Topics

Quarks and leptons

What "fundamental" means

Protons and neutrons are not fundamental — they are built from smaller particles. The standard treats this qualitatively: you need the classification and what it explains, not the mathematics.

A fundamental (or elementary) particle has no internal structure — it is not made of anything smaller.
All known matter particles fall into exactly two families: quarks and leptons.

$The quark composition of the proton and the neutron, with their fractional charges summing to plus one and zero, and a comparison of the quark and lepton families.$

Quarks

Quarks are fundamental particles that feel the strong interaction.
They carry fractional electric charge, in units of the elementary charge $e$ :

Quark	Symbol	Charge
up	u	$+\tfrac{2}{3}$
down	d	$-\tfrac{1}{3}$

There are six quarks in total (up, down, charm, strange, top, bottom), but ordinary matter needs only up and down.
Quarks are never found alone. Separating two quarks takes so much energy that new quarks are created instead, so quarks only ever appear bound into composite particles.
A particle made of three quarks is called a baryon. Protons and neutrons are baryons.

Building the proton and the neutron

Proton = uud: $+\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$
Neutron = udd: $+\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$

The charges of the proton and the neutron are therefore not fundamental facts to memorise — they are consequences of which quarks each contains.

Quark 1:

Quark 2:

Quark 3:

Total charge +1e — a proton: the nucleus of ordinary hydrogen. Turn one d into a u and a neutron becomes a proton — that single flavour change is beta-minus decay.

Switch a single d to a u and a neutron becomes a proton. That one flavour change is beta-minus decay, seen from the inside.

Leptons

Leptons are fundamental particles that do not feel the strong interaction.
The electron is a lepton, and it is genuinely fundamental — unlike the proton, it has no internal structure.
Leptons come in three pairs: the electron, muon and tau, each with its own neutrino.
Neutrinos are electrically neutral, almost massless, and interact only very weakly — which is why they pass through matter almost undisturbed.

The two families compared

	Quarks	Leptons
Feel the strong interaction	yes	no
Electric charge	fractional ( $\pm\tfrac{1}{3}$ , $\pm\tfrac{2}{3}$ )	whole ( $-1$ , or $0$ for neutrinos)
Found on their own	never	yes
Examples in ordinary matter	up, down	electron, electron neutrino

What this picture explains

Why the nucleus holds together. The strong interaction between nucleons is a residual effect of the much stronger force binding quarks inside each nucleon — rather as the forces between neutral molecules are a leftover of the electrical forces inside them.
Beta-minus decay. A neutron (udd) becomes a proton (uud) when a down quark changes into an up quark, emitting an electron and an antineutrino. Charge is conserved because the quark's charge changes by $+1$ while an electron of charge $-1$ is created.
Why the electron is different. The electron is a lepton, feels no strong force, and is not made of quarks — which is why electrons sit outside the nucleus while quarks are locked inside nucleons.
Why neutrinos were predicted before they were found. Beta decay appeared to lose energy and momentum; a neutral, nearly massless lepton carrying them away restored both conservation laws.

Quarks and leptons

Covers AS91525

What "fundamental" means

A fundamental (or elementary) particle has no internal structure — it is not made of anything smaller.
All known matter particles fall into exactly two families: quarks and leptons.

$The quark composition of the proton and the neutron, with their fractional charges summing to plus one and zero, and a comparison of the quark and lepton families.$

Quarks

Quarks are fundamental particles that feel the strong interaction.
They carry fractional electric charge, in units of the elementary charge $e$ :

Quark	Symbol	Charge
up	u	$+\tfrac{2}{3}$
down	d	$-\tfrac{1}{3}$

There are six quarks in total (up, down, charm, strange, top, bottom), but ordinary matter needs only up and down.
Quarks are never found alone. Separating two quarks takes so much energy that new quarks are created instead, so quarks only ever appear bound into composite particles.
A particle made of three quarks is called a baryon. Protons and neutrons are baryons.

Building the proton and the neutron

Proton = uud: $+\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$
Neutron = udd: $+\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$

The charges of the proton and the neutron are therefore not fundamental facts to memorise — they are consequences of which quarks each contains.

Quark 1:

Quark 2:

Quark 3:

Total charge +1e — a proton: the nucleus of ordinary hydrogen. Turn one d into a u and a neutron becomes a proton — that single flavour change is beta-minus decay.

Switch a single d to a u and a neutron becomes a proton. That one flavour change is beta-minus decay, seen from the inside.

Leptons

Leptons are fundamental particles that do not feel the strong interaction.
The electron is a lepton, and it is genuinely fundamental — unlike the proton, it has no internal structure.
Leptons come in three pairs: the electron, muon and tau, each with its own neutrino.
Neutrinos are electrically neutral, almost massless, and interact only very weakly — which is why they pass through matter almost undisturbed.

The two families compared

	Quarks	Leptons
Feel the strong interaction	yes	no
Electric charge	fractional ( $\pm\tfrac{1}{3}$ , $\pm\tfrac{2}{3}$ )	whole ( $-1$ , or $0$ for neutrinos)
Found on their own	never	yes
Examples in ordinary matter	up, down	electron, electron neutrino

What this picture explains

Why the nucleus holds together. The strong interaction between nucleons is a residual effect of the much stronger force binding quarks inside each nucleon — rather as the forces between neutral molecules are a leftover of the electrical forces inside them.
Beta-minus decay. A neutron (udd) becomes a proton (uud) when a down quark changes into an up quark, emitting an electron and an antineutrino. Charge is conserved because the quark's charge changes by $+1$ while an electron of charge $-1$ is created.
Why the electron is different. The electron is a lepton, feels no strong force, and is not made of quarks — which is why electrons sit outside the nucleus while quarks are locked inside nucleons.
Why neutrinos were predicted before they were found. Beta decay appeared to lose energy and momentum; a neutral, nearly massless lepton carrying them away restored both conservation laws.