Ch23HW - 504 Chapter 23 The Early Atom (511cc!) Electron:...

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Unformatted text preview: 504 Chapter 23 The Early Atom (511cc!) Electron: The basic constituent of atoms that has a negative charge. electron volt: A unit of energy equal to the kinetic energy ac- quired by an electron or proton falling through an electric po- tential difference ofl volt. Equal to 1.6 X lO'm‘joule. emission spectrum: The collection of discrete wavelengths emitted by atoms that have been excited by heating or electric currents. ion: An atom with missing or extra electrons. nucleus: The central part of an atom containing the positive charges. photoelectric effect: The ejection of electrons from metallic surfaces by illuminating light. I. Today, we think of the periodic table as being arranged in order of increasing proton (or electron) number. Why did Mendeleev not use this approach? 2. If Mendeleev was ordering the elements according to their masses, why did he not produce a table with only one row? 3. What element in Mendeleev’s periodic table is most similar to silicon (Si)? Vhat elements would you expect to have chemical proper— -/ lies similar to chlorine (Cl)? 5. V‘fliy are the spectral lines for elements sometimes called “atomic fingerprints”? 6. How could you determine if there is oxygen in the Sun? 7. Would you obtain an emission or an absorption spectrum at location A in the setup shown in the figure? “White “light” in Questions 7 and 8 8. Would yOu obtain an emission or an absorption spectrum at location B in the setup shown in the figure? CONCEPTUAL QUESTIONS photon: A particle of light. The energy of a photon is given by the relationshipE : hf, wherefis the frequency of the light and h is Planck's constant. quantum (pl., quanta): The smallest unit of" a discrete prop. erty. For instance, the quantum ofcharge is the charge on the proton. quantum number: A number giving the value of a quantized quantity. For instance, a quantum number specifies the angular momentum of an electron in an atom. Shell: A collection of electrons in an atom that have approxi- mately the same energy. X ray: A high-energy photon with a range of frequencies in the electromagnetic spectrum lying between the ultraviolet and the gamma rays. The emission spectra shown in the figure were all obtained with the same apparatus. What eleinent(s) can you identify in sample (a) ? Are there any that you cannot identify? Element A Element B Element C Questions 9 and 10 I0. What element(s) can you identify in sample (b) of the figure? Are there any that you cannot identify? I I. How does the number of lines in the absorption spectrum for an element compare with the number in the emission spectrum? I2. What are the differences between an emission spectrum and an absorption spectrum? I3. When your authors were students, their textbooks did 1101 have color. Could the spectra in Figure 23—3 still be used to identify atoms if they were in black and white? Explain. a, 2.3 {d e 4 Conceptual Questions 505 I4. Two graphs of brightness versus wavelength are shown in I9. In the Millikan oil-drop apparatus shown in Figure 23-8.:111 ‘ the figure. Identify which is an absorption spectrum and electric field provides a force that balances the gravita- which is an emission spectrum. tional force on charged oil drops. Millikan found that he needed the electric field to point down toward the floor. Was the net charge on the oil drops a result of an excess or Sputum! :\ a deficit of electrons? 20. Why was it not necessary for Millikan to use oil drops charged to only one electron charge to determine the charge of a single electron? 2'l . Millikan’s oil-drop experiment was used to determine the charge on a single electron. Why should Millikan also get credit for determining the electron’s mass? 22. What do we mean when we say that a certain physical quan- tity is quantized? 2! How does Rutherford‘s model-of an atom explain why most alpha particles pass right through a thin gold foil? How \\'.1\g-ii-13Lr{lt 1 does it account for why some alpha particles are scattered bac kward? (Q In Rutherforcl’s model of the atom, nothing separates the i ‘ negative electrons from the positively charged nucleus but empty space. “Thy don’t the electrons just rush right into Spitt'tlum ll the nucleusr 25. Rutherford’s model predicted that atoms should be unsta- {f\ blc (the electrons should spiral into the nucleus) over very ‘ \ short time periods. What caused this instability in Ruther- l \h lord’s model? .