Atomic and nuclear physics in IB Physics centres on a manageable set of testable ideas: quantized atomic energy levels, radioactive decay, mass defect, binding energy, fission, fusion, and stellar physics. To earn marks consistently, you must connect each physical model to equations, nuclear notation, data interpretation, and the command term used in the question.
In the current course, first assessed in 2025, these ideas appear in Theme E: Nuclear and quantum physics. This guide explains the essential theory, the calculations examiners expect, and the mistakes that commonly cost marks.
Where atomic and nuclear physics fits in the IB course
The official IB Physics subject brief divides Theme E into five topics:
| Topic | Status | Main focus |
|---|---|---|
| E.1 Structure of the atom | SL and HL, with additional HL content | Atomic spectra, energy levels, nuclear structure |
| E.2 Quantum physics | HL only | Photons, matter waves, and quantum behaviour |
| E.3 Radioactive decay | SL and HL, with additional HL content | Decay processes, half-life, activity, and binding energy |
| E.4 Fission | SL and HL | Energy release, chain reactions, and nuclear reactors |
| E.5 Fusion and stars | SL and HL | Fusion, stellar equilibrium, and stellar properties |
The IB allocates a recommended 23 hours at SL and 39 hours at HL to the whole theme. These are teaching recommendations, not an indication of how many marks must appear on an examination.
Under the current assessment structure, nuclear and atomic ideas may appear in Paper 1A multiple-choice questions, Paper 1B data-based questions, or Paper 2 short and extended responses. Paper 1 represents 36% of the final grade and Paper 2 represents 44%, so preparation must go beyond memorizing definitions.
Atomic structure and line spectra
Atoms contain a positively charged nucleus surrounded by electrons. The nucleus contains protons and neutrons, collectively called nucleons. In nuclide notation, A is the mass number and Z is the proton number, so the number of neutrons is A - Z.
Isotopes have the same proton number but different neutron numbers. They are therefore the same element and have broadly similar chemical behaviour, but they can differ in mass and nuclear stability.
Discrete energy levels
Electrons in atoms can occupy only particular energy states. They cannot possess energies between these allowed levels, which is why atomic energy is described as quantized.
An electron moving from a higher level to a lower level emits a photon. An upward transition requires the atom to absorb a photon with exactly the required energy:
ΔE = hf = hc/λ
Here, ΔE is the difference between the two energy levels, not the energy of one level by itself. NIST's atomic spectra database documents the characteristic wavelengths and energy levels measured for different atoms and ions.
For example, if the energy difference is 3.0 eV, convert to joules only if the remaining constants are in SI units:
ΔE = 3.0 × 1.60 × 10^-19 = 4.8 × 10^-19 J
Then use λ = hc/ΔE. A frequent error is mixing electronvolts and joules within the same substitution.
Emission and absorption spectra
An emission spectrum contains bright lines produced when excited atoms emit photons. An absorption spectrum contains dark lines where photons from a continuous spectrum have been absorbed by cooler gas.
Both are evidence for discrete energy levels because only specific photon energies are emitted or absorbed. Since every element has a distinctive set of levels, its spectral pattern can be used to identify it.
When asked to explain why a line spectrum is produced, a complete answer should state that:
- electrons occupy discrete energy levels;
- transitions occur between particular pairs of levels;
- each transition emits or absorbs a photon with energy equal to the level difference;
- therefore only particular photon frequencies or wavelengths occur.
Radioactive decay and nuclear equations
An unstable nucleus may transform spontaneously. The process is random for an individual nucleus, meaning the time at which it decays cannot be predicted, but a large sample follows a statistically predictable exponential pattern.
| Radiation | Nuclear change | Penetration and ionization |
|---|---|---|
| Alpha, α | A decreases by 4 and Z decreases by 2 | Strongly ionizing, weakly penetrating |
| Beta minus, β⁻ | Neutron becomes proton, so A is unchanged and Z increases by 1 | Moderately ionizing and penetrating |
| Gamma, γ | Excited nucleus loses energy; A and Z are unchanged | Weakly ionizing, strongly penetrating |
In a nuclear equation, both nucleon number and charge must balance. Gamma emission changes the nuclear energy state, not the identity of the nuclide.
Do not say that alpha or beta radiation is already stored inside the nucleus. These particles are produced or emitted as part of the decay process. In beta-minus decay, an electron and an electron antineutrino are produced when a neutron changes into a proton.
Decay constant, activity, and half-life
For a radioactive sample:
N = N0 e^(-λt)A = λNA = A0 e^(-λt)T1/2 = ln(2)/λ
N is the number of undecayed nuclei, A is activity in becquerels, and λ is the decay constant. Because λt must be dimensionless, the units used for time and decay constant must be reciprocal.
For an integer number of half-lives, repeated halving is usually faster than exponential substitution. After three half-lives, for example, the remaining fraction is (1/2)^3 = 1/8.
Paper 1B may give a graph of count rate against time. If background count is present, subtract it before finding the half-life. Otherwise, the curve approaches the background value rather than zero, causing a systematically incorrect result.
Mass defect and binding energy
The measured mass of a bound nucleus is less than the total mass of its separate nucleons. This difference is the mass defect:
Δm = mass of separate nucleons - mass of nucleus
The corresponding binding energy is:
Eb = Δmc²
Binding energy is the energy required to separate a nucleus completely into its nucleons. Equivalently, it is the energy released when those nucleons form the nucleus.
When mass is given in unified atomic mass units, the useful conversion is approximately:
1 u c² = 931.5 MeV
Therefore, E in MeV = Δm in u × 931.5. Keep additional digits during the calculation and round only the final result to an appropriate number of significant figures.
