1. ATOMS & RADIOACTIVITY ATOMS & RADIOACTIVITY THE ATOM THE ATOM

2. THE HISTORICAL DEVELOPMENT OF ATOMIC MODELS Thomson’s model Rutherford’s model

3. Thomson’s Model The atom is like a raisin bun, with negative raisins in a positive bun. The atom is like a raisin bun, with negative raisins in a positive bun.

4. Rutherford’s Model Rutherford used alpha particles to investigate the composition of gold foil (i.e. to explain the model of an atom). Rutherford used alpha particles to investigate the composition of gold foil (i.e. to explain the model of an atom). He fired alpha particles through a piece of gold foil and used a zinc sulphide detector to detect the scattered alpha particles and their location. He fired alpha particles through a piece of gold foil and used a zinc sulphide detector to detect the scattered alpha particles and their location.

5. Apparatus Rutherford’s alpha scattering apparatus:   -source vacuum gold foil zinc sulphide detector

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28. Rutherford’s experiment found that: Most of the alpha particles passed through the gold foil undeviated. A few alpha particles were deflected from their path but continued through the gold foil. A small number of alpha particles rebounded. Results

29. As most alpha particles passed through the gold foil atoms undeviated, Rutherford concluded that most of the atom was actually empty space. From the results of his experiment, Rutherford explained: The deviation of some alpha particles from their original path were due to positive charges within the foil. Conclusion

30. From the results of his experiment, Rutherford explained: Conclusion A small number of alpha particles had rebounded because they collided with something much larger and heavier and which contains a concentrated region of positive charge.

31. As a result of his observations, Rutherford suggested that the atom had a positively charged centre which contained most of the mass. He called the heavy positively charged centre the nucleus. He went on to suggest that the nucleus was surrounded by orbiting electrons required for electrical neutrality. Conclusion

32. As a result of his observations, Rutherford suggested that the atom had a positively charged centre which contained most of the mass. He called the heavy positively charged centre the nucleus. He went on to suggest that the nucleus was surrounded by orbiting electrons required for electrical neutrality. Conclusion

33. As a result of his observations, Rutherford suggested that the atom had a positively charged centre which contained most of the mass. He called the heavy positively charged centre the nucleus. He went on to suggest that the nucleus was surrounded by orbiting electrons required for electrical neutrality. Conclusion

34. As a result of his observations, Rutherford suggested that the atom had a positively charged centre which contained most of the mass. He called the heavy positively charged centre the nucleus. He went on to suggest that the nucleus was surrounded by orbiting electrons required for electrical neutrality. Conclusion

35. As a result of his observations, Rutherford suggested that the atom had a positively charged centre which contained most of the mass. He called the heavy positively charged centre the nucleus. He went on to suggest that the nucleus was surrounded by orbiting electrons required for electrical neutrality. Conclusion

36. Modern measurements show that the average nucleus has a radius in the order of 10 -15 m. This is 100, 000 times smaller than the radius of a typical atom.

37. Bohr’s Model Electrons revolve around the nucleus in circular orbits The centripetal force is provided by the electrostatic attraction between the electrons and nucleus. Electrons exist only in certain orbits. The radius of the orbit depends on the energy of the electron ;high energy electrons have large orbits.