- . ,_ n \ BI in litltt'ss ll \ ;‘ ii ll . 26. Rutherl'ord‘s model provided an explanation for the emis- ‘ It \ sion oflight from atoms. What was this mechanism and why I was it unsatisfactory? \\ 27. Why can the cones for the intensities ol‘ the colors emitted by hot solid objects not serve as “atomic fingerprints" ol‘the f i i l l l l i _l l l l titalt'rials? “'“dmil‘m 28. If all objects emit radiation, why don’t we see most of them in the dark? IS. TO which of the brightness versus wavelength graphs . I _ _ in Question 14 does the line Spectrum in the figure 29. As you move to the rlght along the-horizontal axrs of Fig correspond; ure 23—14, 18 the frequency mcreasmg or decreasmg.‘ Ex- plain your reasoning. 30. For an object at a temperature of 8000 K, use Figure 23-14 to determine whether the light intensity is greater for light in the ultraviolet or in the infrared. 3 i. You measure the brightness of two different hot objects; IG. Sketch the brightness versus wavelength graph for the hy first with 2|. blue filtflr and then With 3. rfld filter. YOU find drogen spectrum shown in Figure 23—3, that object A has a brightness of 25 in the blue and 20 in the red. Object B has a brightness of 12 in the blue and 3 in the red. The brightness units are arbitrary but the same for all measurements. Which is the hotter of the two l1. Suppose you were a nineteenth-century scientist who had just discovered a new phenomenon known as Zeta rays (yes, we’re making this up). What experiment could you , perform to determine whether Zeta rays were charged Objeas' particles or an electromagnetic wave? Could this experi— 32. The curves in Figure 23-14 show the intensities of the vari- ment distinguish between neutral particles and an electro. ous wavelengths emitted by an object at three different ‘ magnetic wave? temperatures. The region corresponding to visible light is indicated. How would the color of the object at 8000 K Compare to the color of the object at 4000 K? l8. Imagine that you determined that the Zeta rays from Ques— tion 17 were charged particles. How would you determine the sign of the charge? 506 Jerry Schad/thtn Researchers, Inc. 33. 34. 35. 36. 37. 39. 40. 4|. Chapter 23 The Early Atom Questions 33 and 34 Why do astronomers often use the terms color and tempera- ture interchangeably when referring to stars? Why are blue stars thought to be hotter than red stars? What assumption(s) did Planck make that enabled him to obtain the correct curve for the spectrum ot‘light emitted by a hot object? ' What assumption (5) did Einstein make that enabled him to account for the experimental obsenrations of the photo- electric effect? What property of the emitted photoelectrons depends on the intensity of the incident light? What property of the emitted photoelectrons depends on the frequency of the incident light? If a metal surface is illuminated by light at a single fre— quency, why don’t all the photoelectrons have the same ki- netic energy when they leave the metal’s surface? How is it possible that ultraviolet light can cause sunburn but no amount ofvisible light will? You find that if you shine ultraviolet light on a negatively charged eiectroscope, the electroscope discharges even if the intensity of the light is low. Red light! however, will not discharge the electroscope even at high intensities. How do you account for this? EXERCISES ®What is the charge-to-mass ratio for a cathode ray? 2. 3. What is the charge—to-mass ratio for a hydrogen ion (an isolated proton)? Given that the radius ofa hydrogen atom is 5.29 X 10—11 111 and that its mass is 1.682 X 10’27 kg, what is the average density oFa hydrogen atom? How does it compare with the density ol‘water? 42. 43. 44. 45. 46. 47. 48. 49. 50. j. @ Radon (element 86) is a gas. Would you expect the mole- 53. 54. 55. 56. 57. 58. 