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

Fundamental Particles

Learning Objective

Students will be able to recall that protons and neutrons are not fundamental particles and describe their composition in terms of quarks.

Why Protons and Neutrons Are Not Fundamental

• They have an internal structure that can be probed at high energies.
• Experiments (deep‑inelastic scattering) reveal point‑like constituents inside nucleons.
• They are classified as baryons, a family of particles made of three quarks.
• Fundamental particles, in the Standard Model, are the quarks, leptons, gauge bosons and the Higgs boson.

Quark Composition of Nucleons

The two nucleons that make up atomic nuclei are:

1. Proton (\$p\$)
2. Neutron (\$n\$)

Understanding the Quark Charges

Quarks carry fractional electric charges:

\$\$\begin{aligned}

u\text{-quark} &: +\frac{2}{3}e \\

d\text{-quark} &: -\frac{1}{3}e

\end{aligned}\$\$

Therefore, the total charge of a proton is:

\$Q_p = \frac{2}{3}e + \frac{2}{3}e - \frac{1}{3}e = +1e\$

And the total charge of a neutron is:

\$Q_n = \frac{2}{3}e - \frac{1}{3}e - \frac{1}{3}e = 0e\$

Key Points to Remember

• Protons and neutrons are composite particles (baryons) made of three quarks each.
• The proton’s quark composition is two up quarks and one down quark (\$uud\$).
• The neutron’s quark composition is one up quark and two down quarks (\$udd\$).
• Quark charges add up to give the observed integer charges of nucleons.
• Because they are made of quarks, protons and neutrons are not considered fundamental.