What mistakes do students make with Fundamental particles?

Swapping the compositions, giving the proton udd and neutron uud. Put it right by remembering proton is uud. Adding quark charges incorrectly and getting a fractional charge for the proton. Put it right by summing +2/3, +2/3 and −1/3 to get +1. Thinking the neutron has no quarks because it is neutral. Put it right by noting it contains udd.

What are the exam tips for Fundamental particles in Physics OCR A-Level?

Memorise uud for proton and udd for neutron as a pair. Show the charge sum explicitly when asked to verify proton or neutron charge. Remember baryon number is +1 for both nucleons. Check for strange quarks before stating strangeness.

OCR · A-Level · Physics

Course version: H556 · Version from Sept 2015

(f) quark model of the proton (uud) and the neutron (udd)

Full specification wording

Fundamental particles: (f) quark model of the proton (uud) and the neutron (udd)

The proton and neutron are baryons, each built from three quarks in the simple quark model.

Checked against the OCR H556 specification (opens in a new tab) on 10 November 2026.

Point 6 of 12
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Proton quark composition is uud.
Neutron quark composition is udd.
Proton charge is +1 e from (+2/3) + (+2/3) + (−1/3).
Neutron charge is 0 from (+2/3) + (−1/3) + (−1/3).
Neither proton nor neutron contains a strange quark, so strangeness is 0.

Common mistakes and how to put them right

Swapping the compositions, giving the proton udd and neutron uud.
Put it right by remembering proton is uud.
Adding quark charges incorrectly and getting a fractional charge for the proton.
Put it right by summing +2/3, +2/3 and −1/3 to get +1.
Thinking the neutron has no quarks because it is neutral.
Put it right by noting it contains udd.
Assigning strangeness to the proton or neutron.
Put it right by noting neither contains s or s̄.

Exam tips

Memorise uud for proton and udd for neutron as a pair.
Show the charge sum explicitly when asked to verify proton or neutron charge.
Remember baryon number is +1 for both nucleons.
Check for strange quarks before stating strangeness.

Try the lesson

The start of the Fundamental particles: (f) quark model of the proton (uud) and the neutron (udd) lesson, made from checked teaching notes for this specification point.

Listen

The proton and neutron are baryons, each built from three quarks in the simple quark model. The proton has composition uud: two up quarks and one down quark. The neutron has composition udd: one up quark and two down quarks. Using quark charges of u = +2/3 e and d = −1/3 e, the proton charge is (+2/3) + (+2/3) + (−1/3) = +1 e, and the neutron charge is (+2/3) + (−1/3) + (−1/3) = 0. Both have baryon number +1 because each quark has baryon number +1/3. Neither contains a strange quark, so both have strangeness 0. These compositions explain why the proton is charged and the neutron is neutral despite similar masses. A strong answer earns marks for these points. Proton quark composition is uud. Neutron quark composition is udd. Proton charge is +1 e from (+2/3) + (+2/3) + (−1/3). Neutron charge is 0 from (+2/3) + (−1/3) + (−1/3). A common slip: Swapping the compositions, giving the proton udd and neutron uud. Instead, remembering proton is uud. Exam tip: Memorise uud for proton and udd for neutron as a pair.

See it

A top-down flowchart. A node labelled Nucleon branches into two nodes: Proton and Neutron. Proton leads to a node reading uud: two up, one down, which leads to a node reading Charge: (+2/3) + (+2/3) + (−1/3) = +1 e. Neutron leads to a node reading udd: one up, two down, which leads to a node reading Charge: (+2/3) + (−1/3) + (−1/3) = 0. Both the uud node and the udd node also lead to a shared node reading Baryon number +1; strangeness 0. — It shows the proton and neutron as nucleons with their uud and udd quark compositions, the resulting charge sums, and their shared baryon number and strangeness.

Key words

Say what each word means before you turn the card.

Nucleon quark composition

Try answering aloud or in your head before turning the card.

Strangeness of nucleons

Try answering aloud or in your head before turning the card.

See every definition

Nucleon quark composition: The proton has composition uud, two up quarks and one down quark, and the neutron has composition udd, one up quark and two down quarks.
Strangeness of nucleons: Neither the proton nor the neutron contains a strange quark, so both have strangeness 0.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. A node labelled Nucleon branches into two nodes: Proton and Neutron. Proton leads to a node reading uud: two up, one down, which leads to a node reading Charge: (+2/3) + (+2/3) + (−1/3) = +1 e. Neutron leads to a node reading udd: one up, two down, which leads to a node reading Charge: (+2/3) + (−1/3) + (−1/3) = 0. Both the uud node and the udd node also lead to a shared node reading Baryon number +1; strangeness 0.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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