You already know that an atom is a nucleus of protons and neutrons, wrapped in a cloud of electrons. For a long time those looked like the end of the road — the smallest, indivisible bricks of everything. But they are not. Fire two protons at each other hard enough and they shatter into a spray of other particles. The proton, it turns out, is not a solid dot: it has parts.

So what is genuinely at the bottom? A particle is called fundamental when it is point-like — as far as any experiment can tell, it has no size and is made of nothing smaller. When physicists finally sorted the whole subatomic zoo, only two families of matter particle survived as truly fundamental: the quarks and the leptons. Protons and neutrons did not make the cut — they are built out of quarks. This page is about that deeper layer: what the world is really made of.

Quarks — and their strange fractional charges

Quarks are the fundamental particles that build the heart of every atom. The two you meet everywhere — the ones inside every proton and neutron in your body — are the up quark and the down quark. Their most surprising feature is their electric charge. Every particle you have met so far carries a whole number of charges (+1, -1, 0). Quarks break that rule: their charges are fractions of the electron's charge.

• Up quark (u) — charge +\tfrac{2}{3}.
• Down quark (d) — charge -\tfrac{1}{3}.

There are in fact six flavours of quark, grouped in three "generations" of increasing mass: up and down, then charm and strange, then top and bottom. The top row (up, charm, top) all carry +\tfrac{2}{3}; the bottom row (down, strange, bottom) all carry -\tfrac{1}{3}. Everyday matter is built entirely from the lightest pair — the heavier quarks appear only fleetingly in cosmic rays and particle accelerators, then decay.

And here is a rule with no everyday analogy: you can never catch a quark on its own. However hard you pull two quarks apart, the force between them does not fade — so quarks are always locked together in groups. This is called confinement, and it is why no experiment has ever seen a lone, free quark drifting about.

Building a proton and a neutron from quarks

Now the payoff. A proton is not fundamental — it is a bundle of three quarks: two ups and a down, written uud. A neutron is also three quarks: one up and two downs, udd. The magic is that the fractional charges add up to exactly the whole numbers we already knew.

Proton (uud):

Neutron (udd):

The proton comes out at +1 and the neutron at 0 — precisely the charges you used when balancing nuclear equations. The odd-looking thirds were hiding inside the nucleons all along, and they cancel perfectly. Swap one of the proton's up quarks for a down quark and you have turned it into a neutron — a fact we will use to explain beta decay further down.

Hadrons: baryons and mesons

Any particle built out of quarks is called a hadron. Because quarks are confined, hadrons are the only way quarks ever show themselves. They come in two kinds, depending on how many quarks are inside:

• Baryons — three quarks bound together. Protons (uud) and neutrons (udd) are the two baryons you already know; there are many heavier, short-lived ones too.
• Mesons — a quark and an antiquark (more on antiquarks below). Mesons are all unstable and quickly decay; the pion and kaon, seen streaming out of high-energy collisions, are the famous examples.

So the family tree so far: quarks are fundamental; three of them make a baryon, a quark–antiquark pair makes a meson, and both baryons and mesons are hadrons.

The two families of fundamental matter

• Quarks — six flavours (up/down, charm/strange, top/bottom), with fractional charge +\tfrac{2}{3} or -\tfrac{1}{3}. They feel the strong force and are never found alone; they clump into hadrons (baryons of three quarks, mesons of a quark and an antiquark).
• Leptons — the electron, muon and tau, plus their three neutrinos. They are fundamental, point-like, and do not feel the strong force.
• Every one of these particles also has an antiparticle with the opposite charge and the same mass.

Leptons — the other fundamental family

The leptons are the second family of truly fundamental particles, and you have known one of them all your life: the electron. Like the quarks, the leptons come in three generations, each with a charged particle and a ghostly, almost-massless partner called a neutrino:

• Charged leptons — the electron (e^-), the muon (\mu^-) and the tau (\tau^-), each with charge -1. The muon and tau are just heavier, unstable copies of the electron.
• Neutrinos — the electron-neutrino (\nu_e), muon-neutrino (\nu_\mu) and tau-neutrino (\nu_\tau), each with charge 0. Neutrinos barely interact with anything — trillions from the Sun stream through your body every second without touching a single atom.

The defining feature of a lepton is what it ignores: leptons do not feel the strong nuclear force. That is the sharpest line between the two families. Quarks are gripped by the strong force and locked inside hadrons; leptons are free, solitary particles that the strong force cannot touch. An electron zips around outside the nucleus precisely because nothing strong is holding it in.

Explore the two families

Use the switch below the figure to flip between four views. Build a proton and a neutron from their three quarks and watch the fractional charges add up to +1 and 0; then lay out all six quarks and all six leptons to compare the two fundamental families side by side.

An interactive diagram. In proton view, three discs labelled u, u, d show charges +2/3, +2/3, −1/3 summing to +1; neutron view shows u, d, d summing to 0. The quark view arranges up, charm, top (charge +2/3) above down, strange, bottom (charge −1/3); the lepton view arranges electron, muon, tau (charge −1) above their three neutrinos (charge 0).

Antiparticles and annihilation

Every particle has a shadowy twin called its antiparticle — identical in mass but with opposite charge (and opposite other quantum properties). The electron's antiparticle is the positron (e^+), charge +1 but exactly the electron's mass. The antiproton has charge -1. Even neutral particles have antiparticles: an antiquark carries the opposite fractional charge (an anti-up is -\tfrac{2}{3}), which is what pairs with a quark to make a meson.

When a particle meets its antiparticle, something dramatic happens: they annihilate. Both particles vanish and their entire mass is converted into energy — usually a burst of gamma-ray photons — following Einstein's famous relation:

Because c^2 is enormous, even a speck of matter meeting a speck of antimatter releases a colossal amount of energy. Annihilation runs the other way too: given enough energy, a collision can create a matter–antimatter pair out of pure energy. This two-way street between mass and energy is the everyday business of every particle accelerator.

Beta decay, seen from underneath

You met beta-minus decay as "a neutron turns into a proton, throwing out an electron." Now you can see why. A neutron is udd and a proton is uud, so the only difference is a single quark. In \beta^- decay, one down quark changes into an up quark:

The quark's charge jumps from -\tfrac{1}{3} to +\tfrac{2}{3} — a change of +1 — so an electron (charge -1) is emitted to keep the books balanced, along with an electron antineutrino \bar{\nu}_e. Turn one of the neutron's downs into an up and udd becomes uud: the neutron has become a proton, and the familiar beta particle flies out. The nuclear rule you already knew is just this tiny quark flip, viewed from the outside.