Fire alpha particles at gold foil and watch history unfold in cinematic slow motion. Discover the nuclear model of the atom — exactly as Rutherford did — one observation at a time.
The flashes of light on the ZnS screen tell a remarkable story.
From these three simple observations, Rutherford overturned the Thomson "plum pudding" model and proposed the nuclear atom in 1911.
Since ~98.5% of particles pass straight through, the atom cannot be a solid sphere. The positive charge and mass are not spread uniformly — they must be concentrated elsewhere.
The backscattering (~0.1%) is only possible if there is an extremely concentrated region of positive charge and mass at the atom's centre — the nucleus. Its diameter is ~10⁻¹⁵ m, while the atom is ~10⁻¹⁰ m across.
The nearly massless electrons orbit at large distances from the nucleus, making the atom mostly empty. They contribute negligible deflection to the heavy alpha particles.
The angle of deflection depends on the distance of closest approach (impact parameter b). Particles passing close to the nucleus experience stronger Coulomb repulsion: cot(θ/2) = 4πε₀ · m·v²·b / 2Ze²
Try the controls above — here's the real physics behind each change.
Alpha particles (charge +2, mass 4u) are heavy enough to show clear scattering statistics. The Coulomb force between α and nucleus causes measurable deflection. This is the original Rutherford setup.
Gold has atomic number Z=79 — a highly charged, heavy nucleus. The strong Coulomb repulsion produces a clear scattering pattern. Gold is also very malleable, allowing extremely thin foils (≈1000 atoms thick).
Increasing thickness means more nuclear layers. More chance of interaction per particle. Back-scatter rate increases proportionally with thickness. However, too thick and particles lose too much energy before scattering.
Higher kinetic energy means the particle spends less time near the nucleus. Coulomb force has less time to act. The distance of closest approach decreases: d = Ze²/(4πε₀·KE). More energetic particles get closer before being repelled.
Key concepts and formulae for your FYUGP examination.
The differential cross-section. Z₁=2 (alpha), Z₂=79 (gold), E=kinetic energy, θ=scattering angle. Predicts the fraction of particles scattered into each angular range.
For a head-on collision. For 5.5 MeV alpha on gold: d ≈ 42 fm. This sets an upper bound on the nuclear radius. The actual nucleus is much smaller (~7 fm for gold).
b = impact parameter (perpendicular distance from nucleus to particle path). Small b → large θ (backscattering). Large b → small θ (grazing, slight deflection). b → ∞ → θ → 0 (no deflection).
J.J. Thomson's "plum pudding" model (1904) imagined a diffuse, uniform positive charge with electrons embedded. If this were true, the maximum deflection for any alpha particle would be <1° — no backscattering possible. Rutherford's experiment definitively disproved this.
This experiment in 1909–1911 is considered one of the most important in physics. It established that: