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For chemistry, the atom has three ingredients: protons, neutrons and electrons. But protons and neutrons aren’t the end of the story. Each is built from smaller particles called quarks, held together by particles called gluons. Electrons, by contrast, really do seem to be fundamental. This article explains what’s inside a proton, how scientists found out, and why, remarkably, most of the mass of everything around you comes not from the quarks themselves but from the energy binding them.
The Standard Model in brief
Physicists describe the fundamental building blocks of matter with the Standard Model of particle physics. The matter particles fall into two families:
- Quarks: six types (up, down, charm, strange, top, bottom). Only up and down quarks appear in ordinary matter.
- Leptons: six types, including the electron and the nearly massless neutrinos.
Forces are carried by other particles: photons (electromagnetism), gluons (the strong force), and W and Z bosons (the weak force responsible for beta decay). The Higgs boson, discovered in 2012, is linked to how fundamental particles get their mass.
For all of chemistry, you need only three fundamental particles: up quarks, down quarks and electrons.
Quarks and their strange charges
Quarks carry fractional electric charges:
| Quark | Charge |
|---|---|
| up (u) | +⅔ |
| down (d) | −⅓ |
Protons and neutrons each contain three quarks:
| Particle | Quarks | Total charge |
|---|---|---|
| proton | u u d | +⅔ + ⅔ − ⅓ = +1 |
| neutron | u d d | +⅔ − ⅓ − ⅓ = 0 |
That’s why the proton has exactly the same size of charge as the electron (but positive), and why the neutron is neutral. See the proton and the neutron.
Gluons and the strong force
Quarks carry a property called colour charge (nothing to do with visible colour; it’s just a name). The force between colour charges is the strong force, carried by gluons.
The strong force behaves very differently from gravity or electromagnetism:
- At very short distances inside the proton, quarks move almost freely.
- As quarks are pulled apart, the force between them doesn’t weaken; it stays roughly constant, like a stretched elastic band that never snaps.
Why quarks are never found alone
If you try to pull a quark out of a proton, the energy stored in the “gluon string” grows as you pull. Before the quark escapes, there’s enough energy to create a new quark–antiquark pair (E = mc² in action). The string snaps, but each end grabs one of the new quarks, forming new composite particles. So you never get a lone quark. This is called confinement. Quarks are only ever seen inside composite particles such as protons, neutrons and short-lived particles called mesons.
The strong force between nucleons
The force that holds protons and neutrons together in the nucleus is a “leftover” of the strong force between their quarks, a bit like how neutral molecules still attract each other through leftover electrical forces. This residual strong force (the nuclear force) is very strong but acts only over about a femtometre. See inside the atomic nucleus.
Where does the proton’s mass come from?
Here’s one of the most surprising facts in physics:
- An up quark has a mass of about 2 MeV/c²; a down quark about 5 MeV/c².
- Two ups and a down add up to only about 9 MeV/c².
- But the proton’s mass is about 938 MeV/c².
So the quarks’ own masses make up only about 1% of the proton’s mass. The other 99% comes from the energy of the quarks moving at high speed inside the proton and the energy of the gluon fields binding them, converted to mass by E = mc².
In other words, most of the mass of your body, and of every object you’ve ever held, is energy locked inside protons and neutrons.
How quarks were discovered
Predicting them (1964)
By the early 1960s, physicists had found dozens of short-lived particles in cosmic rays and accelerators, a confusing “particle zoo”. In 1964, Murray Gell-Mann and, independently, George Zweig showed that the patterns could be explained if these particles were built from a few smaller constituents. Gell-Mann named them quarks, borrowing the word from a line in James Joyce’s Finnegans Wake (“Three quarks for Muster Mark!”), which fitted neatly with protons containing three.
At first, many physicists regarded quarks as a mathematical bookkeeping device rather than real particles.
Seeing them (late 1960s)
At the Stanford Linear Accelerator Center (SLAC), physicists fired very high-energy electrons at protons, an experiment similar in spirit to Rutherford’s gold foil experiment. Just as Rutherford found that some alpha particles bounced back from a hard nucleus, the SLAC team found that electrons scattered from the proton as if it contained small, hard, point-like objects. This deep inelastic scattering was the first direct evidence that quarks are real. Jerome Friedman, Henry Kendall and Richard Taylor received the 1990 Nobel Prize in Physics for this work.
Heavier quarks
The other four quarks (charm, strange, top and bottom) are heavier and form particles that decay in tiny fractions of a second. They’re produced in cosmic-ray collisions and particle accelerators but play no part in ordinary chemistry. The top quark, discovered in 1995, is the heaviest known fundamental particle, with a mass similar to an atom of gold.
Why chemists can (mostly) ignore quarks
Chemistry involves energies of a few electronvolts, the energies of electrons moving between orbitals and forming bonds. Changing anything inside a proton or neutron requires millions of electronvolts. So, for chemical purposes:
- Protons and neutrons behave as simple particles with fixed charges and masses.
- The number of protons (atomic number) fixes the element.
- The details of quarks and gluons never show up in chemical reactions.
Quarks do matter indirectly: the nuclear masses used in mass defect and binding energy calculations, and the conversion of neutrons to protons in beta decay (a down quark turning into an up quark), both come from quark-level physics. See alpha, beta and gamma radiation.
Is the electron made of anything?
As far as experiments can tell, no. The electron behaves as a point particle with no internal structure, down to the smallest distances ever probed (below about 10⁻¹⁸ m). It’s one of the truly fundamental particles, along with quarks. See the electron.
A neat way to remember it: chemistry is the physics of electrons; nuclear physics is the physics of protons and neutrons; particle physics is the physics of quarks.
Key takeaways
- Protons (uud) and neutrons (udd) are made of up and down quarks with charges of +⅔ and −⅓.
- Gluons carry the strong force, which confines quarks: they’re never found on their own.
- Quark masses supply only about 1% of the proton’s mass; the rest comes from the energy of quarks and gluons.
- Quarks were predicted in 1964 and detected by deep inelastic scattering of electrons in the late 1960s.
- For chemistry, protons and neutrons can be treated as simple particles; electrons appear to be truly fundamental.
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