2. Chapter 4 Atoms and their structure

3. Early Theories of Matter (4.1) n 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

4. Early Theories of Matter (4.1) n Democritus - Famous philosopher –Mater is composed of empty space through which atoms move –Atoms are solid, homogeneous, indestructible, and invisible. –Different atoms have different sizes and shapes.

5. Dalton’s Atomic Theory  All matter is made of tiny indivisible particles called atoms.  Atoms of the same element are identical, those of different atoms are different.  Atoms of different elements combine in whole number ratios to form compounds.  Chemical reactions involve the rearrangement of atoms.  Atoms cannot be created or destroyed.

6. n Atom- The smallest particle of an element that retains the properties of the element.

7. Subatomic Particles and the Nuclear Atom (4.2) n J. J. Thomson - English physicist. 1897 n Made a piece of equipment called a cathode ray tube –It is a vacuum tube - all the air has been pumped out. –A limited amount of other gases are put in

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

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

10. Thomson’s Experiment n By adding an electric field + -

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

12. Thomson’s Model n Found the electron. n Electron - Negatively charged particles of the atom. –Mass = 9.1 x 10^-28 grams

13. Millikan’s Experiment Atomizer Microscope - + Oil Metal Plates

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

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

16. - Millikan’s Experiment +

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

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

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

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

21. Rutherford’s Experiment n Ernest Rutherford English physicist. (1910) n Used radioactivity. n Alpha particles - positively charged pieces given off by uranium. n Shot them at gold foil which can be made a few atoms thick.

22. Rutherford’s experiment n When the alpha particles hit a florescent screen, it glows. n Here’s what it looked like (pg 95)

23. Lead block Uranium Gold Foil Fluorescent Screen

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

25. What he expected

26. Because

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

29. What he got

30. 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.

31. Rutherford’s experiment (cont.) +

32. Modern View n The atom is mostly empty space. n Two regions. n Nucleus- protons and neutrons. n Electron cloud- region where you might find an electron.

33. Other pieces n Proton - positively charged pieces 1840 times heavier than the electron. n Neutron - no charge but the same mass as a proton.

34. Subatomic particles Electron Proton Neutron NameSymbolCharge Relative mass Actual mass (g) e-e- p+p+ n0n0 +1 0 1/1840 1 1 9.11 x 10 -28 1.67 x 10 -24

35. Structure of the Atom n There are two regions. n The nucleus. –With protons and neutrons –Positive charge. –Almost all the mass. n Electron cloud- most of the volume of an atom. –The region where the electron can be found.

36. How Atoms Differ n Atoms are small. n Nucleus tiny compared to atom. n IF the atom was the size of a stadium, the nucleus would be the size of a marble.

37. Counting the Pieces n Atomic Number = number of protons –# of protons determines kind of atom. –the same as the number of electrons in the neutral atom. n Mass Number = the number of protons + neutrons.

38. Isotopes n Dalton was wrong. n Atoms of the same element can have different numbers of neutrons. n different mass numbers. n Isotopes- Atoms with the same number of protons but different number of neutrons.

39. Symbols n Contain the symbol of the element, the mass number and the atomic number.

40. Symbols X Mass number Atomic number

41. Naming Isotopes n Put the mass number after the name of the element. n carbon- 12 n carbon -14 n uranium-235

42. Symbols n Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass Number –Name Na 24 11

43. Symbols n Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass Number –Name Br 80 35

44. Symbols n if an element has an atomic number of 34 and a mass number of 78 what is the –number of protons –number of neutrons –number of electrons –Complete symbol –Name

45. Symbols n if an element has 91 protons and 140 neutrons what is the –Atomic number –Mass number –number of electrons –Complete symbol –Name

46. Symbols n if an element has 78 electrons and 117 neutrons what is the –Atomic number –Mass number –number of protons –Complete symbol –Name

47. Atomic Mass n How heavy is an atom of oxygen? n There are different kinds of oxygen atoms. n More concerned with average atomic mass. n Based on abundance of each element in nature. n Don’t use grams because the numbers would be too small.

