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500 BCE
The Alchemists
AlChemists-500BC-1720 develop the theory that all metals are composed of mercury and sulfur and that it is possible to change base metals into gold. Dalton's most important contribution to science was his theory that matter is composed of atoms of differing weights and combine in simple ratios by weight. -
427 BCE
Plato
Plato introduced the atomic theory in which ideal geometric forms serve as atoms, according to which atoms broke down mathematically into triangles, such that the form elements had the following shape fire (tetrahedron), air (octahedron), water (icosahedron), earth (cube). -
400 BCE
Democritus
The theory of Democritus held that everything is composed of atoms, which are physically, but not geometrically, indivisible that between atoms, there lies empty space that atoms are indestructible, and have always been and always will be in motion. -
330 BCE
Solar system model
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322 BCE
Aristotle
Aristotle did not believe in the atomic theory and he taught so otherwise. He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. He believed all substances were made of small amounts of these four elements of matter. -
Robert Boyle
Robert Boyle's major contribution to the atomic theory was that he helped develop a definition of an element any substance that can be broken into 2 or more substances is not an element and helped with " the death" of the four elements. Also he helped change the way people think of science. -
Lavoisier
A later breakthrough in the discovery of the atomic model came through the work of French chemist Antoine Lavoisier who through a series of experiments found that the total mass of products and reactants in a chemical reactions is always the same. This led to the theory of the law of conservation of mass. -
John dalton
Dalton's atomic theory proposed that all matter was composed of atoms, indivisible and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of differing size and mass. -
Solid Sphere of billard ball
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Dmitri Mendeleev
Mendeleev found that, when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements. -
Robert Millikan
His oil drop experiment helped to quantify the charge of an electron, which contributed greatly to our understanding of the structure of the atom and atomic theory. -
The Curies
In 1898 French physicists Pierre and Marie Curie discovered the strongly radioactive elements polonium and radium, which occur naturally in uranium minerals. Marie coined the term radioactivity for the spontaneous emission of ionizing, penetrating rays by certain atoms. -
JJ Thomson
J.J. Thomson is credited with the discovery of the electron, the negatively charged particle in the atom. He is known for the Thomson atomic theory. In 1904, Thomson proposed a model of the atom as a sphere of positive matter with electrons positioned based on electrostatic forces. -
Plum pudding model
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Albert Einstein
Einstein also in 1905 mathematically proved the existence of atoms, and thus helped revolutionize all the sciences through the use of statistics and probability. Atomic theory says that any liquid is made up of molecules. Furthermore, these molecules are always in random, ceaseless motion. -
Ernest Rutherford
Ernest Rutherford postulated the nuclear structure of the atom, discovered alpha and beta rays, and proposed the laws of radioactive decay. He received the Nobel Prize in Chemistry in 1908. -
Niels Bohr
In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. -
Henry G. J. Mosely
Henry Moseley was an English physicist who contributed to atomic theory and lived from 1887 to 1915. This is because an element's atomic number tells us the number of protons in its nucleus and that number never changes, which takes care of the problem brought on by isotopes, which have different numbers of neutrons. -
Electron cloud model
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Werner Heisenburg
Werner Heisenberg contributed to atomic theory through formulating quantum mechanics in terms of matrices and in discovering the uncertainty principle, which states that a particle's position and momentum cannot both be known exactly.
