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500 BCE
The Alchemists
The Alchemists in 500BC through 1720 developed the theory that all metals are composed of mercury and sulfur and that it is possible to change base metals to gold. -
450 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. His views about elements are incorrect. -
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
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. -
22
John Dalton
"1. Daltons Atomic Theory states that all elements are composed of atoms that are indivisible (they can not be created, divided, or destroyed)
2. All atoms of the same element are identical.
3. Atoms of different elements are different (they always have different numbers of protons and they may also differ in mass numbers.
4. Atoms of different elements can combine with each other in simple whole number ratios to form compounds" -Chemistry Notes -
Robert Boyle
Boyle's Law states that if the volume of a gas is decreased, the pressure increases proportionally. Understanding that his results could be explained if all gases were made of tiny particles, Boyle tried to construct a universal 'corpuscular theory' of chemistry. -
Lavoisier
Antoine Lavoisier created the Law of Conservation of Mass
This states that mass is neither created nor destroyed in chemical reactions. We now understand why matter is conserved -- atoms are neither created nor destroyed in a chemical reaction. -
Dmitri Mendeleev
Mendeleev created the first Periodic Table. The Periodic Law states that when elements are arranged according to their atomic number, elements with similar properties will appear at regular intervals. -
J.J. Thomson
John Dalton was incorrect that the atom was invisible. J.J. Johnson made this discovery and also revised other parts of Dalton's atomic model. Electrons were also founded by J.J. Johnson. -
The Curies
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. -
Albert Einstein
Einstein mathematically proved the existence of atoms and he helped revolutionize all the sciences through the use of statistics and probability. -
Robert Millikan
His earliest major success was the accurate determination of the charge carried by an electron, using the elegant “falling-drop method”; he also proved that this quantity was a constant for all electrons, thus demonstrating the atomic structure of electricity. -
Ernest Rutherford
Ernest Rutherford found and named the nucleus. He found this information out by doing an experiment. He used a sheet of gold foil and aimed a beam of alpha particles. He noticed a few of the particles were greatly deflected. -
Henry G. J. Mosely
Mosely used self-built equipment to prove that every element's identity is uniquely determined by the number of protons it has. His discovery revealed the true basis of the periodic table and enabled Moseley to predict confidently the existence of four new chemical elements, all of which were found. -
Neils 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. Bohr's theory could explain why atoms gave off light in fixed wavelengths. -
Werner Heisenberg
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.
