2. Chapter 2 (Part I) Atoms, Molecules, and Ions

3. History n Greeks n Democritus and Leucippus - atomos n Aristotle- elements n Alchemy n 1660 - Robert Boyle- experimental definition of element. n Lavoisier- Father of modern chemistry n He wrote the book- used measurement

4. Laws n Conservation of Mass n Law of Definite Proportion- compounds have a constant composition by mass. n They react in specific ratios by mass. n Multiple Proportions- When two elements form more than one compound, the ratios of the masses of the second element that combine with one gram of the first can be reduced to small whole numbers.

5. What?! n Water has 8 g of oxygen per g of hydrogen. n Hydrogen peroxide has 16 g of oxygen per g of hydrogen. n 16/8 = 2/1 n Small whole number ratios

6. Example of Law Of Multiple Proportions n Mercury has two oxides. One is 96.2 % mercury by mass, the other is 92.6 % mercury by mass. n Show that these compounds follow the law of multiple proportion. n Speculate on the formula of the two oxides.

7. Your Turn n Nitrogen and oxygen form two compounds. Show that they follow the law of multiple proportions Amount N Amount O Compound A 1.206 g 2.755 g Compound B 1.651g 4.714 g

8. Dalton’s Atomic Theory n 1. Elements are made up of atoms n 2. Atoms of each element are identical. Atoms of different elements are different. n 3. Compounds are formed when atoms combine. Each compound has a specific number and kinds of atom. n 4. Chemical reactions are rearrangement of atoms. Atoms are not created or destroyed.

9. n Gay-Lussac- under the same conditions of temperature and pressure, compounds always react in whole number ratios by volume. n Avagadro- interpreted that to mean n at the same temperature and pressure, equal volumes of gas contain the same number of particles n (called Avagadro’s hypothesis) A Helpful Observation

10. Experiments to determine what an atom was n J. J. Thomson- used Cathode ray tubes

11. Thomson’s Experiment Voltage source +-

12. Thomson’s Experiment Voltage source +-

13. Thomson’s Experiment Voltage source +-

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

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

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

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

18. Thomson’s Experiment n By adding an electric field

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

20. Voltage source Thomson’s Experiment n By adding an electric field + -

21. Voltage source Thomson’s Experiment n By adding an electric field + -

22. Voltage source Thomson’s Experiment n By adding an electric field + -

23. Voltage source Thomson’s Experiment n By adding an electric field + -

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

25. Thomsom’s Model n Found the electron n Couldn’t find positive (for a while) n Said the atom was like plum pudding n A bunch of positive stuff, with the electrons able to be removed

26. Millikan’s Experiment Atomizer Microscope - + Oil

27. Millikan’s Experiment Oil Atomizer Microscope - + Oil droplets

28. Millikan’s Experiment X-rays X-rays give some drops a charge by knocking off electrons

29. Millikan’s Experiment +

30. They put an electric charge on the plates + + --

31. Millikan’s Experiment Some drops would hover + + --

32. Millikan’s Experiment + ++ +++++ -- -----

33. Measure the drop and find volume from 4/3πr 3 Find mass from M = D x V + + --

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

35. Radioactivity n Discovered by accident n Bequerel n Three types –alpha- helium nucleus (+2 charge, large mass) –beta- high speed electron –gamma- high energy light

36. Rutherford’s Experiment n Used uranium to produce alpha particles n Aimed alpha particles at gold foil by drilling hole in lead block n Since the mass is evenly distributed in gold atoms alpha particles should go straight through. n Used gold foil because it could be made atoms thin

37. Lead block Uranium Gold Foil Florescent Screen

38. What he expected

39. Because

40. Because, he thought the mass was evenly distributed in the atom

41. What he got

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

43. +

44. Modern View n The atom is mostly empty space n Two regions n Nucleus- protons and neutrons n Electron cloud- region where you have a chance of finding an electron

45. Sub-atomic Particles n Z - atomic number = number of protons determines type of atom n A - mass number = number of protons + neutrons n Number of protons = number of electrons if neutral

46. Symbols X A Z Na 23 11