Nuclear force is the force that holds the particles of atomic nuclei, protons and neutrons, together. It is a fundamental force able to overcome the electric force that would otherwise force the nucleus apart within extremely small distances. Unlike the electrical force, the nuclear force reduces extremely quickly with distance, dropping to zero before it interacts with the atom's own electrons.
Opposite electrical charges attract each other, and like charges repel each other with the electric force. In every element but hydrogen, this force is present within the nucleus. Every element except hydrogen has more than one proton, and each proton has an identical positive charge. This produces an electric force pushing the protons apart. If some force did not counter the electric force, all atomic nuclei would dissipate, leaving hydrogen as the only element in existence.
Nuclear forces also hold neutrons, particles with a similar mass to protons, within the nucleus. Every element but hydrogen has neutrons in its nucleus. The number of neutrons is somewhat variable, with each different number of neutrons in a nucleus representing a different isotope of that element. There are hydrogen isotopes that possess neutrons, but they are much less common than the standard isotope without neutrons. The nuclear force is not always powerful enough to hold certain isotopes together, which results in a radioactive isotope.