What is the primary force responsible for the attraction between protons and neutrons in the nucleus?

The primary force responsible for the attraction between protons and neutrons in the nucleus is the strong nuclear force. This force acts at very short ranges, typically less than a femtometer (1 x 10^-15 meters), and is extremely powerful compared to other fundamental forces. It is the strongest of the four fundamental forces in nature and is essential for holding the nucleus together, overcoming the repulsive electromagnetic force that exists between the positively charged protons.

While the electromagnetic force acts between charged particles like protons, it actually works against the stability of the nucleus due to the repulsion between protons. The gravitational force is significant on large scales like planets and stars, but it is negligible at the scale of subatomic particles. The weak nuclear force is involved in processes such as radioactive decay but does not play a role in binding protons and neutrons together in the nucleus.

Thus, the strong nuclear force is the key interaction that ensures protons and neutrons can stay closely packed within the nucleus, providing the binding energy that stabilizes atomic structure.