In simple terms
A friendly intro before the formal notes — no formulas yet.
Atoms, nuclei and radiation
Cambridge 9702 Paper 2 — Atoms, nuclei and radiation (11.1). Senpai Corner diagram-backed pilot with premium structure and live visuals.
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11.1 Atoms, nuclei and radiation.
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α-particle scattering provided the proof of the structure of the atom.
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When α-particles are fired at thin gold foil, most of them go straight through but a small number bounce straight back.
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Atoms of all elements are made up of three types of particles: protons, neutrons, and electrons.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 11.1.1
Infer from the results of the -particle scattering experiment the existence and small size of the nucleus
- 11.1.2
Describe a simple model for the nuclear atom to include protons, neutrons and orbital electrons
- 11.1.3
Distinguish between nucleon number and proton number
- 11.1.4
Understand that isotopes are forms of the same element with different numbers of neutrons in their nuclei
- 11.1.5
Understand and use the notation for the representation of nuclides
- 11.1.6
Understand that nucleon number and charge are conserved in nuclear processes
- 11.1.7
Describe the composition, mass and charge of -, - and -radiations (both (electrons) and (positrons) are included)
- 11.1.8
Understand that an antiparticle has the same mass but opposite charge to the corresponding particle, and that a positron is the antiparticle of an electron
- 11.1.9
State that (electron) antineutrinos are produced during decay and (electron) neutrinos are produced during decay
- 11.1.10
Understand that -particles have discrete energies but that -particles have a continuous range of energies because (anti)neutrinos are emitted in -decay
- 11.1.11
Represent - and -decay by a radioactive decay equation of the form
- 11.1.12
Use the unified atomic mass unit (u) as a unit of mass
Explore the concept
Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.
Step-synced diagram — highlights what to look for in the simulation above.
Rutherford scattering showed most mass is…
Rutherford scattering showed most mass is in a tiny, positive nucleus.
6 more simulations for this topic — run them in the Simulations section below
Simulations
Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.
6 simulations · 4 to start with
Start herein this order — each one shows a different piece of the topic
- PhETStart here · 19702 11.1 · 23.2 · IB E.4
Build a Nucleus
Add protons and neutrons to a nucleus and watch which decays it undergoes.
Why this one: Build carbon-14 and uranium-238 and read the new Z and A after beta and alpha decay.
Try this
- Build carbon-14 (6p, 8n) — the sim shows β⁻ decay; what does it become?
- Build a heavy nucleus like uranium-238 — watch the α decay and the new Z and A.
- On the “Chart”, trace a decay chain across the valley of stability.
Look for α: Z−2, A−4. β⁻: Z+1, A same. Too many neutrons → β⁻; too heavy → α.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
- 3JCN PhysicsStart here · 29702 23.2 · 9702 11.1 · IB E.3
Alpha Decay
Watch an alpha particle tunnel out of a heavy nucleus
Why this one: Count the change: the nucleus loses two protons and four nucleons.
Try this
- Watch the alpha particle tunnel out of the nucleus.
- Count the change in proton and nucleon numbers.
Look for The nucleus loses two protons and four nucleons.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- 3JCN PhysicsStart here · 39702 23.2 · 9702 11.1 · IB E.3
Beta Decay
Watch a neutron become a proton with beta and antineutrino emission
Why this one: A neutron becomes a proton, an electron and an antineutrino; Z rises by one and A is unchanged.
Try this
- Watch the neutron become a proton.
- Identify the beta particle and the antineutrino.
Look for Nucleon number stays the same while proton number rises by one, with charge conserved.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- 3JCN PhysicsStart here · 49702 23.2 · 9702 11.1 · IB E.3
Gamma Decay
See an excited nucleus de-excite by emitting a gamma photon
Why this one: Gamma emission changes neither Z nor A, only the energy of the nucleus.
Try this
- Watch the excited nucleus emit the gamma photon.
- Compare the nucleus before and after.
Look for Gamma emission changes neither proton nor nucleon number, only the energy of the nucleus.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
More simulations2 more on this topic — core ones first
- PhETCoreJava · best on a laptop9702 23.2 · 11.1–11.2 · IB E.3
Beta Decay
Watch hydrogen-3 or carbon-14 nuclei undergo beta decay, emitting an electron and an antineutrino, with half-life timing.
Try this
- Watch one hydrogen-3 nucleus decay — count the particles that leave.
- Switch to carbon-14 — it waits much longer on average.
- Start many nuclei and read the time when half have decayed.
Look for In β⁻ decay a neutron becomes a proton plus an electron and an antineutrino; Z rises by one.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
- 3JCN PhysicsCore9702 11.1 · IB E.1
Rutherford Scattering
Fire alpha particles at gold foil; count scattering angles and infer the nucleus
Try this
- Fire alpha particles at the foil and count the scattering angles.
- Compare how many pass straight through with how many bounce back.
Look for Most alphas pass through undeflected and a tiny fraction scatter at large angles, so the positive charge is concentrated in a small nucleus.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
Key formulas
Tap any symbol to reveal exactly what it means and its units.
Full topic notes
Formal explanation with the rigour you need for the exam.
