The proton and neutron are not the end of the story: each is built from smaller particles called quarks. A handful of quarks and leptons, with simple fractional charges, compose all the matter in the everyday world.
Protons and neutrons are not fundamental; they are made of quarks. The up quark has charge +2/3 e and the down quark −1/3 e. A proton is uud and a neutron is udd. Hadrons are particles made of quarks: baryons are three quarks (qqq) and mesons are a quark and an antiquark. In beta-minus decay a down quark changes to an up quark; in beta-plus an up changes to a down. Leptons, such as the electron and the neutrino, are fundamental and not made of quarks.
Combine up and down quarks and read off the total charge. Make a proton from uud, a neutron from udd, and see why a baryon needs three quarks and a meson a quark-antiquark pair.
Charges add to give the particle charge.
| Particle | Charge |
|---|---|
| up quark | charge +2/3 e |
| down quark | charge −1/3 e |
| proton = uud | +2/3 +2/3 −1/3 = +1 |
| neutron = udd | +2/3 −1/3 −1/3 = 0 |
Four quick checks tied to this lesson. Each correct answer earns XP and lights this skill on your star map.
The charge on an up quark is:
A proton is composed of the quarks:
A meson consists of:
In beta-minus decay, a down quark changes into:
Two families of matter, built very differently.
Quark charges are fractional: up is +2/3 e and down is −1/3 e, and they add to give the whole-number charge of the particle. A baryon is three quarks while a meson is a quark-antiquark pair, so do not confuse the two. Leptons (electron, neutrino) are fundamental: they are not made of quarks, unlike the proton and neutron.
Unlocks once the checks above are done. Worth more XP, written to AS Paper 1 and 2 standard.
A neutron has the quark composition udd. Its total charge is:
A baryon is a hadron made of:
Which of these is a fundamental particle (a lepton)?
This skill is now lit gold on your star map. You have finished the lessons of Topic 11, the last AS topic in this set.