Rutherford
Is an atom solid, or mostly empty?
That time, that place
The son of a New Zealand farmer. The atomic model of the day was Thomson's: electrons embedded like raisins in a spread-out pudding of positive charge.
Why this question
Rutherford had his assistant Geiger and the student Marsden fire alpha particles at very thin gold foil.
The expectation was obvious. Against a pudding-like atom the particles would pass almost straight through, deflected only slightly. It hardly seemed worth checking.
What was found
Most passed through as expected. But roughly one in eight thousand bounced back hard, and some came almost straight back.
Rutherford said it was as if you had fired a shell at tissue paper and it had come back and hit you.
To come back, a particle must strike something very heavy and very hard head-on. That this happens so rarely means the something is very small.
Conclusion: nearly all the atom's mass sits in a tiny center, and the rest is empty. If an atom were a stadium, the nucleus would be a grain of rice at midfield.
The atom was a uniformly filled ball
About one alpha particle in eight thousand thrown sharply back
A new problem arrived at once: an orbiting electron should radiate away its energy and spiral into the nucleus. Then no atom could exist