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Atomic Theory Timeline

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BC: As of April 6, 2010, team of Russian and American scientists has discovered a new element that has long stood as a missing link among the heaviest bits of atomic matter ever produced. The element, still nameless, appears to point the way toward still more massive elements with chemical properties no one can predict. Dawn A. Shaughnessy, who is on the team says if the treand continues toward a theorized "island of stability" at high masses, the work could generate a wide array of new materials with unimagined scientific and practical uses.

FC: Atomic Theory Timeline By: Madison Harper Period 2 Honors Chemistry

1: 430 B.C. Democritus proposed the first idea of the atom, all substances are made of tiny indestructible, indivisible particles called 'atomos'.

2: 1788 Lavoisier developed the Law of Conservation of Mass. It said that matter can not be created nor destroyed, only rearranged.

3: Proust developed the Law of Definite Proportions. He said that pure substance will always contain the same element conbined in the same proportion by mass. | 1794

4: 1. All matter is made up of indivisible atoms 2. Atoms of the same element are the same 3. Atoms of different elements are different 4. Atoms combine in whole number ratios to form compounds. He also came up with the first atom structure, a solid sphere. | 1803 John Dalton proposes the first Atomic Theory. It had the following four criteria:

5: 1804 Gay-Lusac used Dalton's Atomic Theory to develop his Law of Combining Volumes: at constant temperature and pressure, volumes of reacting gases and gascous products are in the ratio of small whole numbers.

6: 1. All matter is made up of indivisible atoms 2. Atoms of the same element are the same 3. Atoms of different elements are different 4. Atoms combine in whole number ratios to form compounds. He also came up with the first atom structure, a solid sphere. | 1803 John Dalton proposes the first Atomic Theory. It had the following four criteria:

7: 1804 Gay-Lusac used Dalton's Atomic Theory to develop his Law of Combining Volumes: at constant temperature and pressure, volumes of reacting gases and gascous products are in the ratio of small whole numbers.

8: Dalton developed the Law of Multiple Proportions. IT stated that the same two elements can combine in different ratios to form different compounds. The ratio of the first element is fixed and the second varies. | 1810

9: 1811 Amadeo Avogadro created Avogadro's Hypothethis. It said equal volumes of gases, under the same conditions, have the same number of particles. The number of particles were defined as one mole.

10: 1896 Henri Becquerel discovered that uranium ores emit radiation resembling x-rays and that is exposed film. His assistants, Marie and Pierre Curie, isolated two new elements, polonium and radium, from uranium. These radioactive substances ionized air, causing phosphorescent substances to glow, kill bacteria and small organisms, and increase the temperature of the air surrounding the substance.

11: 1897 J.J. Thomson used cathode rays to discover the electron. He measured the bending of the rays to determine the charge to mass ratio of the electron. This led to modified cathode ray tubes to discover a positive charge. These particles were named protons. | Thomson also developed the plum pudding model of the atom.

12: 1900 Mark Planck proposed Quantum Theory after studying light and heat, saying that energy is not emitted continuously, but in small packets called quanta.

13: 1905 Albert Einstein studied the relationship between energy and mass. He developed the Law of Conservation of Mass-Energy, saying that the sum total of mass and energy must be conserved in a reaction. HE also suggest that light also a particle nature when he explained the photoelectric effect. That is when light of a certain frequency shines on a metal, electrons are emitted.

14: 1911 Robert Millikan performed the "oil drop experiment" to determine the charge on a electron. He used Thomson's charge to mass ratio to calculate the mass of an electron. 1. Sprayed oil into chamber, electrons transferred to droplets 2. Oil drops fall through chamber due to gravity 3. Charge in plates adjusted to off set gravity/suspend drop 4. Charge of drops calculated

15: 1911 Ernest Rutherford conducted the "gold foil experiment." 1. Gold foil was bonbarded with alpha particles 2. Most passed through the foil, but some deflected He concluded that an atom is mostly empty space, with a very small positive core, the nucleus. This led to his model of the atom-the nuclear model.

16: 1913 Henry Moseley used x-ray experiments to discover the atomic number of an element. The results showed that wavelengths of an x-ray emitted depended on the metal used, the wavelength was unique to that element, and it depends on the number of protons in an atom.

17: 1913 Niels Bohr improved the atomic model by attempting to answer the question of why electrons are not pulled into the nucleus. HE created the planetary model of the atom. He said that electrons travel in definite energy levels without radiating energy, electrons in each orbit have a certain amount of energy, energy increases as the distance from the nucleus increases, and electrons lose energy by dropping to the lower energy levels.

18: 1920 Niels Bohr and Wolfgang Pauli developed the Aufbau principle or "building up." It said that electrons occupy the lowest energy level possible.

19: 1923 Louis De Broglie developed DeBRogle's Hypothesis during the Davisson-Germer experiment. It said that if waves can behave like particles, then particles can behave like waves.

20: 1925 Wolfgang Pauli constructed the PAuli Exclusion Principle. This principle says that no two electrons can have the same set of four quantum numbers.

21: 1926 Erwin Schrodinger created his Wave Equation. He said that electrons should be treated as a wave. His equation determined the probability of finding an electron in any given place around the nucleus. His solutions were quantum numbers (n, l, ml, ms). He also developed the most recent atomic model-the Quantum Model.

22: 1927 Friedrich Hund developed Hun'ds Rule of "fair share". That meant that each orbital gets one electron before it gets a second electron.

23: 1927 Werner Heisenberg developed the Uncertainty Principle. He stated that it is impossible to know the position and velocity of the electron at the same time.

24: 1932 James Chadwick discovered the third subatomic particle, the neutron. The discovery of subatomic particles led to the revision of Dalton's Atomic Theory: Atoms were not indivisible and indestructible; they are made up of smaller particles. Discovery of isotopes led to another revision: Atoms of the same element are not exactly alike, they have the same number of protons, but may have different numbers of neutrons.

25: As of July 2, 2010, a team of Western Michigan University researchers are among the first to show what the world's most powerful x-ray laser can do. Scientific results show the x-ray laser's unique ability to control the behaviors of individual electrons within simple atoms and molecules by stripping them away, one by one--in some cases creating hollow atoms. Controlling those changes will be critical to achieving the atomic-scale images of biological molecules and movies of chemical processes

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