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Atomism; 5th C BCE. Matter is not made of atoms, it is only made of the four elements (Earth, Air, Fire, wind).
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Alchemists tried to turn base metals (like lead) into gold in the 1700s. They based their assumptions based on the world and everything in it being composed of four basic elements (air, earth, fire and water).
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Law of conservation of mass; 1774. Matter can be changed from one form into another, mixtures can be separated or made, and pure substances can be decomposed, but the total amount of mass remains constant.
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Law of definite Proportions; 1798-1804. A chemical compound always contains exactly the same proportion of elements by mass.
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Dalton introduced a theory that proposed that elements differed due to the mass of their atoms. It also stated that all compounds were composed of combinations of these atoms in defined ratios. He discovered this atomic theory in 1803.
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All matter is made of atoms; 1803
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Henri Becquerel discovered radioactivity in 1896. He used naturally fluorescent minerals to study the properties of x-rays.
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JJ Thomson discovered the electron using a cathode ray tube in 1897. He showed that the rays were composed of a previously unknown negatively charged particle.
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their achievements included a theory of radioactivity techniques for isolating radioactive isotopes, and discovery of two elements, radium and uranium
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He was responsible for a remarkable series of discoveries in the fields of radioactivity and nuclear physics and he postulated the nuclear structure of the atom
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E=MC^2; 1905. Origin of energy from nuclear experiments.
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Millikan discovered the oil drop experiment in 1909. It was to measure the charge of the electron.
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He developed the application of X-ray spectra to study atomic structure. His discoveries resulted in more accurate positioning of elements in the Periodic Table by closer determination of atomic numbers
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Bohr developed the Bohr model of the atom,in which he proposed that energy levels of electrons are discrete and that the electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level to another
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He considered to be initial founder of quantum theory. He realized that light and other electromagnetic waves were emitted in discrete packets of energy that he called “quanta” in 1923
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James Chadwick discovered the neutron in 1932. He used scattering data to calculate the mass of this neutral particle.
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Lise Meitner, Otto Hahn, and Fritz Straussman discovered nuclear fission in 1938. It was the process by which an atom splits into lighter atoms.
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In the 1970s, these men successfully developed methods that combined quantum and classical mechanics to calculate the courses of chemical reactions using computers.