The Atomic Foundation: Rutherford and Structure
For centuries, the atom was considered an indivisible unit. However, Ernest Rutherford's pivotal gold foil experiment shattered this view. By firing positively charged alpha particles at a thin gold sheet, he observed that most passed straight through, but a small fraction were deflected significantly, some even bouncing back. This led to the revolutionary conclusion that an atom is mostly empty space with a tiny, dense, positively charged nucleus at its centre. This nucleus is held together by the strong nuclear force, which overcomes the electrostatic repulsion between the positively charged protons.
11.1 Atoms, nuclei and radiation.
α-particle scattering provided the proof of the structure of the atom.
When α-particles are fired at thin gold foil, most of them go straight through but a small number bounce straight back.
Atoms of all elements are made up of three types of particles: protons, neutrons, and electrons.
The proton number (Z) is the number of protons in an atom, defining the element.
Almost all the mass of an atom is concentrated in the nucleus, which is held together by the strong nuclear force.
Understanding Isotopes and Specific Charge
Not all atoms of the same element are identical. Isotopes are variations that share the same number of protons but differ in their neutron count. This means they have the same atomic number () but different nucleon numbers (). Another crucial concept is specific charge, which tells us how much charge a particle has per unit of its mass. This ratio (Charge/Mass) helps compare fundamental particles like protons and electrons.
Radioactive Decay: Alpha, Beta, Gamma
Unstable atomic nuclei undergo radioactive decay, transforming into more stable forms by emitting particles or energy. The three main types are alpha (), beta (), and gamma () radiation, each with distinct properties regarding ionising ability and penetrating power. These emissions fundamentally change the composition of the nucleus, altering its proton and nucleon numbers according to strict conservation laws.
Alpha (): Helium nuclei (), highly ionising but low penetrating power. Nucleon number decreases by 4, proton number by 2.
Beta-minus (): Electron () emitted, moderate ionising and penetrating power. A neutron changes to a proton; proton number increases by 1.
Beta-plus (): Positron () emitted. A proton changes to a neutron; proton number decreases by 1.
Gamma (): High-energy photons, massless and uncharged, very high penetrating power. No change to nucleon or proton number.
During all nuclear processes, total charge, nucleon number, and lepton number are strictly conserved.
The Subatomic Zoo: Quarks and Leptons
Delving deeper into matter, we find that protons and neutrons aren't fundamental particles themselves. They are made of even smaller constituents called quarks. There are six 'flavours' of quarks, but 'up' (charge ) and 'down' (charge ) are most common. Particles made of quarks are called hadrons. Separately, fundamental particles like electrons and neutrinos, which don't feel the strong force, are known as leptons.
Quarks: Fundamental particles with fractional elementary charges (e.g., up quark: , down quark: ).
Hadrons: Composite particles made of quarks. Baryons (three quarks, e.g., proton 'uud', neutron 'udd') and mesons (one quark and one antiquark).
Leptons: Elementary subatomic particles that do not experience the strong interaction (e.g., electrons, muons, neutrinos).
Weak Interaction: This fundamental force is responsible for quark 'flavour' changes, which drive processes like beta decay (e.g., down up).
Always remember the conservation laws for nuclear reactions! Total charge, nucleon number, and lepton number must always balance on both sides of a decay equation. This is a common check for correctness in exam questions.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Calculate the specific charge of a proton, given its charge is and its mass is .
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Recall the formula: Specific Charge = Charge / Mass.
A nucleus of Carbon-14 () has a mass of approximately kg. Calculate its specific charge. (The elementary charge, e, is C).
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Identify the charge: The proton number (Z) for Carbon () is 6. The nucleus contains 6 protons. The total charge is the number of protons multiplied by the elementary charge: Charge = .
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Key terms for this topic — skim now; the Check step will test them.
- strong nuclear force
This nucleus is held together by the strong nuclear force, which overcomes the electrostatic repulsion between the positively charged protons.
- Isotopes
Not all atoms of the same element are identical. Isotopes are variations that share the same number of protons but differ in their neutron count.
- specific charge
Another crucial concept is specific charge, which tells us how much charge a particle has per unit of its mass.
- Alpha particle
An alpha particle is a helium nucleus, consisting of two protons and two neutrons ().
- Gamma radiation
Gamma radiation consists of high-energy electromagnetic photons, typically emitted from an excited nucleus after an alpha or beta decay event.
- Antiparticle
An antiparticle has the same mass as its corresponding particle but opposite charge and quantum numbers.
- Alpha radiation
Alpha radiation is the most ionising, beta is moderately ionising, and gamma is the least ionising.
Name it
Read the meaning, then pick which of this lesson’s terms it describes. Miss one and you see what your choice really means.
is the ratio of a particle's charge to its mass (Charge/Mass).
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
If you can explain it simply, you own it — gaps here are marks you’d lose.
Teach it back
Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.
Revision flashcards
Guess first, then flip — retrieval beats re-reading.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
11.1 Atoms, nuclei and radiation.
α-particle scattering provided the proof of the structure of the atom.
When α-particles are fired at thin gold foil, most of them go straight through but a small number bounce straight back.
Atoms of all elements are made up of three types of particles: protons, neutrons, and electrons.
The proton number (Z) is the number of protons in an atom, defining the element.
Almost all the mass of an atom is concentrated in the nucleus, which is held together by the strong nuclear force.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
This isotope of samarium decays to an isotope of neodymium (Nd). Give the radioactive decay equation for this decay. Include the nucleon and proton numbers of all the particles involved.
Describe β⁺ decay in terms of the fundamental particles involved.
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do 9702/23 · Q6(b)(ii) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.
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