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Thomson believed that all atoms electrons. He proposed the plum pudding model of the atom, which had electrons within a positively-charged "soup."

He had experiments with cathode ray tubes showed that all atoms containing  negatively charged subatomic particles (electrons.)

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Daltom believes that matter is made up of atoms. All atoms in a given element are the same. Atoms can form into compounds and can not be created or destroyed.

Dalton discovered that oxygen combined with either one or two volumes of nitric oxide in closed vessels over water

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Rutherford described the atom as a tiny, dense, and positively charged core (nucleus.) He established that the mass of the atom is concentrated in the nucleus. The negatively charged electrons circulated around this nucleus.

Rutherford did his gold foil experiment. A beam of alpha particles was pointed at a piece of gold foil. Most alpha particles passed through the foil, but some scattered backward. This showed that most of the atom is empty space surrounding the nucleus.

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Bohr theory modified the atomic structure model from Rutherford by saying that electrons move in fixed orbitals and not anywhere in between and he also explained that each orbit has fixed energy.

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Heisenberg conducted a thought experiment. He tried to measure the position of an electron with a gamma ray microscope. The rays to illuminate the electron would give it a kick, changing its momentum.