Geiger and Marsden expected alpha particles to drift gently through Thomson's diffuse positive pudding. Instead, a tiny fraction rebounded like bullets off steel. The atom is mostly empty space with a dense core doing all the work. Drag the slider to fire alpha particles and watch the scattering pattern change between models.
Thomson's plum pudding model pictured the atom as positive charge spread throughout a sphere, with electrons embedded like raisins. When alpha particles (helium nuclei, doubly positive) plough through this diffuse charge, they experience gentle, distributed electrostatic forces from all directions. The model predicts tiny deflections—perhaps a degree or two—but never sharp rebounds, because there is no concentrated charge to deliver a localised, powerful repulsion. The spreading positive pudding cannot turn a massive, fast-moving alpha particle around.
The one-in-thousands rebound rate is quantitative evidence that nearly all the atom's positive charge sits in a tiny, dense nucleus, with electrons orbiting through otherwise empty space.