recall that protons and neutrons are not fundamental particles and describe protons and neutrons in terms of their quark composition

Fundamental Particles – Cambridge IGCSE/A‑Level (9702) – Section 11.2

Learning Objective

Students will be able to:

• Recall that protons and neutrons are not fundamental particles.
• Describe protons and neutrons in terms of their quark composition.
• Identify all six quark flavours, their electric charges and the corresponding antiquarks.
• Classify nucleons as baryons (three‑quark hadrons) within the Standard Model.

1. The Quark Family

Quarks are elementary (fundamental) particles. They also carry a colour charge, which means they feel the strong nuclear force and are permanently confined inside hadrons.

2. Hadrons – the Particles that Feel the Strong Force

A hadron is any particle that experiences the strong nuclear interaction. Hadrons are made of quarks and are always colour‑neutral (the colour charges cancel).

• Baryons – three quarks (qqq). Protons and neutrons belong to this family.
• Mesons – a quark–antiquark pair (q̄q). Mentioned for completeness; they are not required for the present learning objective.

3. Why Protons and Neutrons Are Not Fundamental

• Deep‑inelastic scattering experiments (e.g., SLAC electron‑proton scattering) revealed point‑like constituents inside nucleons.
• These point‑like constituents are the quarks listed above, showing that nucleons have internal structure.
• Consequently, protons and neutrons are classified as baryons, not as elementary particles.

4. Quark Composition of the Nucleons

The two nucleons that make up atomic nuclei are:

Charge Calculation

Using the fractional charges of the up‑ and down‑quarks:

\[

\begin{aligned}

Q_{\text{proton}} &= \frac{2}{3}e + \frac{2}{3}e - \frac{1}{3}e = +1e,\\[4pt]

Q_{\text{neutron}} &= \frac{2}{3}e - \frac{1}{3}e - \frac{1}{3}e = 0e.

\end{aligned}

\]

5. Colour Confinement (Brief Context)

• Quarks carry one of three colour charges (red, green, blue). The strong force binds them together so that the overall colour of a hadron is neutral (white).
• Because of colour confinement, isolated quarks are never observed; they always appear inside baryons or mesons.

6. Key Points to Remember

• Quarks (u, d, s, c, b, t) are fundamental particles; each has a corresponding antiquark with opposite charge.
• Hadrons are particles that feel the strong interaction; baryons are three‑quark hadrons.
• Proton = two up quarks + one down quark (uud).
  Neutron = one up quark + two down quarks (udd).
• The fractional charges of the constituent quarks add up to the integer charges observed for nucleons.
• Quarks are permanently confined by the strong force; they cannot be isolated.