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. Fire more alpha particles, switch between models and watch the scattering pattern change.
Rutherford’s nuclear model packs all the positive charge and nearly all the mass into a tiny core, the nucleus, tens of thousands of times smaller across than the atom. Most alpha particles pass through the empty space around it and barely bend. An alpha that passes close to the nucleus feels a strong repulsion, two positive charges pushing apart at close range, and is deflected sharply. The closer the approach, the bigger the deflection. An alpha heading almost straight at the nucleus is slowed, stopped and pushed back the way it came. This is rare, about one in eight thousand, because the nucleus is so small, but the rebounds are clear evidence of concentrated charge.
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.