History and Discoveries of Atoms and Molecules

1. Chapter 2Atoms, Molecules, and Ions AP Chemistry West Valley High School Mr. Mata

2. History • Greeks • Democritus and Leucippus – “atomos” • Aristotle- elements (earth, water, fire, air) • Alchemy (tried to convert Pb Au) • 1660 - Robert Boyle- experimental definition of element. • Lavoisier- Father of modern chemistry • He wrote the book- used measurement

3. Laws • Conservation of Mass • Law of Definite Proportion- compounds have a constant composition by mass. • They react in specific ratios by mass. • Multiple Proportions- When two elements form more than one compound, the ratios of the masses can be reduced to small whole numbers.

4. What?! • Water (H2O) has 16 g of O per 2 g of H or 8 g O per 1 g of H. • Hydrogen peroxide (H2O2) has 32 g of O per 2 g of H or 16 g of O per 1 g of H. • If both H2O and H2O2 form from reactants, they would have a 16 g/8 g ratio = 2/1 or a 2:1 ratio. • Small whole number ratios.

5. Dalton’s Atomic Theory • 1. Elements are made up of atoms • 2. Atoms of each element are identical; atoms of different elements are different. • 3. Compounds are formed when atoms combine. Each compound has a specific number and kinds of atom. • 4. Chemical reactions are rearrangements of atoms. Atoms are not created nor destroyed.

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

7. Experiments to determine what an atom was • J. J. Thomson- used Cathode ray tubes

8. Voltage source Thomson’s Experiment - +

9. Voltage source Thomson’s Experiment - +

10. Voltage source Thomson’s Experiment - +

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

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

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

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

15. Voltage source Thomson’s Experiment • By adding an electric field

16. Voltage source Thomson’s Experiment + - • By adding an electric field

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

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

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

23. Atomizer + - Oil Microscope Millikan’s Experiment

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

25. Millikan’s Experiment X-rays X-rays give some drops a charge by knocking offelectrons

26. + Millikan’s Experiment

27. Millikan’s Experiment - - + + They put an electric charge on the plates

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

29. + Millikan’s Experiment - - - - - - - + + + + + + +

30. Millikan’s Experiment - - + + Measure the drop and find volume from 4/3πr3 Find mass from M = D x V

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

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

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

34. Florescent Screen Lead block Uranium Gold Foil

35. What he expected

36. Because

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

38. What he got

39. + 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

40. +

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

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

43. Symbols A X Z 23 Na 11

44. Chemical Bonds • The forces that hold atoms together. • Covalent bonding - sharing electrons. • makes molecules. • Chemical formula- the number and type of atoms in a molecule. • C2H6 - 2 carbon atoms, 6 hydrogen atoms, • Structural formula shows the connections, but not necessarily the shape.

45. Structural Formula H H H C C H H H

46. There are also other model that attempt to show three dimensional shape • Ball and stick (see the models in room) • Space Filling

47. Ions • Atoms or groups of atoms with a charge. • Cations- positive ions - get by losing electrons(s). • Anions- negative ions - get by gaining electron(s). • Ionic bonding- held together by the opposite charges. • Ionic solids are called salts.

48. Polyatomic Ions • Groups of atoms that have a charge. • Yes, you have to memorize them. • List on page 65 of Zumdahl (9th edition).

49. Periodic Table

50. Metals • Conductors • Lose electrons • Malleable and ductile