Neutrons are located in the nucleus of an atom, alongside protons. The nucleus is a very small, dense region at the centre of an atom that contains most of its mass. Neutrons are subatomic particles that have no electric charge and are neutral. They are bound together with protons by the strong nuclear force, which is much stronger than the electromagnetic force that binds electrons to the nucleus. The number of neutrons in an atom can vary, even for atoms of the same element. These variations are called isotopes.