---
title: "Quarks, Leptons and the Conservation Laws"
description: "Protons and neutrons are built from up and down quarks, while electrons and neutrinos are leptons with nothing smaller inside them. Whether a proposed reaction can happen is settled by checking charge"
canonical: https://lightmysky.com/learn/science/quarks-leptons-and-the-conservation-laws-mt_kCGc5Neub_
source: https://lightmysky.com/learn/science/quarks-leptons-and-the-conservation-laws-mt_kCGc5Neub_.md
retrieved: 2026-09-12
---

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# Quarks, Leptons and the Conservation Laws

Protons and neutrons are built from up and down quarks, while electrons and neutrinos are leptons with nothing smaller inside them. Whether a proposed reaction can happen is settled by checking charge, baryon number and lepton number.

Subject: Science · Area: Matter & Materials · Ages 17 to 18
Page: https://lightmysky.com/learn/science/quarks-leptons-and-the-conservation-laws-mt_kCGc5Neub_

## Ready when they can

- Gives the quark content of a proton and of a neutron and checks that the charges add up
- Writes beta-minus decay at quark level, as a down quark turning into an up quark
- Tests a proposed reaction against charge, baryon number and lepton number

## Lesson: The quarks inside protons and neutrons

Protons and neutrons are built from up and down quarks. An up quark carries plus two thirds of a unit, a down quark minus one third. A proton is up, up, down: two thirds plus two thirds minus one third equals plus one. A neutron is up, down, down: two thirds minus one third minus one third equals zero.

**Example.** Watch beta-minus decay at quark level. One down quark inside a neutron flips into an up quark. The neutron becomes a proton, and the spare charge and energy leave as an electron plus an antineutrino. Quark flavour changed, yet every tally still balances.

Quarks feel the strong force and group into hadrons such as protons and neutrons. Leptons, the family holding electrons and neutrinos, feel no strong force and hide nothing smaller inside. Whether any proposed reaction can happen is settled by three tallies: charge, baryon number, and lepton number, matched on both sides.

**Tip.** Run the tallies line by line and mind the antiparticles, which flip every sign. The classic slip is writing neutron to proton plus electron and stopping there: lepton number then reads zero against one. Add the antineutrino and all three tallies close.

**Recap.** Two quark flavours build the nucleons, one quark flip drives beta decay, and three tallies judge every reaction.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Particles, Antiparticles and Annihilation](https://lightmysky.com/learn/science/particles-antiparticles-and-annihilation-mt_-Ud20HRNJz)
- [Inside the Atom: Protons, Neutrons and Electrons](https://lightmysky.com/learn/science/inside-the-atom-protons-neutrons-and-electrons-mt_WqDXAr2IkA)

## Opens up

- [Exchange Particles and Reading a Feynman Diagram](https://lightmysky.com/learn/science/exchange-particles-and-reading-a-feynman-diagram-mt_7Ka5cVpTVL)
- [The Standard Model and the Four Interactions](https://lightmysky.com/learn/science/the-standard-model-and-the-four-interactions-mt_GYB4UMS0D9)