What holds all the protons and neutrons in the nucleus?

It is called the residual nuclear force. Its force carriers, pions, are themselves composite particles, made up of two quarks each (protons and neutrons are each made up of three quarks). read more

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 like 10^-13 centimeters. read more

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. read more

Protons and neutrons are in the center (nucleus) of the atom. You may want to mention that hydrogen is the only atom that usually has no neutrons. The nucleus of most hydrogen atoms is composed of just 1 proton. read more