Aristotle believed all matter was made up of water, fire, aether, air and earth.
Democritus believed that you could break a piece of matter continuously in half, is there an end?
Did experiments with different chemicals that showed matter, seemed to consist of atoms. Although he did not know about their structure, he knew that the evidence pointed to something fundamental.
J.J. Thomson discovered the electron and proposed a model for the structure of the atom. He also did the Plum Pudding Model.
showed that when you vibrate atoms strong enough, you can measure the energy only in discrete units.
Niels Bohr came up with a theory that the electrons don't spiral into the nucleus and came up with some rules for what does happen.
identified the particles of the nucleus as discrete positive charges of matter.
Louis de Broglie believed that if light can exist as both particles and waves, why couldn't atom particles also behave like waves?
Werner Heisenberg's matrix mechanics theory explained the behavior of atoms.
Erwin Schrödinger's theory worked kind of like harmonic theory for a violin string except that the vibrations traveled in circles.
James Chadwick discovered the neutron. He found it to measure slightly heavier than the proton with a mass of 1840 electrons and it was neutral