A neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass slightly larger than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave similarly within the nucleus, and each has an associated electron orbiting it outside the nucleus (forming an atom), they are both referred to as nucleons. Their properties and interactions are described by nuclear physics.
The number of protons in the nucleus defines what chemical element the atom belongs to: for example, all atoms with six protons in their nucleus are carbon atoms (atomic number 6). Similarly, all atoms with ninety-two protons in their nucleus are uranium atoms (atomic number 92). The number of neutrons defines how stable an atom is: additional neutrons generally make an atomic nuclide more stable due to extra binding energy via the strong nuclear force; however too many can make it unstable due to excess fissionable material or radiation poisoning from beta decay leading up to instability.