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350
Aristotle
350 BC- Aristotle did not believe in the atomic theory, for he believed that all matter could be broken down infinitely. He believed that all matter consisted of either Earth, Water, Fire, or Air. Aristotle did not complete an atomic model because he did not believe in atoms. -
442
Democritus
This is in BC, so it is before Aristotle (do not mind the order in the timeline)- Democritus created the first atomic model at this time. He believed that the atom was solid and indestructable, and was uncuttable (hence the word atom). The word atom derives from the Greek word "atomos" which translates to "uncuttabe." Democritus' atomic model was merely a round, solid circle consisting of only solid matter on the inside. This is the beginning of the Atomic Theory. -
442
Democritus Atomic Model
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Antoine Lavoisier's Law of Conservation of Mass
This law states that "mass is neither created nor destroyed in any ordinary chemical reaction." -
John Dalton
John Dalton was an English chemist who created the Atomic Theory of Matter. He used Democrtius' teachings and findings to aid his research; Dalton believed that atoms were in fact real, and all atoms cannot be created nor destroyed. Dalton's theory of spherical, solid atoms was based on measurable properties of mass. Dalton's theory stated that matter is composed of atoms of differing weights and combine in simple ratios. -
John Dalton Atomic Model
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Law of Definite Proportions
In 1806, French chemist Joseph Proust stated that "I shall conclude by deducing from these experiments the principle I have established at the commencement of this memoir, viz. that iron like many other metals is subject to the law of nature which presides at every true combination, that is to say, that it unites with two constant proportions of oxygen. In this respect it does not differ from tin, mercury, and lead, and, in a word, almost every known combustible." Every element is proportional. -
Henri Becquerel
Becquerel studied different properties of the atom, including radioactivity. In 1903, he was awarded many Nobel Prizes for Physics for his finding of radioactvity. Becquerel's discovery later helped scientists to create the ideal atomic model. -
Marie and Pierre Curie
Marie and her husband, Pierre, also studied radioactivity. They studied two radioactive elements, uranium and thorium, and discovered the two elements called polonium and radium. Marie gave direction that the discovery of radioactivity be used first for the treatment of cancer. -
J.J. Thomson- Cathode Ray Tube Experiment
Thomson discovered the electronby performing the cathode ray tube experiment, which had changed the model of the atom at that time. When he discovered the electron, the atom was still said to be a solid ball, and Thomson redefined that theory. He was awarded the Nobel Prize for Physics in 1906 for his discovery of electrons. Thomson, with his discovery of electrons, created the plum pudding atomic model. This is basically just a circle with positive energy and electrons roaming about inside. -
J.J. Thomson Plum Pudding Model
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Max Planck and his Quantum Theory
Planck was the founder of the Quantum Theory. He discovered that wave form energy can only be allowed in specific quantities, which helped with the finding of energy levels in atoms. This discovery added to the growth of the atomic model. -
Robert Millikan
He was an American physicist who studied the properties of electrons. Millikan discovered the overall charge of electron by conducting experiments called the "falling drop method." He concluded that the charge of electrons was constant throughout all of them. He inspired scientists to explore other parts of the atomic structure. -
Rutherford's Gold Foil Experiment- Discovery of Nucleus
In 1909, Rutherford and his team performed this experiment. In this experiment, the exsistence of a nucleus became evident. -
Ernest Rutherford
He was an English physicist that thought of J.J Thomson's model as incorrect. He created his own atomic model, giving the atom a nucleus and surounding the nucleus with orbitting electrons. He also ststed that the atom is mostly empty space, and he was the first person to split the atom. -
Rutherford's Nuclear Model
In this model, Rutherford did not include the presence of neutrons. Unlike neutrons, protons were included in the model. Rutherford stated that the nucleus is made up of positive energy while the negative energy is in the electron cloud. -
Modified Nuclear Model (may be out of order)
This date of this model is not correct, for it is only this date because it will go along with Rutherford on the timeline. In this model, he included the neutron after Chadwick's discovery. -
Henry Mosely
Henry Mosely was a student of Ernest Rutherford, and he was an English chemist. In order to study atomic structure better, he created the application of X-ray spectra.This allowed him to have a closer determination of atomic numbersnd give better positioning on the periodic table. -
Neils Bohr
In 1922, he received a Nobel Prize for his contributions to the atomic structure and quantum mechanics. Bohr had helped solve the lasting problems with the nuclear model of the atom and he said that electrons moved in fixed orbits around the nucleus. -
Neils Bohr Atomic Model
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Uncertainty Principle
Heisenburg stated that it was impossible to measure the velocity and the precision of an electron at the same time -
Warner Heisenburg
Heisenburg had published a book in 1925 that proposed his theory on quantum mechanics. He proved the uncertainty in Borhr's model by stating that one cannot know the momentum and velocity of an electron at the same time. He then stated that because of this, you can't know the exact location of an electron. -
Erwin Schrodinger
Erwin Schrodinger was an Austrian scientist that disagreed with Bohr's theory. Because of this, he created his own theory, which lead to the discovery of the Quantum Mechanical Model. -
Quantum Mechanical Model
The date may be off, but it was Schrodinger's model discovery. This model had a nucleus along with an electron cloud to hold the electrons. -
James Chadwick
He was the Britsh physicist who discovered the neutron. He first contemplated on why the number of protons differed from the tomic mass, and then he discovered the neutron. -
James Chadwick Atomic Model- Nucleus
