The atom was supposed to be soft

J. J. Thomson pictured electrons embedded in diffuse positive matter. A heavy, fast alpha particle passing through thin gold foil should then suffer only many tiny deflections.

In Manchester, Hans Geiger and Ernest Marsden counted faint flashes on a screen. Most particles passed straight through. A few turned sharply. A very small number came almost back.

Particle scattering Scientific model diagram Au Au alpha alpha Rare large-angle backscatter Rare large-angle backscatter scintillation scintillation
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Static physics diagram: rutherford_gold_foil.

Like a shell bouncing from tissue paper

Rutherford compared the result to firing artillery at tissue paper and having it rebound. A single strong deflection required positive charge and most atomic mass to be concentrated in a region far smaller than the atom.

Rutherford nuclear atom model Scientific model diagram + + Au · nuclear atom Au · nuclear atom mostly empty space mostly empty space e⁻ e⁻ e⁻ e⁻ e⁻ e⁻ e⁻ e⁻ e⁻ e⁻ e⁻ e⁻ Historical model (1911) Historical model (1911) positive charge and most mass positive charge and most mass r_atom / r_nucleus ≈ 10⁵ r_atom / r_nucleus ≈ 10⁵ schematic, not to scale schematic, not to scale no electron trajectories implied no electron trajectories implied
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Static physics diagram: rutherford_nuclear_atom.

The foil did not photograph a nucleus. Positions of thousands of flashes were interpreted through a model of an invisible structure. Modern particle physics still follows that logic: launch a known probe, measure how it scatters, and infer what it struck.