Light photons strike the radium atom, bumping its electrons into a higher orbit. After the sun sets and it gets dark, those electrons spontaneously drop into lower orbits, emitting a particle and ...

Curie not only discovered radium, she realised what it could be ... were things smaller than atoms and up until this point, the atom was an indivisible thing. MARIE-NOELLE HIMBERT: Yes.

Everyone at that time imagined the atom as a "plum pudding ... emitted by the radioactive element radium) at a sheet of gold foil only 1/3000 of an inch thick, and tracing the particles' paths.