4. You find that if yott shine ultraviolet light on a negatively charged electroscope, the electroscope discharges. Can you discharge a positively charged clectroscope the same wa ’? Why or why not? What are the three assumptions of Bohr‘s model of the atom? Why did Bohr assume that the electrons do not radiate when they are in the allowed orbits? An electron in the n. : 3 energy level can drop to the ground state by emitting a single photon or a pair of pho- tons. How does the total energy of the pair compare with the energy of the single photon? If electrons in hydrogen atoms are excited to the fourth Bohr orbit, how many different frequencies of light maybe emitted? How can the spectrum of hydrogen contain so many spec— tral llllCS when the hydrogen atom only has one electron? What determines the frequency ofa photon emitted by an atom? Why does the spectrum ol‘ lithium (element 3) resemble that of hydrogen? How does Bohr’s model explain that there are more lines in the emission spectrum than in the absorpticm spectrum? How many electrons would you expect to find in each shell for chlorine (Cl)? cules ol‘ radon to consist ol‘a single atom or a pair of atoms? “Thy? Sodium does not naturally occur as a free element. Why? What etfectiye charge do the outer electrons in aluminum “see”? (Aluminum is element 13.) ‘Nhat type of electromagnetic wave has a wavelength about the size of an atom? (The electromagnetic spectrum is given in Figure 22-24.} Are X rays deflected by electric or magnetic fields? Explain. How does an X ray differ from a photon of visible light? Why would you not expect an X-ray photon to be emitted every time an inner electron is removed from an atom? “That is the average density of the hydrogen ion (an iso- lated proton) given that its radius is 1.2 X 10715 m and that its mass is 1.673 X 10‘27 kg? It is interesting to note thPlL Such densities also occur in neutron stars. If you were helping your younger brother build a scale model of an atom for a science fair and wanted it to fit in a box 1 m on each side, how big would the nucleus be? student decides to build a physical model of an atom. 11' L be nucleus is a rubber ball with a diameter of] cm, how far away would the outer electrons be? 7. What is the energy ofa photon ofred lightwith a frequency of4.5 X to” H2? 8. What is the energy of the most energetic photon of visible light? 9. A photon ofgreen light has energy 3.6 X 10719} What is its frequency? E0. An X—ray photon has energy 1.5 X 10—15 What is its frequency? | l. A microwave photon has an energy of 2 X 10—23 J. What is its wavelength? l2. A photon ofyellow light has a wavelength of 6.0 X 10’7 in. What is its energy? l3. What is the angular momentum of an electron in the ground state of hydrogen? 14. What is the angular momentum of an electron in the n : 4 level of hydrogen? What is the radius of the 7? z 4 level of hydrogen? l6. What is the quantum number of the orbit in the hydrogen atom that has 36 times the radius of the smallest orbit? :{JnfoTrafl College Edition 507 Exercises l7. What is the frequency ofa photon of energy 3 eV? [8. What is the energy, in electron volts, ofa yellow photon of wavelength 5.0 X 10’7 m? 19. According to Figure 23-20, it requires a photon with an energy of 10.2 eV to excite an electron from the n : 1 en- ergy level to the n = 2 energy level. What is the frequency of this photon? Does it lie in, above, or below the visible / range? A f When a roton ca tut-es an electron, a hoton with an en— P P P ergy of 13.6 eV is emitted. What is the frequency of this photon? Does it he in, above, or below the visible range? 2| . What is the ratio of the volumes of the hydrogen atom in the n : 1 state compared with those in the n : 2 state? *22. The diameter of the hydrogen atom is 10'10 m. In Bohr’s model this means that the electron travels a distance of about 3 X 10—10 min orbiting the atom once. If the orbital frequency is 7 X 10'5 Hz, what is the speed of the electron? / _ How does this speed compare with that of light? What difference in energy between two atomic levels is re * quired to produce an X ray with a frequency of? X 1013 Hz? 24. V‘l-‘rhat is the frequency of the X ray that is emitted when an electron drops down to the ground state from an excited state with 1000 eV more energy? For additional readings, explore InfoTrac College Edition, your online library. Go to htth//www.infotrac—college.com/t-vadsworth and use the passcode that came on the card with your book. Tiy these search terms: Ernest Rutherford]. Thomson, Max Planck, Mendeleev, Niels Bohr, photoelectric sensors, Robert Millikan. ...
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This note was uploaded on 12/28/2011 for the course PHYSICS 117 taught by Professor Griffin during the Fall '08 term at Maryland.

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Ch23HW - 504 Chapter 23 The Early Atom (511cc!) Electron:...

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