Why do protons and neutrons stay together in the nucleus?
The nucleus of an atom is held together by the strong nuclear force that binds together protons and neutrons. Although the strong nuclear force is the strongest of the four fundamental forces, it acts only over very short - typically nuclear - distances. It binds together the protons and neutrons in the nucleus. It also holds together the quarks that make up those protons and neutrons and the other hadrons.
Mac Mestayer, Staff Scientist (Other answers by Mac Mestayer)