of the mass of the atom as the electrons are far lighter than the proton and neutron, and is a very small region. Nucleons are bounded together by the strong interaction and are themselves made up of quarks. A nucleus can undergo beta decay, where a neutron gets converted to a proton with emitting an electron and anti electron neutrino.