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460 BCE
Democritus
Democritus theorized that materials are made up of invisible atoms and was a central figure in the development of the atomic theory of the universe. -
450 BCE
Aristotle
Aristotle thought that all materials on earth were not made of atoms and were made with earth, fire, water and air. He believed substances had these elements in them. -
427 BCE
Plato
Plato introduced the atomic theory in which atoms break down to geometric forms serve as atoms such as, fire is a tetrahedron, air is a octahedron, water is a icosahedron and earth is a cube. -
400
The Alchemists
The alchemists practiced from 400-1400 AD, to turn common elements like lead into gold. The alchemists used symbols to identify elements, and made many important discoveries which led to the modern science of chemistry. Two of the most famous alchemists were Paracelsus, who is known as the father of pharmacology because of his work in the chemical treatment of ailments, and Agricola, who described the preparation of sulfuric and nitric acids -
Robert Boyle
Robert Boyle discovered that the volume of a gas decreases with increasing pressure and decreasing pressure increases volume which is called Boyle's law. A leading scientist and intellectual of his day, he was a great proponent of the experimental method. -
Lavoisier
Lavoisier discovered that oxygen is important in combustion. Also he recognized oxygen and hydrogen and opposed the phlogiston theory. -
Solid Sphere of Billiard Ball Model
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John Dalton
Dalton's theorized that all matter was composed of atoms which couldn't be seen and were invincible indivisible and indestructible building blocks. -
Dmitri Mendeleev
Dmitri Mendeleev wrote down the symbols for the chemical elements and put them in order with their atomic weights which made the periodic table. -
Neils Bohr
Niels Bohr in 1913 made theory that the hydrogen atom based on the quantum theory is that energy is transferred in certain quantities and electrons move around the nucleus but only in certain orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. -
J.J. Thomson
J.J. Thomson's experiments with cathode ray tubes showed that atoms contain small negatively charged particles and electrons. Thomson made the plum pudding model which represented the Atom and represented negatively-charged electrons embedded within a positively-charged -
The Curies
In 1898 French physicists Pierre and Marie Curie discovered radioactive elements polonium and radium which are in uranium minerals. -
Plum Pudding Model
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Robert Millikan
Robert Millikan got the accurate determination of a charge carried by an electron, using his falling-drop method and proved that this was electrons and demonstrating the atomic structure of electricity. -
Ernest Rutherford
Rutherford in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny and heavy nucleus. -
Henry G.J. Mosely
Henry made equipment to show every element is determined from the number of protons it has. -
Solar System Model
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Electron Cloud Model
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Albert Einstein
Albert Einstein made equation E=mc2 which meant he discovered that energy and mass are interchangeable which helped develop nuclear power and atomic weapons. -
Werner Heisenberg
Werner Heisenberg contributed to atomic theory by formulating quantum mechanics and in discovering the uncertainty principle, which say that a particle's position and momentum cannot be known exactly.
