Atomic Models
Rutherford's Gold Foil Experiment · The Bohr Model of the Atom
IB Physics SL topic guide
Nuclear and Quantum Physics
Nuclear and Quantum Physics is a core part of IB Physics SL. This guide connects the syllabus ideas behind Atomic Models, Emission and Absorption Spectra, Nuclear Notation, Isotopes and Mass Defect, Types of Radioactive Decay, The Decay Law and Half-Life and 5 more units, shows how they appear in worked problems, and points you to the formulas and full lessons needed for exam revision.
What you will learn
Nuclear and Quantum Physics syllabus outline
The units below follow the structure used in the full Study to Learn course. Use the outline to identify exactly which idea needs attention, then work through the public example before continuing to the complete lesson path.
Emission and Absorption Spectra
Energy Levels and Photon Transitions · The Hydrogen Spectrum and Spectral Series
Nuclear Notation, Isotopes and Mass Defect
Nuclear Notation and Isotopes · Mass Defect and Binding Energy
Types of Radioactive Decay
Alpha, Beta and Gamma Decay · Nuclear Equations and Conservation Laws · The Neutrino and Weak Interaction
The Decay Law and Half-Life
Activity and the Decay Constant · Half-Life Calculations · Radioactive Dating and Tracers
Applications and Safety
Medical Applications of Radioisotopes · Biological Effects and Safety
Nuclear Fission
Nuclear Fission and Chain Reactions · Binding Energy and Fission Energy Release
Fission Reactors and Energy
Nuclear Power Stations · Nuclear Waste and Safety
Nuclear Fusion
Nuclear Fusion and the Proton-Proton Chain · Fusion Energy from Mass Defect
Stellar Properties and Evolution
Astronomical Distances and Parallax · Stellar Properties and the H-R Diagram · The Stellar Life Cycle
Free worked preview
Rutherford's Gold Foil Experiment
This complete preview comes from the Atomic Models unit. It introduces the core language, shows the method in context, and gives you a real example of the lesson quality before you create an account.
Discovering the Nucleus
At the turn of the 20th century, the atom was imagined as a diffuse "plum pudding" of positive charge with electrons embedded in it — until Rutherford's gold-foil experiment fired alpha particles at a thin metal sheet and saw a few bounce straight back. This lesson is the starting point for all of nuclear physics: it tells you why the atom is mostly empty space, why its mass is concentrated in a nucleus 100,000 times smaller than the atom itself, and why the plum-pudding model had to be discarded. After this lesson you will explain each of Rutherford's three key observations and recognise the one thing his model could not explain — the stability of electron orbits — which sets the stage for Bohr's quantum solution next.
In 1909-1911, Geiger and Marsden, under Ernest Rutherford's direction, fired a beam of alpha particles (helium nuclei) at a very thin gold foil. Most passed straight through, but about 1 in 8000 was deflected backwards — an astonishing result.
Three Key Observations
- Most α-particles passed through undeflected → the atom is mostly empty space.
- A small fraction were deflected at small angles → the positive charge is concentrated in a tiny region.
- A very small number (∼1 in 8000) bounced back → the nucleus is massive and dense compared with the α-particle.
Rutherford concluded that the atom consists of a tiny, dense, positively charged nucleus (diameter ∼10⁻¹⁵ m) surrounded by electrons at relatively vast distances (atom diameter ∼10⁻¹⁰ m). This overturned J.J. Thomson's "plum pudding" model (a uniform sphere of positive charge with electrons embedded in it), which would predict no large-angle scattering.
Exam TipRutherford's model could NOT explain why atoms have discrete emission spectra (line spectra) or why electrons don't spiral into the nucleus (classically, an accelerating charge should radiate and lose energy). These problems were solved by Bohr.
Reviewed by the Study to Learn editorial team · Updated 2026-07-24