-
400 BCE
Democritus
The Greek scientist, Democritus, developed a theory that everything is composed of "atoms", which are physically indivisible. Nothing had been developed before this so he said they were indestructible and are infinite. The shape and size of the atom varies and it is perfectly solid.He developed a theory to what was made up of all life. The year the model was adopted is unknown, but it is around 400 BCE. -
Period: 400 BCE to
History of the Atom Timeline
-
332 BCE
Aristotle
Another Greek scientist, Aristotle, disagreed with Democritus' theory and instead proposed that all materials on earth were made of the four elements: fire, water, air, and earth instead of atoms.This theory was adopted during the 300's and was taught for nearly 2000 years, with scientists discarding Democritus' idea. He was the only scientist on this timeline to say this. -
Antoine Lavoisier
Over the course of 22 years, between 1772 through 1794, french scientist and experimenter, Antoine Lavoisier. After experimenting for a while, he proposed several theories which included the Combustion theory and the Law of Conservation of Mass. He also combined several molecules together to create elements and compounds. He even created the first periodic table which included 32 elements. He further proved that Democritus' theory was right and started a discussion about the atom. -
John Dalton
English chemist, John Dalton, proved that Democritus' theory was right and brought upon the studying of the atom. He proved that all matter is made up of atoms and atoms are indivisible and indestructible. He also discovered that compounds are formed by a combination of two or more different kinds of atoms. It was over a few years that he adapted this this theory. It enhanced other models like Democritus and Lavoisier. -
Henri Becquerel
French physicist, Henri Becquerel, discovered radioactivity while experimenting with uranium and a photographic slate. He discovered it in 1896 and it made other models better later on with the discovery. It showed the atoms and the particles that were attracted together with the nuclei. Further work was made by the Curies. -
J.J. Thomson
English physicist, J.J. Thomson, discovered the electron by experimenting with a cathode ray. He demonstrated that the ray was negatively charged. He discovered this in 1897, and this helped other scientists use the electron as the first particle of the atom and the most important. -
Marie and Pierre Curie
In 1898, polish scientists, Marie and Pierre Curie, discovered two new elements, which were radium and polonium, and they also were the pioneers of radioactivity. They won the Nobel prize in 1903, and shared it with fellow scientist, Henri Becquerel. This helped the atom and future experiments, so they could record more about the atom, and it also added to the periodic table which was growing exponentially. According to them, the nucleus of an atom contained high amounts of radiation. -
Max Planck
In 1900, German physicist, Max Planck proposed the quantum theory which explained energy is radiated in very minute and discrete quantized amounts or packets, rather in a continuous unbroken wave. This help contributed to the understanding of subatomic and atomic processes and used these packets of energy to determine that the energy of each quantum is equal to the frequency of the radiation multiplied by a universal constant that he derived. Niel Bohrs improved upon this theory. -
Robert Millikan
In 1908, american physicist, 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. His model had the charge of electron figured out and it was on the model of his experiment. This enhanced the work of other scientists to figure other charges of subatomic particles and more and helped the science of the atom overall. -
Ernest Rutherford
In 1911, New Zealand physicist, Ernest Rutherford overturned Thomson's model of the atom after conducting an experiment with gold foil, in which he demonstrated that the atom has a tiny, heavy nucleus. He then experimented using alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. This was the newest model and helped other scientists to make more models. In his model, it shows a nucleus in the center. -
Niels Bohr
In 1913, Danish physicist, Niels Bohr, proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons move around the nucleus, but only in prescribed orbits. He proposed a model of the atom, called the Bohr model based on this evidence of the atomic structure which included energy levels that are discrete and jumped one orbit to another. His teacher was Rutherford and his model was used by the whole world. -
Henry Moseley
In 1913, British physicist, Henry Moseley, developed a law called Moseley's law in X-ray spectra to study atomic structure and discovered a way to organize elements by positioning in the Periodic Table by determination of atomic numbers. His graph showed the range of atomic numbers by square root of X-ray frequency. His work and discoveries enhanced the organization of elements in the periodic table ad changed the scientific world completely. He was a student of Ernest Rutherford. -
Erwin Schrodinger
In 1926, Erwin Schrodinger, an Austrian physicist improved upon the Bohr model. He used mathematical equations to describe the likelihood of finding an electron in a certain position. This model was used for finding electrons and is step up from Bohr's model. He also made an equation that accurately calculated the energy levels of electrons in atoms. -
Werner Heisenburg
In 1927, German physicist, Werner Heisenburg, contributed to the atomic theory using quantum mechanics, which is based on quantum theory, used for interpreting the behavior of elementary particles and atoms. This helped with more quantum mechanic experiments in the future and overall helped the atomic structure of an atom a lot. Niels Bohr introduced him to quantum physics and atomic structure. -
James Chadwick
In 1932, British scientist, James Chadwick, discovered the neutron, a subatomic particle with a neutral charge that had about the same mass as a proton. Earlier, Ernest Rutherford, proposed that such a particle could exist in the atomic nuclei. This enhanced the science world as it drastically changed how an atom's shape is. The Periodic Table changed and the atomic mass was produced as a result of it. The atomic model now added neutrons to it and was found in the nucleus.
