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400
Aristotle
The theory that the Atom was the smallest and indivisible form of matter was proposed in 400 BC,this theory, however, was not supported or accepted by Aristotle. Aristotle's widly accepted theory was that of the elements, he believed that there was no smallest unit of matter, but rather individual proportions of earth, fire, air and water. Since there was no means to prove either theory, Aristotle's theory dominated mainly because his philosophy was widly accepted and admired. -
400
Democritus
Democritus was a philosopher from 400-500 BC that said that matter was made up of particles in motion. He called these paticles atoms, which derived from the Greek word "atomos" meaning "indivisible". Democritus said that they were just that, indivisible, they could not be seen and that they were solid, with no void inside. The atomic model of this time resembled that of a pool ball. Solid and sperical. As there was no means of scientific proof to this theory, it was accepted because of reputati -
John Dalton
1600-1700. Dalton came about his Atomic Theory during his research into gasses in. Dalton developed the 5 fundamental theorems which stated that pure elements consist of particles called atom,atoms of an element are all the same,atoms of different elements can indentified by their atomic weights,atoms of elements unite to form chemical compounds and atoms can neither be created or destroyed in chemical reaction. -
Henri Becquerel
Becquerel was a physicist from Paris who discovered radioactivity in Uranium. This discovery radically changed the way scientists viewed the structure of the atom. This new evidence proved that the atom was not indivisible and, in fact, could change form and could expel an enormous amount of energy. Becquerel unveiled the importance of radioactivty and its relevance to the interpretation of the inside of an atom. -
J.J. Thomson
In the late 1800's Thomson discovered the eledctron and theorized that atoms were neutrally charged because the amount of positive charge and negative charges in an atom cancelled each other out. Thomson also created the plum pudding or chocolate chip cookie model. The plum pudding model suggested that the atom was not a solid sphere. It was called the cookie model because the positive parts of an atom were referred to as the dough of the cookie and the electrons were the chocolate chips. -
Marie and Pierre Curie
Marie and Pierre Curie, together, did incredible work in the field of radioactivity. The two founded two new elements, Polonium and Radium. Radioactivity and its discovery proved that the atom was divisible, disproving the ideas of Dalton and others. The two favored the idea that the atom looked like a planet surrounded by many electron rings. Marie and Pierre Curie were also -
Robert Millikan
In 1909, Robert Millikan conducted his "oil-drop" experiment which allowed him to measure the charge of an electron. Alongside the findings of fellow physicist Thomson, Millikan concluded that e = 1.60 x 10-19 coulombs -
Ernest Rutherford
In 1912, Rutherford discovered the atomic nucleus, and said that the nucleus contained the positive charge and the majority of the mass of the atom within its very small center. It was Rutherford's Gold Foil experiment that led him to the creation of this model, as it disproved J.J. Thomson's accepted "plum pudding" model of the time. Rutherford was also known for discovering the proton. -
Niels Bohr
in 1913 Niels Bohr introduced the Rutherford-Bohr or plenetary model which showed the atom as a small positively charged nucleus surrounded by electrons. Bohr's model was a modification on the Rutherford Model and greatly resembled a model of the solar system. Bohr's model is the most widly accepted Atomic Model and was established when Bohr extended Rutherfords research to further state that "electrons "jump" between energy levels (orbits) in a quantum fashion" -
James Chadwick
In 1932, James Chadwick successfully discovered the neutron, a non charged particle within the nucleus. With the discovery of the neutron, the atomic model became virtually complete and gave scientists the answer to the discrepancy they saw between the atomic mass and atomic number. Chadwick's experiments with Beryllium and alpha particles are what led him to this discovery. -
Quantum Model
The Quantum Model was proposed when Werner Heisenberg came up witht he uncertainty principle which said that it is impossible to calculate momentum or location of an electron in an atom, so it must be based on probability to find an electron in a given space. Quantum model includeds a nucleus containing neutrons and protons. Quantum model deals with orbitals and probability of a given element