Binding energy per nucleon
Binding energy per nucleon is total binding energy divided by mass number. It provides a useful measure of how tightly the average nucleon is bound.
The curve rises sharply for light nuclei, reaches a maximum in the iron and nickel region, and then decreases gradually for heavy nuclei. Energy can therefore be released when:
- light nuclei combine through fusion and move up the curve;
- heavy nuclei split through fission and also move toward the maximum.
A strong explanation refers to the products having a greater binding energy per nucleon and a lower total mass than the reactants. Saying only that mass is “lost” is incomplete because mass-energy is conserved.
Nuclear reactions, fission, and fusion
For any nuclear reaction, calculate the reaction energy or Q-value from the mass difference:
Q = (total reactant mass - total product mass)c²
A positive result means energy is released. If masses are in u, multiply the mass difference by 931.5 MeV u^-1.
Fission and reactor questions
In induced fission, a heavy nucleus absorbs a neutron, becomes unstable, and splits into smaller nuclei while releasing energy and additional neutrons. Those neutrons may initiate further fissions, creating a chain reaction.
Examiners often ask for the functions of reactor components:
- Moderator: slows neutrons, increasing the probability of further fission in suitable fuel.
- Control rods: absorb neutrons to regulate the reaction rate.
- Coolant: transfers thermal energy away from the reactor core.
- Heat exchanger: transfers energy to a separate water system used to produce steam.
- Shielding: absorbs ionizing radiation and protects workers and the environment.
Do not confuse moderator and control rods. The moderator primarily slows neutrons; control rods remove neutrons from the chain reaction.
Fusion and stars
Fusion combines light nuclei into more tightly bound nuclei. Energy is released because the products have lower mass and greater binding energy per nucleon.
Fusion requires very high temperature so positively charged nuclei have enough kinetic energy to approach despite electrostatic repulsion. High density also increases the collision rate. The IAEA identifies temperature, particle density, and sufficient confinement time as central conditions for sustained fusion.
A stable main-sequence star is in approximate equilibrium between inward gravitational forces and outward pressure associated with its hot interior and radiation. Fusion supplies the energy needed to maintain this pressure. If asked why a star is stable, both opposing effects must be identified.
How IB examiners phrase atomic and nuclear questions
Command terms determine the required depth. Match the structure of your answer to the verb:
| Command term | What to provide |
|---|---|
| State | A brief fact, value, or relationship without extended reasoning |
| Calculate | Formula, substitution, numerical result, and unit |
| Determine | A result obtained from supplied information, often including graph work |
| Describe | Relevant features or stages of a process |
| Explain | A linked physical cause and consequence |
| Show that | Working that reaches the stated approximate value |
| Suggest | A plausible answer based on physics and the information given |
For “show that” questions, do not begin with the value supplied in the question. Start from the data, show substitutions, and retain enough precision to demonstrate agreement.
For explanations, use explicit causal links. Instead of writing “fusion releases energy because of mass defect,” write that the fusion products have lower total mass and higher binding energy per nucleon, so the mass difference is transferred to other forms of energy according to E = Δmc².
An exam-focused revision method
Revise this topic through repeated cycles of theory, calculation, and correction:
- Learn nuclear notation, decay changes, and essential definitions.
- Practise photon-energy, half-life, activity, and mass-defect calculations separately.
- Complete mixed questions without being told which equation to use.
- Mark every response by identifying the exact missing statement or calculation step.
- Reattempt incorrect questions several days later without viewing the solution.
Use the Topic E Nuclear and Quantum Physics Questionbank to isolate specific weaknesses. The IB Physics revision notes and IB Physics data booklet resource are useful for checking terminology and equations before attempting timed sets.
Seeing complete solutions is especially valuable because it shows where units, conversions, nuclear balancing, and explanatory statements earn credit. RevisionDojo's Physics video library demonstrates worked methods, while the Physics predicted papers and video solutions provide paper-level practice. Where per-question video solutions are available, attempt the question first and then compare each line of your method rather than passively watching.
Common mistakes that lose marks
- Using
Afor both mass number and activity without defining the intended meaning. - Reversing the proton-number change in beta-minus decay.
- Failing to subtract background count before finding half-life.
- Mixing
eV,MeV, and joules in one calculation. - Using mass defect without stating which masses are reactants and products.
- Claiming mass disappears instead of describing mass-energy conversion.
- Giving identical functions for a moderator and control rods.
- Describing spectra without connecting photon energy to an energy-level difference.
- Rounding intermediate mass differences too early.
- Writing a memorized paragraph that does not answer the command term.
Conclusion
IB atomic and nuclear physics becomes much more predictable once its ideas are organized around energy differences, conservation laws, exponential decay, and binding energy. Secure the physical explanations first, then practise translating them into balanced equations, graph interpretations, and carefully structured calculations.
RevisionDojo can support this process through Topic E notes, targeted Questionbank practice, Jojo AI feedback, and worked video solutions. Finish your preparation with timed IB Physics predicted papers, then use the solutions to diagnose exactly where your method or explanation needs improvement.
Sources and referenced URLs
- Official IB Physics subject brief, first assessment 2025
- Official IB Physics curriculum overview
- Official IB Physics course and assessment updates
- Official IB Physics specimen examinations and markschemes
- NIST Atomic Spectra Database
- NIST CODATA atomic mass unit energy equivalent
- IAEA nuclear physics and reactor theory module
- IAEA introduction to fusion physics
- RevisionDojo Topic E Nuclear and Quantum Physics Questionbank
- RevisionDojo IB Physics revision notes
- RevisionDojo IB Physics data booklet resource
- RevisionDojo IB Physics video library
- RevisionDojo Physics predicted papers and video solutions
