What protons and neutrons are made of

8 minAS Level11.2.3

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can write the quark composition of a proton and of a neutron, and use the quark charges to show that their charges come out at $+1$ and $0$ .

Two nucleons, three quarks each

Protons and neutrons are collectively called nucleons, and neither is fundamental. Each is a bound group of three quarks:

nucleon	quarks	charge from the quarks
proton	$\text{u}\,\text{u}\,\text{d}$	$+\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$
neutron	$\text{u}\,\text{d}\,\text{d}$	$+\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$

The only difference is one quark: a proton has one down quark and a neutron has two. Swap a single up quark for a down quark and a proton becomes a neutron.

The charge is bookkeeping, not magic

You already know a proton is $+1e$ and a neutron is $0$ . The quark model does not just accommodate those numbers, it explains them.

Two up quarks give $+\tfrac{4}{3}$ , and one down quark takes away $\tfrac{1}{3}$ , leaving exactly $+1$ . In a neutron, one up quark gives $+\tfrac{2}{3}$ and two down quarks take away $\tfrac{2}{3}$ , leaving exactly $0$ .

This is a good check on any composition you are given: if the charges do not add to a whole number, the combination is not a real particle.

Worked example: Example: checking a claimed composition

A student claims that a neutron is $\text{u}\,\text{u}\,\text{d}$ . Without looking up the answer, show that this cannot be a neutron.

Have a go first?

Check 1 — writing the compositions

Multiple choiceQuestion 1 · Recall

Answer the 2 questions above to continue.