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Democritus
The date above is incorrect, it was actually around 460 BCE that Democritus "came" up with the atomic theory. He expanded on Leucippus's (his teacher) atomic theory. He stated that the atmosphere was filled with small particles- atoms- that were indivisible and indestructible. Unfortunately, his view was not accepted, as Aristotle's was the one accepted in Greece. Aristotle disagreed with his atomic theory. As a result, the people disagreed with his views, as Aristotle was highly respected. -
Antoine Lavoisier
Lavoisier is considered the father of modern chemistry. He stated that an element was a substance that could not be broken down through a chemical analysis. He first started experimenting with oxygen and realized that it could not be broken down any further. He also developed the very first Periodic Table, in which the elements were organized due to their properties. -
John Dalton
John Dalton had important postulates for the atom. They were: All matter is made of atoms, an element is composed of the same type of atoms, atoms are the units of chemical changes, atoms combine to form molecules, and atoms can be neither created nor destroyed. It slightly relates to Lavoisier's experiments in which the elements were made up of atoms. -
Henri Becquerel
Becquerel was studying phosphorescence when he discovered there was a form of radiation that was only slightly related to x-rays. By covering uranium salts with opaque paper, he left it there. Later, when he found it, the paper had become foggy. Upon experimenting with the other uranium slats, he concluded that the salts ionized that gases. He basically discovered spontaneous radiation. He recieved half of a Nobel prize, shared with the Curies. -
J J Thomson- The Electron
Thomson discovered the electron through the use of a high vacuum cathode ray tube, in which he conducted many experiments with it. Using the cathode ray tube, he noticed the rays were being deflected by magnets and electrically charged plates. Through the use of negatively charged plates, when the rays were deflected, he was able to prove that there were electrons in existence, and they had a negative charge. He also noticed they were extremely small. Through this, he developed a model. -
Max Planck
Planck introduced the concept of the quanta- a unit of measurement of packets of energy emitted by matter. He was a physicist, and devoloped the quanta unit to explain the colors of glowing matter. Niels Bohr used the quanta as a way to develop a new model of the atom. -
Marie and Pierre Curie
Marie and Pierre Curie were a pair who worked on the radiation of atoms. They had heard of Becquerel's uranium rays, and upon doing so, decided to test whether or not the same rays were existence for thorium. They were. The pair decided to test more compounds of uranium, and found that the same rays existed for all the compounds. Then, upon taking smaller samples, they found the energy was even greater. They also found radium's atomic weight. The couple shared the 1903 Nobel with Becquerel. -
Robert Millikan
During 1908-1917, Millikan conducted a series of experiments that were known as the oil drop experiment. He used a drop of oil between capacitator plates to measure the change in charge as it entered the region between the plates. Through the experiment, he developed a way to measure the charge of the electron- and it was the same value for all matter. -
Ernest Rutherford
From 1910-1911, Ernest Rutherford had conducted his gold foil experiment, an experiment that would show that most particles would pass through a piece of gold foil, but some particles- 1 in 8000 were deflected back, leading to the thought that there was something in the middle of an atom. Though he did not say it was the nucleus, what he did discover that caused the particles to be deflected is indeed the nucleus. Through this, he developed his own model of the atom. -
Henry Moseley
Henry Moseley helped develop a more accurate periodic table than of Mendeleev's. His experiments showed that the X-Ray emissions of the atoms didn't coincide with the atomic weight, but more instead with the atomic number. Afterwards, this cleared up some of the confusion due to the ordering of the atomic weight, and with atomic number, the periodic table was reordered. Moseley studied under Rutherford -
Niels Bohr
Bohr based his model of the atom off of Rutherford's model. He first realized there was something wrong with the model- that the atom should be extremely unstable. In order to fix the problem, he used Planck's equation to adjust the electrons into levels. That way, when energy is added or decreased, the electron jumps or drops, but always goes back to the original position it was in. And, each level could contain more electrons, the outermost containing the valence electrons. -
Erwin Schrodinger
Schrodinger developed wave mechanics, which is related to atoms. He had looked at Bohr's atomic model, and had been unsatisfied with the equations there. He proceeded to use mathematical equations to describe how likely an electron was to be in one spot. His model predicts the location of the electron, not the exact position of the electron in its ring. It is shown as an electron cloud around the nucleus. -
Werner Heisenberg
Heisenberg developed the Heisenberg Uncertainty Principle- It is impossible to measure simultaneously the exact position and exact velocity or momentum of a sub-atomic particle like electron and neutron. This came about due to him contradicting Bohr's atomic model, that states the electrons are in exact positions, but according to wave mechanics, electrons position and energy can't be measured exactly at the same time. -
James Chadwick
It had been thought that the nucleus was simply positively charged, that only protons were within it. However, when Chadwick bombarded beryllium with alpha particle, an unknown radiation emitted from the atom. The radiation had a neutral electrical charge, and the approximate mass of a proton. Thus, it was named the neutron. From this, new models of the atom were made. -
Aristotle
Again, the above date is incorrect. It was during 384-322 BCE that Aristotle worked on his science theories. One of them was his atomic theory, in which all the matter of Earth is made up of four elements- fire, earth, air, and water. His contradicted Democritus's theory, as he used elements in his theory, not small, indivisible parts to create matter. His depiction of the atom is like a diamond- each element at a vertex.
