In 1911, Ernest Rutherford and his team (Hans Geiger and Ernest Marsden) conducted the "Gold Foil" experiment, which Backstage Science describes as essentially the .
To look inside the atom, Rutherford needed to fire something at it. He used:
Chosen because gold is incredibly malleable and can be beaten into a sheet only about 1,000 atoms thick.
While most did pass through, Rutherford was shocked to find that:
Positively charged particles emitted from a radioactive source (radium).