How are neutrons and protons held together in a nucleus?

Neutrons and protons are held together in a nucleus by nuclear forces.

Protons and neutrons are both nucleons, and the nuclear force acts between them over extremely short distances. It is a residual effect of the strong interaction, the fundamental force that binds quarks together inside protons and neutrons. At nuclear distances, this attractive force can bind nucleons into a stable atomic nucleus.

The force must overcome an important obstacle: protons all have positive electric charge, so they repel one another through the electromagnetic force. The nuclear force is stronger at distances of roughly a femtometre, allowing it to hold protons and neutrons together. However, it becomes much weaker beyond a few femtometres, which helps explain why nuclei have limited sizes.

A common mix-up is calling the binding force simply “the strong force.” The strong interaction is the underlying fundamental interaction, while “nuclear force” usually refers to the residual force between whole nucleons. Nuclear stability also depends on the balance of protons and neutrons, not just on attraction alone.