How did our model of the atom evolve, and what happens when unstable nuclei decay?
The history of atomic models is a masterclass in how science works. Thomson's plum-pudding model (electrons embedded in a diffuse positive sphere) was overturned by Rutherford's gold-foil experiment in 1909, where most alpha particles passed straight through but a few deflected sharply backward — impossible if charge were spread out. Rutherford concluded that most of the mass and all the positive charge must be concentrated in a tiny nucleus. Bohr then added quantised electron orbits to explain why atoms emit discrete spectral lines, not a continuous spectrum.
Radioactive decay is the process by which unstable nuclei emit radiation to become more stable. The type of decay depends on the nature of the instability: too many protons → beta-plus or alpha; too many neutrons → beta-minus; excess energy → gamma. Half-life is a statistical concept: after one half-life, half the nuclei in a sample have decayed on average. It doesn't predict when any specific nucleus will decay, only the bulk behaviour of large numbers.
Carbon-14 has a half-life of 5700 years. A sample originally contained 240 g of C-14. (a) How much C-14 remains after 22,800 years? (b) Write a balanced nuclear equation for the beta-minus decay of C-14. What element does it become?
Key facts
Nuclear notation: ᴬZX — A = nucleon number (protons + neutrons), Z = proton number
α decay: A decreases by 4, Z decreases by 2 (emits ⁴₂He)
β⁻ decay: Z increases by 1, A unchanged (emits ⁰₋₁e + antineutrino)
γ decay: no change in A or Z (high-energy photon emitted)
HT β⁺ decay: Z decreases by 1, A unchanged (emits ⁰₊₁e + neutrino)
What students must understand
Development of atomic model: Thomson (plum-pudding) → Rutherford (nuclear) → Bohr (orbit shells)
Rutherford scattering: evidence for a small, massive, positively-charged nucleus
Nuclear notation ᴬZX; proton number, neutron number, nucleon number
Isotopes: same element (same Z), different A (different neutron numbers)
Radioactive decay: random, spontaneous; cannot be influenced by physical or chemical conditions
Alpha (α): helium nucleus, least penetrating, stopped by paper; highly ionising
Beta-minus (β⁻): fast electron, stopped by thin aluminium; moderately ionising
Gamma (γ): EM radiation, stopped by thick lead/concrete; least ionising per unit path
Balancing nuclear equations: A and Z both conserved
Half-life: time for half the nuclei to decay; use with integer multiples
Activity: rate of decay (Bq = decays per second); decreases over time
Background radiation: cosmic rays, rocks (radon gas), medical sources
Uses: medical imaging (gamma), cancer treatment, smoke detectors (alpha), carbon dating
Hazards: ionisation damages cells/DNA; contamination vs irradiation distinction
HT β⁺ decay and positrons; nuclear equations for β⁺
HT Ratio of remaining nuclei after given number of half-lives
Linking questions
How does ionisation from radiation connect to DNA damage and cancer? → P6 Waves (EM hazards)
How does the GCSE atomic model compare to the DP treatment? → DP Physics E.1, E.3
Video Support
The Organic Chemistry Tutor
Protons Neutrons Electrons Isotopes - Average Mass Number & Atomic StructureHalf Life Chemistry Problems - Nuclear Radioactive Decay Calculations
Tyler DeWitt
Basic Atomic Structure: A Look Inside the AtomAlpha Decay
Michel van Biezen
Physics - Nuclear Physics (13 of 22) What is Alpha Decay?Physics - Nuclear Physics (10 of 22) Radioactive Decay: How Much is Left?
WNY Tutor — worked problems
Rutherford's scattering experiments - alpha particlesThe nucleus of Be which consists of 4 protons