What is an Atom?

1. What is an Atom? ATOMIC STRUCTURE, NUCLEAR CHEMISTRY & UNIT 6

2. Democritus • He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? • Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.

3. History of the Atom • Aristotle – famous philosopher • Believed all substances made of 4 elements • Blending these in different proportions to get all substances • Fire – hot • air – light • Earth – cool, heavy • Water – wet

4. History of the Atom • Late 1700s – John Dalton • Teacher who summarized results of his experiments and those of others • Combined idea of elements with that of atoms

5. Dalton’s Atomic Theory • All matter is made of tiny, indivisible particles call atoms • Atoms of the same element are identical • Atoms of different elements combine in whole number ratios to form compounds • Chemical reactions involve the rearrangement of atoms. No atoms are created or destroyed.

6. Atoms • Smallest particle of an element that still retains the properties of that element

7. JJ Thompson • In 1897, Thompson discovered the first component part of the atom: the electron, a particle with a negative electric charge.  • In 1904, he proposed an initial model of an atom, since nicknamed "Thompsons pudding".  • He imagined the atom as a sphere full of an electrically positive substance mixed with negative electrons "like the raisins in a cake".

8. Voltage source Thomson’s Experiment Vacuum tube Metal Disks

9. Voltage source Thomson’s Experiment - + Vacuum tube Metal Disks

10. Voltage source Thomson’s Experiment - +

11. Voltage source Thomson’s Experiment - +

12. Voltage source Thomson’s Experiment - +

13. Voltage source Thomson’s Experiment - + Passing an electric current makes a beam appear to move from the negative end to the positive end

14. Voltage source Thomson’s Experiment - + Passing an electric current makes a beam appear to move from the negative end to the positive end

15. Voltage source Thomson’s Experiment - + Passing an electric current makes a beam appear to move from the negative end to the positive end

16. Voltage source Thomson’s Experiment - + Passing an electric current makes a beam appear to move from the negative end to the positive end

17. Voltage source Thomson’s Experiment - + By adding an electric field…

18. Voltage source Thomson’s Experiment - + + - By adding an electric field…

19. Voltage source Thomson’s Experiment - + + - By adding an electric field…

20. Voltage source Thomson’s Experiment - + + - By adding an electric field he found that the moving pieces were negative

21. Atomizer Oil droplets + - Oil Telescope Millikan’s Experiment

22. Millikan’s Experiment X-rays X-rays give some electrons a charge.

23. Millikan’s Experiment From the mass of the drop and the charge on the plates, he calculated the mass of an electron

24. Other Particles • Proton – positively charged pieces 1840 times heavier than electrons, discovered by E. Goldstein • Neutron – no charge but the same mass as a proton, by J. Chadwick • Where are these pieces?

25. Rutherford’s Experiment • Ernest Rutherford – English physicist • Believed in the plum pudding model of the atom

26. Rutherford’s Experiment • Wanted to see how large the positive particles are • Used radioactivity • Shot alpha particles at gold foil • Alpha particles – positively charged pieces,

27. Rutherford’s Experiment • When an alpha particle hits a fluorescent screen it glows

28. Fluorescent Screen Lead block Uranium Gold Foil

29. Rutherford’s Experiment • He expected the alpha particles to pass through without changing direction very much • Because… • The positive charges were thought to be spread out evenly • Not enough to stop the alpha particles

30. What he expected

31. Because

32. He thought the mass was evenly distributed in the atom

33. Since he thought the mass was evenly distributed in the atom

34. What he got

35. + How he explained it • Atom is mostly empty. • Small dense, positive piece at center. • Alpha particles are deflected by it if they get close enough.

36. +

37. Density and the Atom • Since most of the particles went through, it was mostly empty space • Because the alpha particles turned so much, the nucleus was heavy • Small volume, big mass, big density • The small dense positive area is the nucleus

38. The Nucleus

39. Unit 6 – Part 2 Structure of the Nuclear Atom

40. Subatomic Particles

41. Counting the Pieces • Atomic Number – the number of protons in the nucleus • All protons are alike so… • # of protons determines kind of atom • The same as the number of electrons in a neutral atom

42. Counting the Pieces • Mass number = # of protons + # of neutrons • Protons and neutrons account for most of the mass of an atom

43. Symbols • Contain the element symbol, mass number, and atomic number Mass number X Atomic number

44. Symbols • Find: • # of protons • # of neutrons • # of electrons • Atomic # • Mass # F 19 9

45. Symbols • Find: • # of protons • # of neutrons • # of electrons • Atomic # • Mass # Br 80 35

46. Symbols • If an element has 91 protons and 140 neutrons, what is the • Atomic # • Mass # • # of electrons • Complete symbol

47. Isotopes • Dalton was wrong • Atoms of the same element can have different numbers of neutrons • This means different mass numbers • Called isotopes

48. Naming Isotopes • Can also put mass # after name of the element • Carbon-12 • Carbon-14 • Uranium-235

49. Atomic Mass • There are different kinds of oxygen atoms. • More concerned with the average atomic mass

50. Atomic Mass • Based on the abundance of each in nature • Do not use grams because numbers would be too small