What Are Neutrons and Protons Made Of? Inside the Atomic Nucleus
Protons and neutrons are made of quarks held together by the strong interaction. A proton has two up quarks and one down quark among its valence quarks. A neutron has one up quark and two down quarks. Gluons carry the strong interaction between quarks.
The three valence quarks describe each particle’s basic identity, but they are only part of its structure. Protons and neutrons also contain gluons and short-lived quark–antiquark pairs. Their internal structure is described by quantum chromodynamics, the theory of the strong interaction in the Standard Model.
How quarks give protons and neutrons their charge
Up and down quarks have fractional electric charges:
- An up quark has a charge of +2/3.
- A down quark has a charge of −1/3.
Together, these charges produce the familiar totals:
| Particle | Valence quarks | Total charge |
|---|---|---|
| Proton | up, up, down | +1 |
| Neutron | up, down, down | 0 |
For a proton, the sum is +2/3 + 2/3 − 1/3 = +1. For a neutron, it is +2/3 − 1/3 − 1/3 = 0.
Charge is one property of these particles. Their mass has a different explanation. The up and down quarks’ own masses account for only a small part of a proton’s mass. Much of the rest comes from the energy of the quarks and gluons inside it. Because energy contributes to mass, the energetic motion and strong interactions within a proton or neutron account for most of their mass. The Particle Data Group compiles measured properties of these particles.
What gluons do
Gluons carry the strong interaction that binds quarks inside protons and neutrons. Quarks are confined, so they are not observed on their own. When enough energy goes into separating quarks, that energy can produce new quark–antiquark pairs instead of leaving a single free quark.
A simple diagram of a proton shows two up quarks and one down quark. This is a useful way to represent its valence quarks, though the proton’s interior is more active. Gluons and short-lived quark–antiquark pairs also contribute to its structure. Their effects depend on how the proton is probed.
Evidence for quarks
Physicists found evidence for quarks by scattering high-energy electrons from protons. In deep inelastic scattering experiments, some electrons scattered as if they had hit small, concentrated objects inside the proton. The results supported the idea that protons have internal structure and helped establish the quark model.
The simple three-quark picture accounts for the charges of protons and neutrons and fits into a broader description of hadrons, which are particles built from quarks. Baryons, including protons and neutrons, have three valence quarks. Mesons contain a quark and an antiquark.
One example: neutron beta decay
A free neutron can decay into a proton, an electron, and an antineutrino. At the quark level, one of the neutron’s down quarks changes into an up quark. The valence-quark combination changes from up, down, down to up, up, down, the combination found in a proton.
This change also explains the shift in charge. Replacing a down quark, with charge −1/3, with an up quark, with charge +2/3, increases the particle’s charge by one unit. The neutron’s charge of zero becomes the proton’s charge of +1.
The decay is possible because a free neutron is slightly heavier than a proton. Neutrons inside nuclei can behave differently, depending on the nucleus and its energy.
Key Takeaways
- A proton has two up valence quarks and one down valence quark. A neutron has one up and two down.
- Up quarks have charge +2/3; down quarks have charge −1/3. These charges add to +1 for a proton and 0 for a neutron.
- Gluons carry the strong interaction that binds quarks. Gluons and the energy inside the particles account for much of their mass.
- Protons and neutrons also contain gluons and short-lived quark–antiquark pairs.
- Deep inelastic scattering experiments provided evidence for the internal structure of protons.
