Model: The Greek Model (442 BC)
A round, solid sphere
Democritus believed that all matter is made out of tiny particles which cannot be divided, created, or destroyed. Due to the fact that there was no way to scientifically test his ideas and they went against the popular opinion, they were not accepted.
The Plum Pudding Model (1904)
A sphere containing a positively charged "soup" with negatively charged electrons throughout.
Thomson placed one positively charged and one negatively charged plate on opposite sides of a cathode ray tube, and found that the ray bent towards the positively charged plate. He concluded that the rays were made of negatively charged particles called "electrons". His model added this new knowledge to the model of the atom.
Planetary Model (1911)
Mostly empty space, with negatively charged electrons traveling around a positively charged nucleus
In order to test Thomson's model, Rutherford and his students shot alpha-particles at a thin sheet of gold foil which was surrounded by a screen. Instead of the particle going through the sheet and showing a single spot of light on the screen as expected, the particle reflected in many different direction, suggesting some particles hit something dense and reflected, while others passed through. This dense thing was the nucleus.
Bohr Model (1913)
Features electrons orbiting around the nucleus in circular energy levels.
Bohr experimented by adding electricity to hydrogen and studying the released energy. He found that electrons closer to the nucleus have less energy than electrons farther away from the nucleus and that they move in energy levels around the nucleus.
Erwin Schrodinger and Louis de Broglie
Electron Cloud Model (1926)
An election cloud surrounds the nucleus, showing likelihood that electrons will be there.
Broglie discovered that matter has wave properties, and Schrodinger built on the atom using that information, discovering that electrons don't move in set paths, and used mathematical equations to discover the likelihood that an electron will appear in a certain location.