Democritus theorized that matter could not be divided into smaller and smaller pieces forever,eventually the smallest possible piece would be obtained.
Aristotle theorized the matter was made of the four elements: Water, Fire, Earth, and Air. He also believed that there were four quaalities to these elements: dryness, hotness, coldness, and moistness. Based on these beliefs fire would hold the characteristics of being hot and dry, water is wet and cold,air is hot and wet, while the earth is dry and cold.
Dalton theorized that atoms of different elements could be distinguished by differences in their wieghts.
He identified the negatively charged electron in the cathode ray tube. He deduced that the electron was a component of all matter and calcualted the charge to mass ratio for the electron: e/m = -1.76 x 10 (to the 8th power) coulombs/g. He also proposed the plum pudding model.
He was the originator of the Quantum Theory.
He theorized that the atom had a central positive nucleus surrounded by negative orbiting electrons.
Niels theorized that electrons travel in stationary orbits defined by their angular momentum.
He conducted the Oil Drop Experiment. Doing this he was able to determine the charge on electrons.
He proposed that electrons could behave as waves under some conditions.
He calculated the behavior of electrons, and subatomic particles that also make up an atom. He also created the matrix mechanics, the first version of quantum physics.
Chadwick proved the existence of neutrons.
He theorized that rather than electrons being distributed within an electron configuration of shells and energy levels, they were arranged in orbitals which were systematically distributed within electron clouds.