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History of Atomic Theories

Explore the transformative journey of atomic theories throughout history, from Democritus to Dalton to Thomson, unraveling the mysteries of matter and structure. Discover key milestones, experiments, and influential figures that shaped our understanding of the atomic world.

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History of Atomic Theories

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  1. History of Atomic Theories Dr. Chin Chu River Dell Regional High School

  2. History of the Atomic Theory • Remember: a scientific theory explains behaviors and the ‘nature’ of things • Theories can be revised when new discoveries are made • The theory describing the composition of matter has been revised many times

  3. Ancient Times Democritus Aristotle VS.

  4. Democritus (460-370 BC) 1.Matter is made up of “atoms” that are solid, indivisible and indestructible 2.Atoms constantly move in space 3.Different atoms have different size and shape 4.Changes in matter result from changes in the grouping of atoms 5. Properties of matter result from size, shape and movement

  5. Aristotle(384-322 BC ) 1. Four kinds of matter a. Fire – Earth – Water – Air 2. One kind of matter can transform into another 3. Rejected idea of the “atom” (idea then ignored for almost 2000 years 4. This theory was more popular and it was easier to accept

  6. Aristotle’s Theory of Matter

  7. Alchemists • Alchemy, a popular pursuit during the Middle Ages, was the search for a way to transform common metals into gold. • Though not successful to achieve their original goals, alchemists did generate vast amount of data relating to physical and chemical properties of matter, which in turn was instrumental in the development of modern atomic theory later. • Later on in the 17th century, some alchemists began focusing on identifying new compounds and reactions.

  8. Near Modern Time • Important figures: • Antoine Lavoisier (Law of Conservation of Mass) • Joseph Proust (Law of Definite Proportions) • John Dalton (Law of Multiple Proportions)

  9. Antoine Lavoisier (1770s) Experiment: 2 Sn + O2 2 SnO tin oxygen tin (II) oxide mass before reaction = mass after reaction Law of Conservation of Mass Matter cannot be created or destroyed (in a chemical or physical change)

  10. 1788-1799

  11. Joseph Proust (1779) Develops Law of Definite Composition- all samples of a specific substance contain the same mass ratio of the same elements a. ex: all samples of CO2 contains 27.3% carbon and 72.7% oxygen b. therefore ‘elements’ are combining in a whole number ratio

  12. John Dalton (1803) Dalton became a school teacher at the age of 12 (he left school at age 11) Loved meteorology - pioneer in this field Studied works of Democritus, Boyle and Proust Wrote New System of ChemicalPhilosophy in 1808

  13. Develops Law of Multiple Proportions a. describes the ratio of elements by mass in two different compounds composed of the same elements Example: carbon monoxide vs. carbon dioxide 1 part oxygen : 2 parts oxygen *when compared to the same amount of carbon in each compound

  14. Dalton collects data and develops Atomic Theory in 1803 a. Matter is made of small particles-atoms b. Atoms of a given element are identical in size, mass, but differ from those of other elements*. c. Atoms cannot be subdivided or destroyed*. d. Atoms combine in small whole number ratios to form compounds. e. Atoms combine, separate, or rearrange in chemical reactions. * Modified in Modern Atomic Theory

  15. Subatomic Particles A. J.J. Thomson (1887) Experiments with cathode ray tubes

  16. Voltage source - + Vacuum tube Metal Disks

  17. Voltage source - +

  18. Voltage source - +

  19. Voltage source - +

  20. Voltage source - + • Passing an electric current makes a beam appear to move from the negative to the positive end

  21. Voltage source - + • Passing an electric current makes a beam appear to move from the negative to the positive end

  22. Voltage source - + • Passing an electric current makes a beam appear to move from the negative to the positive end

  23. Voltage source - + • Passing an electric current makes a beam appear to move from the negative to the positive end

  24. Voltage source • By adding an electric field

  25. Voltage source + - • By adding an electric field

  26. Voltage source + - • By adding an electric field

  27. Voltage source + - • By adding an electric field

  28. Voltage source + - • By adding an electric field

  29. Voltage source + - • By adding an electric field

  30. Voltage source + - • By adding an electric field he found that the moving pieces were negative

  31. Thomson’s Model of the Atom • a. electrons present (-) • b. atom is like plum pudding - bunch of positive stuff (pudding), with the electrons suspended (plums) • Calculated the ratio between the charge of the electron and its mass: e/m “Chocolate Chip Cookie” or “Plum Pudding Model”

  32. Millikan’s Oil Drop Experiments Robert Milikan (1909) • Oil Drop Experiment • Measured the electrical charge on the electron • Mass can be calculated (Thomson determined the e/m ratio) • Mass is 1/1840 the mass of a hydrogen atom • electronhas a mass of 9.11 x 10-28 g

  33. Millikan’s Oil Drop Experiment (1909)

  34. History of the Atomic Structure – Summary thus far • So, at this point we know: - Atoms are divisible particles • Electrons are negatively charged • The mass of an electron is very small HOWEVER • Atoms should have a (+) portion to balance the negative part - Electrons are so small that some other particles must account for mass

  35. Ernest Rutherford (1871-1937) Experiment (1909) Gold Foil Experiment (Expectations) a. Shot alpha particles at atoms of gold b. expected them to pass straight through

  36. Florescent Screen Lead block Uranium Gold Foil

  37. He thought this would happen:

  38. According to Thomson Model

  39. He thought the mass of the positive charge was evenly distributed in the atom

  40. Here is what he observed:

  41. The positive region accounts for deflection

  42. Gold Foil Experiment Results a. Most positive alpha particles pass right through b. However, a few were deflected c. Rutherford reasoned that the positive alpha particle was deflected or repelled by a concentration of positive charge

  43. Gold Foil Experiment Conclusions a. the atom is mostly empty space b. the atom has a small, dense positive center surrounded by electrons

  44. History of the Atomic Structure • At this point in 1909, we know: • p+ = 1.67 x 10-24 g • e- = 9.11 x 10-28 g • The charges balance! • But, • How are the electrons arranged? • There is still mass that is unaccounted for

  45. Niels Bohr (1913) Electrons orbit nucleus in predictable paths

  46. Chadwick (1891 – 1974) In 1935 1. Discovers neutron in nucleus 2. Neutron is neutral - does not have a charge n0 3. Mass is 1.67 x 10-24 g slightly greater than the mass of a proton

  47. History of the Atomic Theory

  48. History of the Atomic Structure • Charges balanced • Mass accounted for • However – what about the behavior of the electrons?

  49. The Quantum Atom Theory 1. The atom is mostly empty space 2.Two regions: a. Nucleus- protons and neutrons b.Electron cloud- region where you have a 90% chance of finding an electron

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