2. The Structure of the Atom Chapter 4 4.1 - Early Theories of the Atom 4.2 - Subatomic Particles 4.3 - How Atoms Differ 4.4 - Unstable Nuclei & Radioactivity – CHEMISTRY 112

3. History of the atom Not the history of atom, but the idea of the atom Original idea Ancient Greece (400 B.C..) Democritus and Leucippus Greek philosophers Atomos - not to be cut

4. Another Greek Aristotle - Famous philosopher All substances are made of 4 elements Fire - Hot Air - light Earth - cool, heavy Water - wet Blend these in different proportions to get all substances

5. Who Was Right? Greek society was slave based Beneath Famous to work with hands did not experiment Greeks settled disagreements by argument Aristotle was more famous He won His ideas carried through middle ages. Alchemists change lead to gold

6. 4.1 Early Theories of Matter Dalton’s Atomic Theory 1.All matter is composed of extremely small particles called atoms. 2.All atoms of a given element are identical. Atoms of different elements are different from one another. 3.Atoms cannot be created or divided into smaller particles or destroyed. 4.Different atoms combine in simple whole number ratios to form compounds. 5.In a chemical reaction, atoms are separated, combined, or rearranged. Dalton’s Atomic Model Atom - the smallest particle of an element that retains the properties of the element.

7. 4.1 Early Theories of Matter E. Goldstein discovered the proton in 1886. J.J. Thomson discovered the electron in 1897 during cathode ray tube experiments in the late 1890’s.

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

9. Thomson’s Experiment Voltage source +-

10. Thomson’s Experiment Voltage source +-

11. Thomson’s Experiment Voltage source +-

12. Thomson’s Experiment Voltage source +-

13. n Passing an electric current makes a beam appear to move from the negative to the positive end 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. Thomson’s Experiment By adding an electric field

18. Voltage source 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 he found that the moving pieces were negative + -

24. Thomson’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

25. 4.1 Early Theories of Matter Robert A. Millikan determined the mass and charge of the electron in 1916. –one unit of negative charge –mass is 1/1840 of a hydrogen atom

26. 4.1 Early Theories of Matter In 1911 Ernest Rutherford discovered the nucleus during his gold foil experiment.

27. 4.1 Early Theories of Matter Ernest Rutherford’s gold foil experiment.

28. Lead block Uranium Gold Foil Florescent Screen

29. He Expected The alpha particles to pass through without changing direction very much Because The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

30. What he expected

31. Because

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

34. What he got

35. +

36. 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

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

38. 4.1 Early Theories of Matter Neils Bohr developed the planetary model of the atom –Electrons are in a particular path have a fixed energy –Energy level-region around a nucleus where the electron is likely to be moving

39. 4.1 Early Theories of Matter Erwin Schrodinger developed the Quantum Mechanical Model –Describes the electronic structure of the atom as the probability of finding electrons within certain regions of space

40. 4.1 Early Theories of Matter James Chadwick discovered the neutron in 1932. In 1913 Henry Mosley used X-rays to count the number of protons in the atomic nuclei of different atoms.

41. 4.2 Subatomic Particles & the Nuclear Atom Located within the Nucleus –Proton (p + ) Positively charged particle (each carries a charge of +1) Relative mass = 1 amu Actual mass = 1.673 X 10 -27 kg –Neutron (n 0 ) Neutrally charged particle Relative mass = 1 amu Actual mass = 1.675 X 10 -27 kg Serves as the glue that holds the nucleus together as well as a buffer between the charges of protons and electrons

42. Subatomic Particles Located outside the nucleus in the electron cloud –Electron Negatively charged particle (each carries a charge of -1) Relative mass = 1/1840 amu Actual mass = 9.11 X 10 -31 kg The electron is the part of the atom that will function in bonding and reactions

43. 4.3 How Atoms Differ Atomic Number –the number of protons in the nucleus of an atom –indicated at the top of the element’s block on the periodic table 8O15.999 12Mg24.305 Oxygen has an atomic number of 8 There are 8 protons in an atom of Oxygen Magnesium has an atomic number of 12 There are 12 protons in an atom of Magnesium

44. Isotopes Atoms of the same element with the same number of protons, but different numbers of neutrons Since the atoms have different numbers of neutrons, they also have different mass numbers –Mass number = # of protons + # of neutrons

45. Abbreviating Isotopes Hyphen Notation –Simply write the Name of the atom, put a hyphen, and then write the mass number Carbon-12 vs. Carbon-14 –Carbon 12 has 6 protons and 6 neutrons –Carbon 14 has 6 protons and 8 neutrons Nuclear Designation –Element symbol is written in the center –Mass number goes in the upper left corner –Atomic number goes in the lower left corner C 12 6

46. Different Isotopes Identify the number of protons, neutrons, and electrons each of the following have. Boron-10 Boron 11 35 Cl 66 Zn 1730 35 Br 79.904 p + : ________ n o : ________ e - : ________ p + : ________ n o : ________ e - : ________ p + : ________ n o : ________ e - : ________

47. Calculating Atomic Mass Mass Number –the number of protons + neutrons in a given isotope Atomic Mass –The weighted average mass of all of the isotopes of that element [(Mass of isotope A)(percent abundance )] + [(Mass of isotope B)(percent abundance)]

48. Practice Calculating Atomic Mass Calculate the atomic mass of helium given the following information: There are two naturally occurring isotopes of helium: Isotope% AbundanceMass helium-3 0.0001 3.0160 helium-4 99.9999 4.0026

49. Practice Calculating Atomic Mass There are two naturally occurring isotopes of helium: Isotope % Abundance Mass helium-3 0.0001 3.0160 helium-4 99.9999 4.0026 (3.0160 x 0.000 001) + (4.0026 x 0.999999) 0.000 003 0160 + 4.002595997 0.000 003 0160 + 4.0026 = 4.002603016 = 4.0026

50. Practice Calculating Atomic Mass There are three naturally existing isotopes of silicon: silicon-28, silicon-29, and silicon-30. Their percents of natural abundance is listed respectfully: 92.21 %, 4.70 %, and 3.09 %. Calculate the average atomic mass of silicon and express your answer in 4 significant digits.

51. 4.4 Unstable Nuclei & Radioactive Decay Nuclear Reactions –reactions that involve a change in the nucleus of an atom. Radioactivity –the spontaneous release of radiation. Radiation –rays and particles emitted by radioactive materials Radioactive atoms emit radiation because their nuclei are unstable. There are three main types of radiation –Alpha decay –Beta decay –Gamma decay

52. Alpha ( α ) radiation two protons and 2 neutrons Positive charge Symbols:  4  4 He 2 2 reduces the atomic number by 2 reduces the mass by 4

53. Beta ( β ) radiation Fast moving electron Negative charge Symbols:  0  increases the atomic number by 1 does not change the mass

54. Gamma ( γ ) radiation high energy radiation released with alpha and beta radiation symbol:  0  0 does not change the mass or atomic number

55. Half lives The time it takes for 1/2 of the mass of the isotope to be decayed. If I have a 60g sample and the half life is 2 years, how long will it take for there to be 7.5g left of the sample? 60g  30g  15g  7.5g 2 years 4 years 6 years So, it takes 6 years for the 60g sample to decay into 7.5g.