48. Measuring Atomic Mass n Unit is the Atomic Mass Unit (amu) n One twelfth the mass of a carbon-12 atom. n 6 p + and 6 n 0 n Each isotope has its own atomic mass n we get the average using percent abundance.

49. Calculating averages n You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? n Total mass = 4 x 50 + 1 x 60 = 260 g n Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 n Average mass = 4 x 50 + 1 x 60 = 260 g 55 5

50. Calculating averages n Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5 n Average mass =.8 x 50 +.2 x 60 n 80% of the rocks were 50 grams n 20% of the rocks were 60 grams n Average = % as decimal x mass + % as decimal x mass + % as decimal x mass +

51. Atomic Mass n Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu.

52. Atomic Mass n Magnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 25 with a mass of 25.9826 amu. What is the atomic mass of magnesium? n If not told otherwise, the mass of the isotope is the mass number in amu

53. Atomic Mass n Is not a whole number because it is an average. n are the decimal numbers on the periodic table.

54. Unstable Nuclei and Radioactive Decay (4.4) Decay (4.4)  Up to this point the nucleus of the atom has remained unchanged –The protons and neutrons  Nuclear reactions are reactions that involve specifically changes in the nucleus of an atom. Unstable Nuclei and Radioactive Decay (4.4) Decay (4.4)  Up to this point the nucleus of the atom has remained unchanged –The protons and neutrons  Nuclear reactions are reactions that involve specifically changes in the nucleus of an atom.

55.  Nuclear reactions are responsible for producing what we think of as radiation.  There are three different forms of nuclear radiation –Alpha –Beta –Gamma  Nuclear reactions are responsible for producing what we think of as radiation.  There are three different forms of nuclear radiation –Alpha –Beta –Gamma Particles emitted from nucleus  Spontaneous emission of radiation from an atom is known as radioactivity.

56.  Almost all atoms we’ve encountered have stable nuclei – Not radioactive  Radioactive atoms have nuclei that are disintegrating  So why are some nuclei stable and others unstable?  Almost all atoms we’ve encountered have stable nuclei – Not radioactive  Radioactive atoms have nuclei that are disintegrating  So why are some nuclei stable and others unstable? NUCLEAR STABILITY

57.  One of the reasons is the number of protons and neutrons that they contain. –Not all combinations of protons and neutrons are stable.  One of the reasons is the number of protons and neutrons that they contain. –Not all combinations of protons and neutrons are stable.  The nucleus is made up of protons and neutrons densely packed together.  However, protons are positive and the positive charges should and the positive charges should be repelling each other  However, protons are positive and the positive charges should and the positive charges should be repelling each other + + +

58. + + +  So why don’t the protons just fly apart?  Why are most nuclei stable?  So why don’t the protons just fly apart?  Why are most nuclei stable? There are 2 forces at work: The repulsive force & a strong nuclear force. There are 2 forces at work: The repulsive force & a strong nuclear force.  The neutrons in the nucleus act as the glue that holds the nucleus together. A strong nuclear force set up by the neutrons A strong nuclear force set up by the neutrons

59.  All stable nuclei follow a distinct pattern. –For elements with atomic numbers between 1 and 20 stable nuclei have almost equal numbers of protons and neutrons. Beyond 20 protons, nuclei need increasingly more neutrons than protons to be stable.  All stable nuclei follow a distinct pattern. –For elements with atomic numbers between 1 and 20 stable nuclei have almost equal numbers of protons and neutrons. Beyond 20 protons, nuclei need increasingly more neutrons than protons to be stable.  Nuclei are unstable not only if they contain too few neutrons, but also if they contain too many.

