# Particle physics (AS)

> CIE A-Level Physics · CIE 9702 AS Level
> Source: https://www.owlsprep.com/study/cie-9702-u11-overview/
> Weight: n/a

This unit introduces the core structure of atoms, the nature of radioactivity, and the particle model of light, forming the foundation for further nuclear and quantum physics study at A2 level.

**Prerequisites:** Foundational knowledge of energy, motion and waves from earlier AS CIE 9702 units

## Learning objectives

- Describe the structure of the atom and the key properties of protons, neutrons and electrons
- Explain the nature of isotopes and represent nuclides using standard notation
- Compare the penetrating, ionizing and physical properties of alpha, beta and gamma radiation
- Balance nuclear decay equations and describe the spontaneous random nature of radioactive decay
- Calculate photon energy and apply the photon model to basic quantum problems

## Unit at a glance

This unit moves from a macroscopic view of matter to the microscopic structure of atoms and radiation. We start by breaking down the atom into its fundamental subatomic particles, then explore variations in atomic structure through isotopes. Next, we examine the three common types of ionizing radiation emitted by unstable nuclei, before covering the rules of spontaneous radioactive decay. The unit concludes by introducing the particle nature of light (photons), connecting particle physics to core quantum concepts.

Work through the following sub-topics in order to build your understanding:
- [Atomic structure](https://www.owlsprep.com/study/cie-9702-u11-atomic-structure/) — Learn the properties, relative masses and charges of the three core subatomic particles.
- [Isotopes](https://www.owlsprep.com/study/cie-9702-u11-isotopes/) — Understand isotopes and how to represent nuclides with proton and nucleon number notation.
- [Alpha, beta and gamma radiation](https://www.owlsprep.com/study/cie-9702-u11-alpha-beta-and-gamma-radiation/) — Compare ionizing power, penetration, range and deflection of the three main types of nuclear radiation.
- [Radioactive decay](https://www.owlsprep.com/study/cie-9702-u11-radioactive-decay/) — Balance nuclear decay equations and understand the spontaneous random nature of decay.
- [Photons](https://www.owlsprep.com/study/cie-9702-u11-photons/) — Introduce the photon model of light, calculate photon energy and solve basic photon problems.

## Common pitfalls

- **Wrong:** Confusing proton number (Z) with nucleon number (A) when calculating neutron count.
  - Why it fails: Z (lower subscript) is the number of protons, while A (upper superscript) is the total number of protons + neutrons.
  - Correct: Always calculate number of neutrons as $N = A - Z$, double-checking which number corresponds to which.
- **Wrong:** Incorrectly balancing proton and nucleon numbers for beta decay.
  - Why it fails: Beta minus decay changes proton number but leaves total nucleon number unchanged, unlike alpha decay which reduces both.
  - Correct: Balance proton numbers and nucleon numbers separately on both sides of the decay equation to confirm they are equal.

## Cheatsheet

| Key Concept / Formula | Definition / Relationship |
| --- | --- |
| Number of neutrons | $N = A - Z$, $A$ = nucleon number, $Z$ = proton number |
| Specific charge | Specific charge = $\frac{\text{total charge}}{\text{total mass}}$, units: C kg$^{-1}$ |
| Alpha decay general equation | $^{A}_{Z}\text{X} \rightarrow ^{A-4}_{Z-2}\text{Y} + ^{4}_{2}\alpha$ |
| Beta minus decay general equation | $^{A}_{Z}\text{X} \rightarrow ^{A}_{Z+1}\text{Y} + ^{\;\;\;0}_{-1}\beta$ |
| Photon energy | $E = hf = \frac{hc}{\lambda}$, $h = 6.63 \times 10^{-34}$ J s |

## What's next

Start your study of this unit with the first sub-topic on atomic structure, where you will learn the fundamental properties of subatomic particles that underpin all topics in this unit. Work through each sub-topic in sequence to build your knowledge of radioactivity and the particle nature of light. Once you complete all topics here, you will be ready to progress to the next AS unit.

- [Atomic structure](https://www.owlsprep.com/study/cie-9702-u11-atomic-structure/)
- [Isotopes](https://www.owlsprep.com/study/cie-9702-u11-isotopes/)
- [Alpha, beta and gamma radiation](https://www.owlsprep.com/study/cie-9702-u11-alpha-beta-and-gamma-radiation/)

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