Neutron is a subatomic particle. Neutrons, together with subatomic particles called protons, form the nuclei of all atoms except ordinary hydrogen, whose nucleus consists of a single proton (see Proton). Neutrons and protons make up 99.9 percent of an atom's mass. A cloud of electrons around the nucleus accounts for the rest of the mass (see Electron). In the nucleus, neutrons and protons are held together by a force known as the strong interaction or the strong nuclear force.