The Particle Model

P1-1-1 Structure of an atom

Plum Pudding model:

In 1897 J.J. Thompson discovered the electron is much lighter than the atom, and hence realised the atom is not the smallest particle. So he proposed the Plum Pudding model:

The pudding is positive, and the plums in it are negative! 

JJ Thompson did not think of neutrons! 

Nuclear Model (Rutherford Model): 

In 1909 Geiger and Marsden fired some alpha particles towards a thin sheet of gold (alpha particle is made of two protons and two neutrons, so it has a positive charge!).

Because they thought material is made of atoms that are like a plum pudding and hence it is soft like a pudding, they expected all of the alpha particles to pass through the metal sheet. But they saw some alpha particles bounce back! So their supervisor, Ernest Rutherford, said there is a positive nucleus at the centre of the atom which repels the positive alpha particles. He said electrons randomly orbit around the nucleus.