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Democritus was a central figure in the development of the atomic theory of the universe. He theorized that all material bodies are made up of indivisibly small “atoms.” Aristotle famously rejected atomism in On Generation and Corruption.
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In Aristotle's time, atomists held that matter was fundamentally constructed out of atoms. These atoms were indivisible and uniform, of various sizes and shapes, and capable only of change in respect of position and motion, but not intrinsic qualities.
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Isaac Newton contributed to atomic theory through his systematic development of calculus-based object modeling concepts e.g. displacement, velocity, acceleration and energy, as well as gravity, uniform circular motion and angular momentum, etc.
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Dalton's atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties. Dalton based his theory on the law of conservation of mass and the law of constant composition. The first part of his theory states that all matter is made of atoms, which are indivisible.
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Michael Faraday, a British physicist, made one of the most significant discoveries that led to the idea that atoms had an electrical component. Faraday placed two opposite electrodes in a solution of water containing a dissolved compound.
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He made fundamental contributions to the field of analytical geometry and was a pioneer in the investigations of cathode rays that led eventually to the discovery of the electron. He also vastly extended the study of Lamé curves.
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Dmitri Mendeleev devised the periodic classification of the chemical elements, in which the elements were arranged in order of increasing atomic weight.
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He continued work on the new element thallium, isolated it, studied its properties, and determined its atomic weight.
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Eugene Goldstein discovered positive particles by using a tube filled with hydrogen gas (this tube was similar to Thomson's tube). This resulted in The positive particle had a charge equal and opposite to the electron. The positive particle was named the proton.
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J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged
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Marie and Pierre Curi announced the discovery of two new elements, radium and polonium.
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Introduced the theory of "quanta", that is, that radiation consists of quanta with specific energies determined by a new fundamental constant, thereafter called Planck's constant.
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Stoney had calculated the magnitude of his electron from data obtained from the electrolysis of water and the kinetic theory of gases. George Johnstone Stoney was acknowledged for his contribution to developing the theory of electrons by H.A
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Rutherford overturned Thomson's model in with his well-known gold foil experiment in which he demonstrated that the atom has a tiny and heavy nucleus. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure.
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He was the first to announce the concept that atoms can be identical chemically and yet have different atomic weights. These related atoms are called isotopes
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Niels Bohr proposed a theory for the hydrogen atom, based on quantum theory that some physical quantities only take discrete values.