In 1803, John Dalton published his ideas on the atom, describing how all matter is made up of small particles called atoms.
In 1897, J J Thomson discovered the electron and subsequently proposed the atom looked like a plum pudding, with the atom being a positive 'dough' with negative electrons stuck in it like plums.
In 1905, Ernest Rutherford tested the plum pudding experiment by firing a directed beam of alpha particles at a thin gold leaf suspended in a vacuum. This experiment led to the proposal of the nuclear model.
The Rutherford experiment led to the theory of the Nuclear model, which is the model of the atom we know today, nucleus cotaining protons and neutrons surrounded by electrons in energy shells.