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Democritus' theory held that everything is made up of atoms, which are physically, but not geometrically. The atomic model of it is that matter had invisible particles called atoms and empty space. ( 400 B.C.). Duignan, B. (2020, January 14). Democritus. Encyclopedia Britannica. https://www.britannica.com/biography/Democritus.

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He proposed a theory for the hydrogen atom based on the quantum theory that energy is transferred only in certain well-defined quantities. Bohr's atomic model is that electrons must move around the nucleus, but only in prescribed orbits.(1913). Britannica, T. Editors of Encyclopaedia (2020, January 23). Bohr model. Encyclopedia Britannica. https://www.britannica.com/science/Bohr-model.

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James Chadwick bombarded beryllium atoms with alpha particles. Chadwick bombarded beryllium atoms with alpha particles.(1932). Britannica, T. Editors of Encyclopaedia (2020, December 17). James Chadwick. Encyclopedia Britannica. https://www.britannica.com/biography/James-Chadwick.

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Dalton's atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties. He proposed that all matter was made up of atoms, indivisible and indestructible building blocks. (1803). Ross, S. (2020, December 15). John Dalton. Encyclopedia Britannica. https://www.britannica.com/biography/John-Dalton.

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Thomson proposed a model of the atom that consists of positive and negative charges, present in equal amounts so that an atom is electrically neutral. He proposed that the atom was a sphere, but positive and negative charges were embedded within it.(1903). Helmenstine, Anne Marie, Ph.D. (2020, August 26). J.J. Thomson Atomic Theory and Biography. Retrieved from https://www.thoughtco.com/j-j-thomson-biography-607780

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Ernest Rutherford envisioned the atom as a miniature solar system, with electrons orbiting a massive nucleus, and as mostly empty space, with the nucleus occupying only a very small part of the atom.(1911). Britannica, T. Editors of Encyclopaedia (2020, November 9). Rutherford model. Encyclopedia Britannica. https://www.britannica.com/science/Rutherford-model

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Werner Heisenberg contributed to atomic theory by formulating quantum mechanics in terms of matrices and discovering the uncertainty principle, which states that the position and momentum of a particle cannot be known exactly.(1927). Beyler, R. (2021, January 21). Werner Heisenberg. Encyclopedia Britannica. https://www.britannica.com/biography/Werner-Heisenberg.

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Erwin Schrödinger showed that the quantification of the energy levels of the hydrogen atom that appeared in Niels Bohr's atomic model could be calculated from the Schrödinger equation, which describes how the wave of a quantum mechanical system works.(1926).Bernstein, J. (2021, January 11). Erwin Schrödinger. Encyclopedia Britannica. https://www.britannica.com/biography/Erwin-Schrodinger.