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

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

4. Ancient Times DemocritusAristotle VS.

5. 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

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

7. Aristotle’s Theory of Matter

8. Alchemy, a popular pursuit during the Middle Ages, was the search for a way to transform common metals into gold. 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. 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 17 th century, some alchemists began focusing on identifying new compounds and reactions. Later on in the 17 th century, some alchemists began focusing on identifying new compounds and reactions. Alchemists

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

10. Antoine Lavoisier (1770s) Experiment: Experiment: 2 Sn + O 2  2 SnO 2 Sn + O 2  2 SnO tin oxygen tin (II) oxide 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)

11. 1788-1799

12. 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 CO 2 contains 27.3% a. ex: all samples of CO 2 contains 27.3% carbon and 72.7% oxygen carbon and 72.7% oxygen b. therefore ‘elements’ are combining b. therefore ‘elements’ are combining in a whole number ratio in a whole number ratio

13. 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 Chemical Philosophy in 1808

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

15. 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 * Modified in Modern Atomic Theory Dalton collects data and develops Atomic Theory in 1803

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

17. Voltage source +- Vacuum tube Metal Disks

18. Voltage source +-

19. +-

20. +-

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

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

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

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

25. By adding an electric field By adding an electric field

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

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

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

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

30. Voltage source n By adding an electric field + -

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

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

33. Millikan’s Oil Drop Experiments Robert Milikan (1909) –Oil Drop Experiment Oil Drop ExperimentOil 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 –electron has a mass of 9.11 x 10 -28 g

34. Millikan’s Oil Drop Experiment (1909)

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

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

37. Lead block Uranium Gold Foil Florescent Screen

38. He thought this would happen:

39. According to Thomson Model

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

42. Here is what he observed:

43. The positive region accounts for deflection

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

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

46. At this point in 1909, we know: 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, But, –How are the electrons arranged? –There is still mass that is unaccounted for History of the Atomic Structure

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

48. In 1935 In 1935 1. Discovers neutron in nucleus 2. Neutron is neutral - does not have a charge n 0 not have a charge n 0 3. Mass is 1.67 x 10 -24 g slightly greater than the mass of a proton slightly greater than the mass of a proton Chadwick (1891 – 1974)

49. History of the Atomic Theory History of the Atomic Theory 18031897190919131935Today solidparticleelectronproton e- orbit nucleus neutron Quantum Atom theory DaltonThomsonRutherfordBohrChadwick Schrodinger and others

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

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

52. The Current Model for Atoms

53. 1.All matter is made up of small particles called atoms. 2.Atoms of the same element have the same chemical properties while atoms of different elements have different properties 3.Not all atoms of an element have the same mass, but they all have a definite average mass which is characteristic. (isotopes) Modern Atomic Theory

54. 4. Atoms of different elements combine to form compounds and each element in the compound loses its characteristic properties. 5. Atoms cannot be subdivided by chemical or physical changes – only by nuclear changes changes Modern Atomic Theory (cont.)