Rutherford Model

Rutherford's alpha-particle scattering experiment (Geiger-Marsden, 1909): bombarded thin gold foil with α-particles, observed scattering pattern.

Most α's passed through nearly undeflected ⇒ atom is mostly EMPTY space.

A small fraction scattered at LARGE angles (some > 90°) ⇒ atom has a tiny, dense, positively charged core (nucleus).

Result: nuclear (planetary) model. Positive charge and almost all mass concentrated in nucleus (~10⁻¹⁵ m); electrons orbit at ~10⁻¹⁰ m.

Distance of closest approach for head-on collision: r_min = (1/4πε₀) · (2Ze²/K_α). For 5 MeV α on gold: r_min ~ 3×10⁻¹⁴ m.

Impact parameter b vs scattering angle: cot(θ/2) = (4πε₀·2K·b)/(2Ze²).

Limitations of Rutherford's model: orbiting electrons would radiate, spiral in, atoms would collapse — needs quantum fix (Bohr).