Nuclei with the same Z but different numbers of neutrons.
Photon
A quantum of electromagnetic radiation with energy E = hf.
Energy level
An allowed energy of an electron in an atom.
Electronvolt (eV)
Energy gained by an electron through 1 V: 1 eV = 1.60 × 10⁻¹⁹ J.
Geiger–Marsden–Rutherford experiment: alpha particles fired at gold foil. Most passed straight through (atom mostly empty); a few were deflected through large angles (small, dense, positive nucleus).
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Convert eV to J by multiplying by 1.60 × 10⁻¹⁹ before using E = hc/λ.
E.2Quantum physics HL
Key terms
Photoelectric effect
Emission of electrons from a metal surface when light above a threshold frequency falls on it.
Work function (Φ)
Minimum energy needed to remove an electron from a metal surface.
Threshold frequency
Minimum frequency of light that causes photoemission: f₀ = Φ/h.
Stopping potential
The reverse potential that just stops the fastest photoelectrons.
de Broglie wavelength
λ = h/p: every particle has wave properties.
Photoemission happens instantly, only above the threshold frequency, and the maximum kinetic energy depends on frequency, not intensity. Wave theory cannot explain this; the photon model can: one photon gives all its energy to one electron.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: The intensity of light controls how many electrons; the frequency controls how energetic. Say both in explanation questions.
E.3Radioactive decay
Key terms
Mass defect
Difference between the mass of the separate nucleons and the mass of the nucleus.
Binding energy
Energy needed to separate a nucleus into its nucleons: E = Δmc².
Half-life
Time for half the radioactive nuclei in a sample to decay (or for activity to halve).
Activity (A)
Number of decays per second, in Bq.
Ionising radiation
Alpha, beta and gamma radiation, which remove electrons from atoms.
Alpha: helium nucleus, strongly ionising, stopped by paper. Beta-minus: electron (with antineutrino), stopped by a few mm of aluminium. Beta-plus: positron (with neutrino). Gamma: high-energy photon, weakly ionising, reduced by thick lead.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Always check charge (Z) and nucleon (A) numbers balance on both sides of a decay equation.
E.4Fission
Key terms
Nuclear fission
Splitting of a heavy nucleus into two smaller nuclei, releasing energy and neutrons.
Chain reaction
Neutrons from one fission cause further fissions.
Critical mass
The minimum mass of fissile material for a self-sustaining chain reaction.
Moderator
Material (water, graphite) that slows fast neutrons so they are more likely to cause fission.
Control rods
Rods (boron, cadmium) that absorb neutrons to control the reaction rate.
Each U-235 fission releases about 200 MeV, mostly as kinetic energy of the fragments, which heats the fuel.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Explain each reactor component in terms of neutrons: what it does to them and why that matters.
E.5Fusion and stars
Key terms
Nuclear fusion
Joining of light nuclei to form a heavier nucleus, releasing energy.
Stellar equilibrium
Balance between inward gravitational pressure and outward radiation and gas pressure.
Hertzsprung–Russell diagram
A plot of stellar luminosity against surface temperature (decreasing to the right).
Parsec (pc)
Distance at which a star has a parallax angle of 1 arcsecond: 1 pc = 3.26 ly.
Main sequence
The band on the HR diagram where stars fuse hydrogen to helium in their cores.
Fusion needs very high temperatures (about \(10^{7}\) K) so nuclei move fast enough to overcome electrostatic repulsion, and high density so collisions are frequent.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: On an HR diagram, temperature increases to the left: hot blue stars are on the left, cool red stars on the right.