Protons and neutrons are held together in a nucleus of an atom by the strong force. The strong force gets it name by being the strongest attractive force. It is 137 times more powerful than electromagnetic which by the way cannot hold neutrons to protons because neutrons are not charged. It is 100,000 times more powerful than the weak force and 6,000 billion billion billion billion (6 followed by 39 zeroes) times more powerful than gravity which by the way has almost no effect at atomic scales. Hope I helped u.. Plz mark as the BRAINLIEST answer..