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400
Early Greek philosophers (Democritus, Leucippus)
Atomism; 5th C BCE. Matter is not made of atoms, it is only made of the four elements (Earth, Air, Fire, wind). -
Alchemists tried to turn base metals (like lead) into gold
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). -
Antoine Lavoisier
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. -
Joseph Proust
Law of definite Proportions; 1798-1804. A chemical compound always contains exactly the same proportion of elements by mass. -
John Dalton
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. -
John Dalton
All matter is made of atoms; 1803 -
Henri Becquerel- Discovery of radioactivity
Henri Becquerel discovered radioactivity in 1896. He used naturally fluorescent minerals to study the properties of x-rays. -
JJ Thompson- Discovery of the electron using a cathode ray tube
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. -
Marie and Pierre Curie- Discovered radioactivity if radium and uranium
their achievements included a theory of radioactivity techniques for isolating radioactive isotopes, and discovery of two elements, radium and uranium -
Ernest Rutherford
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 -
Albert Einstein
E=MC^2; 1905. Origin of energy from nuclear experiments. -
Robert Millikan
Millikan discovered the oil drop experiment in 1909. It was to measure the charge of the electron. -
Henry Moseley- Discovered that electron exist in orbits at different distances from the nucleus
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 -
Niels Bohr- Dicovered that each element has a specific,unique atomic number
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 -
Max Planck- Discovered that energy is given off in "packets" called quanta or photons
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 -
James Chadwick- Discovery of the neutron
James Chadwick discovered the neutron in 1932. He used scattering data to calculate the mass of this neutral particle. -
Lise Meitner, Otto Hahn, and Fritz Straussman- Discovery of nuclear fission
Lise Meitner, Otto Hahn, and Fritz Straussman discovered nuclear fission in 1938. It was the process by which an atom splits into lighter atoms. -
Martin Karplus, Michael Levitt, and Arieh Warshel
In the 1970s, these men successfully developed methods that combined quantum and classical mechanics to calculate the courses of chemical reactions using computers.
