In chemistry, a neutron is a subatomic particle that is one of the primary constituents of atomic nuclei. It has no electric charge and a rest mass slightly greater than that of a proton. Neutrons and protons, commonly called nucleons, are bound together in the dense inner core of an atom, the nucleus, where they account for 99.9 percent of the atom’s mass. Unlike protons and electrons, which are electrically charged, neutrons have no charge—they are electrically neutral. The neutron is a necessary constituent of any atomic nucleus that contains more than one proton, as interacting protons have a mutual electromagnetic repulsion that is stronger than their attractive nuclear interaction, so proton-only nuclei are unstable. The presence of neutrons is a must for the stability of atomic nuclei, except for the protium (hydrogen-1) nucleus. Neutrons are produced copiously in nuclear fission and fusion and are a primary contributor to the nucleosynthesis of chemical elements within stars through the process of neutron capture. The neutron plays an important role in many nuclear reactions, including the fissioning of elements like uranium-235 and plutonium-239, which is caused by their absorption of neutrons. In several nuclear reactions, the neutron plays a significant role, and neutron capture often results in the activation of neutrons, which, in turn, induces radioactivity.