61.  Radioactive elements emit different kinds of radiation –Alpha –Beta –Gamma  Each differs in charge, mass, and penetrating power  Radioactive elements emit different kinds of radiation –Alpha –Beta –Gamma  Each differs in charge, mass, and penetrating power

62.  Alpha radiation consists of a stream of high-energy alpha particles  Consists of 2 protons and 2 neutrons and is identical to a helium-4 nucleus  Can be represented by the symbol  Alpha radiation consists of a stream of high-energy alpha particles  Consists of 2 protons and 2 neutrons and is identical to a helium-4 nucleus  Can be represented by the symbol Alpha Particles

63.  Alpha particles do not have much penetrating power.  They are able to travel only a few centimeters through air and are easily stopped by paper or clothing  Not normally harmful to humans  Alpha particles do not have much penetrating power.  They are able to travel only a few centimeters through air and are easily stopped by paper or clothing  Not normally harmful to humans Alpha Particles

64.  Beta radiation consists of a stream of high-speed electrons  A neutron changes into a proton and an electron –The proton remains in the nucleus and the electron is propelled out of the nucleus  Beta radiation is represented by the symbol  Beta radiation consists of a stream of high-speed electrons  A neutron changes into a proton and an electron –The proton remains in the nucleus and the electron is propelled out of the nucleus  Beta radiation is represented by the symbol Beta Particles

65.  The mass number is zero  100 times more penetrating then alpha radiation  Can damage the skin  The mass number is zero  100 times more penetrating then alpha radiation  Can damage the skin Beta Particles

66.  A Gamma ray is highly energetic light, similar to x-rays  Does not consist of particles  Gamma radiation accompanies alpha and beta radiation  Much more penetrating than either of  or   It is able to penetrate deeply into solid material, including body tissue  Symbolized by:  A Gamma ray is highly energetic light, similar to x-rays  Does not consist of particles  Gamma radiation accompanies alpha and beta radiation  Much more penetrating than either of  or   It is able to penetrate deeply into solid material, including body tissue  Symbolized by: Gamma Rays

67. Properties of Some Radiations PropertyAlphaRadiationBetaRadiation Gamma Radiation Composition Alpha Particle Beta Particle High-energy photon Symbol , He , e  Charge2+1-0 Mass (amu) 41/18370 4 2 0

68. Lead, concrete Metal foil Paper, clothing Shielding Very high (penetrates body easily) Moderate (4mm body tissue) Low (0.05mm body tissue) Power 1MeV 0.05 to 1 MeV 5 MeV Appox.energy Cobalt-60Carbon-14Radium-226 Common source Gamma Radiation BetaRadiationAlphaRadiationProperty Properties of Some Radiations

69.  When an atom emits one of these kinds of radiation, it is said to be decaying.  An atom may undergo an alpha or beta decay.  The radiation is called decay because the nucleus decomposes to form a new nucleus, called transmutation  The best way to understand a decay is with a nuclear equation  When an atom emits one of these kinds of radiation, it is said to be decaying.  An atom may undergo an alpha or beta decay.  The radiation is called decay because the nucleus decomposes to form a new nucleus, called transmutation  The best way to understand a decay is with a nuclear equation Radioactive Decay

70.  An alpha particle is a particle composed of 2 protons and 2 neutrons.  With each expulsion of an alpha particle from the atom’s nucleus the atom loses 4 units of mass & 2 protons (+2 charged particle)  Any change in #’s of protons changes the type of atom, this is transmutation.  An alpha particle is a particle composed of 2 protons and 2 neutrons.  With each expulsion of an alpha particle from the atom’s nucleus the atom loses 4 units of mass & 2 protons (+2 charged particle)  Any change in #’s of protons changes the type of atom, this is transmutation. Alpha Decay Equations

72.  If you remember a  is an e - that is expelled from an atom  This electron is the result of one of the atom’s neutrons decomposing into a proton and an electron.  This results in the atom having one more proton which causes it to mutate into a different atom.  If you remember a  is an e - that is expelled from an atom  This electron is the result of one of the atom’s neutrons decomposing into a proton and an electron.  This results in the atom having one more proton which causes it to mutate into a different atom. Beta Decay Equations