1. The Atomic Model

2. The atomic model is an idea of what an atom looks like. It is not a model like a model airplane. Scientific models are simply working representations. Model of the Atom

3. There are two different types of models. 1. Conceptual 2. Mathematical

4. Scientific models of any kind are just tools to help us visualize real-life phenomena. Model of the Atom

5. History of the Atomic Model 1. Leucippus

6. Leucippus was a Greek who believed that there is a smallest particle. This belief implies that the atom is indivisible. Leucippus

7. History of the Atomic Model 1. Leucippus 2. Democritus

8. Democritus was a student of Leucippus. He called the smallest particle the atom. (Atomos means “indivisible.”) He believed that atoms make up all matter. Democritus

10. Democritus was right! There is a basic unit of matter called the atom. However, the atom is not indivisible, and it has not always existed. Democritus

11. Democritus made no observations, so he cannot be called a scientist. He reasoned, not experimented, to create his model. Democritus

12. Question What did Democritus do? 1.He discovered the atom. 2.He theorized that atoms exist. 3.He discovered the electron. 4.He discovered the proton.

13. Question Democritus was right when he said 1.matter is composed of atoms. 2.atoms are indestructible. 3.atoms are large. 4.matter is composed of protons and electrons.

14. As more information was discovered about the atom, scientists made changes to the atomic model. Model change represents a revolution in the way scientists are thinking. The Atom Rediscovered

15. History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton

16. Dalton created the first experimental model by weighing substances and applying the Law of Definite Proportions. Dalton

17. The Law of Definite Proportions, discovered by Joseph Proust, states that every compound has a definite composition by mass. For example, water’s composition by mass is a 1:8 ratio. Dalton

18. Dalton used Proust’s Law and further experiments to come up with his model called the core-envelope model. Dalton

19. core heat envelope

20. Dalton’s Model Atoms are neither created nor destroyed. Atoms cannot be subdivided.

21. Elements are made only of atoms. All atoms of an element are the same. Atoms of elements differ in their masses. Dalton’s Model

22. History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton 4. Thomson

23. Thomson found that the atom is not solid—it has negative particles in it. He discovered the electron (e – ). Thomson

24. The electrons are in a positive (+) gel-like substance. The electrons and the positive substance charges cancel out. Electrons do not vary in mass or charge. Electrons can be removed from the atom. Thomson’s Model

25. Thomson’s model is known as the plum-pudding model. Thomson’s Model

26. positive “pudding” negative electrons “plums” atom

27. Thomson’s model can also be called the chocolate chip cookie model. Thomson’s Model

28. Question Thomson’s model looked like 1.peanut brittle. 2.roast beef. 3.plum pudding. 4.a hot dog.

29. Question Thomson’s cathode rays were 1.protons. 2.neutrons. 3.quarks. 4.electrons.

30. History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton 4. Thomson 5. Rutherford

31. Rutherford shot alpha particles at thin gold foil.  Alpha particles are relatively heavy and positively charged. Some of the alpha particles bounced straight back, revealing that atoms have a nucleus made of positive protons (p + ). Rutherford

32. glass plate vacuum chamber microscope zinc sulfide screen alpha particle beam scattered alpha particles gold foil rotating housing

33. Rutherford’s model is called the nuclear model. Rutherford’s Model

34. Protons form a small, central nucleus only 1/100,000 the size of the atom. Protons are many times larger than electrons. Electrons are whirling somewhere around the nucleus. Most of the atom is empty space. Rutherford’s Model

35. tiny dense positive nucleus electrons occupy space around nucleus

36. Question Rutherford’s experiment involved 1.a cathode ray. 2.alpha particles. 3.neutron emission. 4.atomic absorption diffraction.

37. Question Rutherford discovered 1.protons. 2.neutrons. 3.quarks. 4.electrons.

38. History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton 4. Thomson 5. Rutherford 6. Chadwick

39. Chadwick discovered the neutron (n°). The neutron is as big as the proton but it has no charge. The neutron is also in the nucleus. Chadwick

40. History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton 4. Thomson 5. Rutherford 6. Chadwick 7. Bohr

41. Bohr knew that atoms give off colors when heated (like fireworks). Looking through a prism, he could see different colors being emitted. Bohr

42. Bohr thought that the electrons jumped further from the nucleus when heated and then fell back down, giving off energy as they fell. The electrons occupy specific levels at their normal energies. Bohr

43. Each atom has a different arrangement of electrons, so it gives off different colors of light. Electrons can jump from level to level. Bohr’s Model

44. Bohr’s model looked like the solar system, so it was called the planetary model. Bohr’s Model

45. Kr

46. History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton 4. Thomson 5. Rutherford 6. Chadwick 7. Bohr 8. Quantum model

47. The quantum model replaces Bohr’s exact orbits with regions (like whirling fan blades) where electrons are most likely to be found. These regions are called orbitals. Quantum Model