# Why Protons and Neutrons Stick Together in the Atomic Nucleus

Protons and neutrons aren’t electrically attracted to each other, so have you ever wondered why they stick together in the atomic nucleus? The strong interaction produces the strong nuclear force, which holds matter together when the particles are close enough.

### The Strong Force Keeps the Nucleus Together

The strong force is one of the four fundamental forces. The other three are electromagnetism, the weak force, and gravity. Particles need to be very close to one another to feel the strong force, but at a distance of one femtometer (10−15 m), the strong force is 137 times more powerful than electromagnetism, a million times stronger than the weak force, and 100 undecillion (1038) times stronger than gravity.

At a range of 1 femtometer to 3 femtometers, the strong force binds nucleons (protons and neutrons) together to form the atomic nucleus. It’s also the force that forms protons and neutrons from their quarks. At a distance of about 0.8 femtometers, massless particles called gluons carry the force to make protons and neutrons. Around 99% of the mass of a proton or neutron results from the strong force field energy. The quarks only contribute about 1% of the measured mass!

### Protons, Neutrons, and Electrons in an Atom

Consider an atom:

Electrons bear a negative electrical charge, so they are attracted to the protons in the atomic nucleus. But, electrons are tiny and move very quickly. They fall around the nucleus, much like a satellite falls around Earth. It’s theoretically possible for electrons to even pass through the nucleus, but they can’t stick. While the nucleus is more dense than the rest of the atom, it actually contains very little matter and the electrons move too fast to do more than visit.

Protons and neutrons aren’t electrically attracted to each other, but when they get close enough they can exchange particles called mesons and become bound together by the strong force. Considerable energy is required to separate nucleons once they are bound.

Protons electrically repel each other because of their positive charge. If they approach each other at high speed or are forced together by pressure, they get close enough for the strong force to bind them together. The electrical repulsion still exists, so it’s easier to add neutrons to an atomic nucleus than it is to add protons.

### References

• Christman, J.R. (2001). “MISN-0-280: The Strong Interaction” (PDF). Project PHYSNET
• Griffiths, David (1987). Introduction to Elementary Particles. John Wiley & Sons. ISBN 978-0-471-60386-3.
• Halzen, F.; Martin, A.D. (1984). Quarks and Leptons: An Introductory Course in Modern Particle Physics. John Wiley & Sons. ISBN 978-0-471-88741-6.
• Kane, G.L. (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 978-0-201-11749-3.