Ernest Rutherford (1871 - 1937)

Rutherford beamed positively charged alpha particles at a sheet of really thin gold foil. Since so many particles could pass right through the foil he concluded that an atom is made up of mostly empty space. However, some of the particles deflected so he hypothesized that an atom must have a small dense positively charged mass which he names the nucleus. 

He discovered positively charged  nucleus particles named protons and that electrons were scattered in empty space around the nucleus. 

Rutherford developed the planetary model of the atom also named the Nuclear Model, because he discovered the nucleus, where all the protons were in the nucleus and the electrons orbited around the nucleus like planets around the sun. Due to his discovery of the nucleus, he proved Thomson's Plum Pudding Model to be incorrect.