Why Do Protons and Neutrons Stick Together?

Nucleus and atoms

An atom contains protons, neutrons, and electrons. The nucleus of an atom consists of bound protons and neutrons (nucleons). The negatively-charged electrons are attracted to the positively-charged protons and fall around the nucleus, much like a satellite attracted to the gravity of the Earth. The positively-charged protons repel each other and aren't electrically attracted or repelled to the neutral neutrons, so you may wonder how the atomic nucleus sticks together and why protons don't fly off.

The reason protons and neutrons stick together is the strong force. The strong force is also known as the strong interaction, color force, or strong nuclear force. The strong force is much more powerful than the electrical repulsion between protons, but the particles have to be close to each other for it to stick them together.

How the Strong Force Works

Protons and neutrons are made up of smaller subatomic particles. When protons or neutrons get close enough to each other, they exchange particles (mesons), binding them together. Once they are bound, it takes considerable energy to break them apart. To add protons or neutrons, the nucleons either have to be moving at high speed or they need to be forced together under great pressure.

Although the strong force overcomes electrostatic repulsion, protons do repel each other. For this reason, it's usually easier to add neutrons to an atom than to add protons.