Atomic Theory Timeline

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Atomic Theory Timeline

Atomic Theory Timeline

460 bc

development of the atomic theory

1897: Cathode Ray Tube ExperimentJ.J. Thompson's Cathode Ray Experiment was used to investigate whether or not the negative charge could be separated from the cathode ray by magnets. To test this, Thompson used a cathode ray tube with metal cylinders (electrodes) on either end that had very low pressure. He then passed an eletric current through the tube, which produced a glow. He then put the positive side of a magnet to the edge of the tube and the cathode ray was drawn towards it. As a result, he was able to conclude that there were particles with a negative charge, which came to be known as electrons.

1906: Plum Pudding Atomic ModelJ.J. Thompson originally believed that an atom's electrons must make up its mass, but an atom made up of thousands of electrons would have a very high, negative electric charge. This is odd because atoms are usually without charge. In 1906, Thompson suggested that an atom consisted of much fewer electrons than he had originally though. The electrons had to be balanced out by a positive charge. He then proposed a plum pudding model with positive and negative charges filling a sphere of about 1 10 billionth of a meter across. This was generally accepted, but later proved incorrect by Thompson's own student, Ernest Rutherford.

1911: Rutherford and The Gold Foil ExperimentWorking at Manchester University in 1910, his investigations into the scattering of alpha rays and the nature of the inner structure of the atom, which caused such scattering, led to the suggestion of his concept of the "nucleus". According to Rutherford, almost the whole mass of the atom and all positive charge of the atom is concentrated in the atom’s center. He aimed a beam of alpha particles at a piece of gold foil. In accordance to the J.J. Thomson model of an atom, the alpha particles should have passed directly through the gold foil for all instances. To confirm this activity, a zinc sulfide screen was placed behind the foil as a backdrop for the alpha particles to appear upon. Rutherford fired the beam of alpha particles through the piece of foil and observed the location at which the particles landed on the screen. Each particle should have gone directly through the foil if the plum pudding model was correct. For the most part, the alpha particles passed straight through the gold foil. There were, however, a small handful of particles that deflected slightly from the straight path by about one or two degrees. But the biggest discovery was made when 1 in 20,000 particles would deflect approximately 90 degrees or more from the parent beam. Two conclusions: one, an atom was much more than just empty space and scattered electrons (J.J. Thomson model argued), and two, an atom must have a positively charged center that contains most of its mass (which Rutherford termed as the nucleus). rutherford and the Gold Foil Experiment helped further knowledge on subatomic particlues and their placement, also introducing the nucleus.

Sophie DeemAbbie HollernMaddie Reeves

1803: John Dalton and The Atomic TheoryHe was originally a meteorologist who switched to chemistry when he saw the applications for chemistry of his ideas about the atmosphere.Proposed the atomic theory in 1803.Atomic Theory:1. All elements are composed of tiny indivisible particles called atoms.2. Atoms oft he same element are identical. The atoms of any one element are different from those of any other element. 3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. 4. Chemical reactions occur when atoms are separate joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction. Dalton's Atomic Theory helped give a more specific vision of the atom, as well as a testable hypothesis for future scientists to test and prove or disprove.


460 B.C.: Democritus Democritus believed atoms were indivisible and indestructible. His ideas agreed with later atomic theories, but they didn't explain chemical behavior. The approach was not based on scientific method, so there was no experimental support. This helped set up a basic idea of the atom for future scientists to work with.








Democritus' model of an indivisible, indestructible atom.

Thomson's Plum Pudding Model

John Dalton

1890s Robert Millikan:Millikan used a small drop of oil suspended between two electrical plates to measure the incremental charge of an electron. He thought oil would gain a charge due to the friction as it moved between the plates. He ionized the surrounding atmosphere as well as monitored the motion of different drops of oil, which allowed him to compare the time the drop took under gravity and with the electrical plates off, against the time it took the drop to climb under the voltage. The drop with the electrical field occurred in discreet units, which said that the charge of an electron was a single value and it would be the same for all forms of electricity. His experiment settled the dispute over the charge and charge to mass ratio. Both of these discoveries let Niels Bohr calculate the Rhydenberg constant and provided the first and most important proof of the new atomic theory.

1913: Niels Bohr and His Planetary ModelNiels Bohr was a Danish scientist who developed a planetary model for the atom. His model shows that the electrons in an atom are in orbitals, differing in the amounts of energy, around the nucleus. He said that the energy of an electron can have 1 energy level or another, but nothing in between. The energy level an electron normally occupies is called the ground state. The energy level an electron occupies when it is at a higher energy level and less stable is called the excited state. Bohr discovered that the various energy levels hold differing numbers of electrons in order to be considered stable.


1932: James ChadwickAs atomic disintegration was being studied, scientists noticed that the number of protons were about half of the atomic mass, but Chadwick kept in mind the idea of an atom without a charge. Chadwick became interested in the methods of Frederic and Irene Joliot - Curie, which they had used to track particle radiation. He used the same expereriment , but was able to find the neutral particle which had no charge, but a mass almost equal to that of a proton. This was the third subatomic particle to be discovered, and was named the neutron.

1926: Erwin SchrodingerSchrodinger was the first to begin thinking of the movement of an electron as a wave. He used mathematical equations to describe the likelihood of finding an electron in a specific position. From his studies, he is able to put together the quantum mechanical model. His model was different from Niels Bohr's in that it predicted the odds. He came to the realization that the nucleus is surrounded by an electron cloud, thus making his Electron Cloud Model, and where the electron cloud is most dense, the probability increases. This model introduced the concept of sub energy levels.

CAthode Ray Tube Experiment

Niels Bohr's Planetary Model

1785: Antoine Lavoisier and the Law of Conservation of MassLavoisier discovered the law of conservation of mass in 1785. The law of conservation of mass states that the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction. This stands for all reactants and products in any chemical reaction. The discovery of the law of conservation of mass aided in the development of the modern day atomic theory. Daltons second law of the atomic theory and this theory are somewhat similar some what similar. Dalton states in his theory,”Atoms can neither be created nor destroyed," and the law of conservations of mass states “ the mass of reactant before a chemical reaction begins is the same as the mass of the products of the reaction." The law of conservation of mass really fueled the curiosity of scientists around the world.

1869: Dimitri Mendeleev and The Periodic TableMendeleev arranged the elements in the periodic table in order of increasing atomic mass. He left spaces win the table here he predicted new elements would be discovered. The periodic table is the basis for all chemistry. Once scientists learned the structure of the atoms, the periodic table was arranged in order of increasing atomic number.


1911: Ernest Rutherford and His Ruthermodel Rutherford pondered over the experiments of his colleage and professor, and concluded that they were not completely accurate. He theorized that the only way the alpha particles could be deflected backwards was if most of the mass in an atom was concentrated in the nucleus he then developed model of an atom with protons in the nucleus and the electrons orbiting around the nucleus, which became known as the Ruthermodel.


Mendeleev's Periodic Table

1913: Henry Moseley Moseley discovered the wavelengths of the x-rays of elements became shorter as the atomic weight increased, which meant that both the frequency and energy were increasing because of the inverse relationship between both wavelengths and frequency. He rearranged the periodic table by atomic number instead of atomic mass, the new table left holes for elements that would soon be discovered and is still the basis for our periodic table today.



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