There was polemic about how was the charge of an atom distributed, so base on the Thomson´s theory he made an experiment:
WIth a gold leaf in plum puddin only some aplha particles will deviat and some others will pass.
In a nuclear atom the particles shoud pass staright through it, every time they pass a zinc light will shine.
1 out of 1800 lighted, the hypthesis was correct. Most of the atom is just empty space, the center (nucleous) is positive and it is orbited by small negative particles (electrons).
He published it´s results on 1911.