Chapter 5. Electrons in Atoms

Transcription

1 Chapter 5 Electrons in Atoms

2 Warm - Up What kind of light causes you to sunburn? Why does only this type of light burn your skin and not other light? What does sunscreen do on a chemical level?

3 Today s Agenda Question for Today: How are the wavelength and frequency of a wave related? Understand and utilize mathematical electromagnetic wave relationships. Continuous vs. electromagnetic spectra. Homework: Read Section 5.1 problems evens Due tomorrow! Lab books due Thurs

4 Development of Atomic Structure electron nucleus protons and neutrons Certain elements emit light when heated. Why? Li, Na, and K all react violently with water. Why? Chemical behavior related to the arrangement of the electrons.

5 Light as a WAVE Electromagnetic radiation A form of energy exhibiting wavelike behavior as it travels through space. Examples: Microwaves, X rays, radio waves

6 Parts of a Wave Wavelength (lambda) shortest distance between to peaks (or troughs) on a wave. Expressed in nm (1x10 9 nm = 1m) Frequency (nu) number of waves that pass a given point/second (1/s = Hz) Amplitude height from origin. peak trough

7 Parts of a Wave Amplitude Which wave has a shorter Which wave has a higher frequency?

8 What s nu? If wavelength is short, is frequency high or low? INVERSELY PROPORTIONAL - as increases decreases, Both related to the speed of light c = 3x10 8 m/s c = Pretend, c = 10, = 5, whats Now c = 10, = 2, whats c = c =

9 Practice 1. Rearrange c = to solve for (include units!) 2. Calculate the frequency of each of the following wavelengths of electromagnetic radiation: nm (632.8x10-9 m), and 503 nm. 3. Rearrange c = to solve for (include units!) 4. Calculate the wavelength of each of the following frequencies of electromagnetic radiation: x 10 6 Hz, and 1035 khz

10 More c = Practice! What is the wavelength of a microwave with a frequency of 1.56 x 10 9 Hz? The red-colored light in a fireworks display might be produced when Strontium salts are heated. What is the frequency of such red light with a wavelength of 6.5x10-7 m? After careful analysis, an electromagnetic wave is found to have a frequency of 7.8x10 14 Hz. What is the wavelength of the wave?

11 Electromagnetic Spectrum VIB G YOR

12 Continuous Spectrum White light is separated into its visible components by the prism. Continuous because each color corresponds to a unique wavelength and frequency.

13 Vocabulary to Know Emit Electromagnetic radiation Wavelength Frequency Amplitude Electromagnetic wave relationship Electromagnetic spectrum

14 Vocabulary to Know Emit to give off (energy, light, heat, etc.) Electromagnetic radiation energy that travels through space as a WAVE Wavelength distance from peak to peak of a wave Frequency # of waves that pass a point per second Amplitude height of wave Electromagnetic wave relationship c = Electromagnetic spectrum all wavelengths of energy from radio to gamma (visible nm)

15 Energy of Waves We want to be able to understand the energy given off by these waves Are they harmful to us? (too much energy) Can we use them for medicinal imaging? Technology? Transmission of information? What is the relationship between and ENERGY?!

16 What else is nu? Energy, E = h h = Plank s constant = x Js Named for Max Plank German physicist. Whats J? Joule = (kg x m 2 )/s 2 unit of energy

17 Practice Rearrange E = h to solve for How much energy is possessed by a single photon of UV-A electromagnetic radiation with a frequency x Hz? What is the frequency of a wave with an energy of 1.64 x J?

18 Energy and We know c = and E= h Rearrange both equations to solve for = c/ = E/h c = E/h E = hc/ We can relate wavelength, energy, and frequency with those relationships!

19 Practice Problem A Zn salt emits orange light at a wavelength of 450 nm. What is the energy of this light? 1. Identify the appropriate equation. E = hc/ 2. Rearrange to solve for unknown. Check (E is unknown) 3. Solve and show unit cancellations. E = 6.626x10-34 Js x 3x10 8 m s 450 x 10-9 m = 4.42 x J

20 Warm Up What is the energy? (show equations) Light with a wavelength of 640 nm. Wave with a frequency of 3.24 x Hz.

21 Today s Agenda Question of the Day: If light transfers energy like a wave, how does light also act like a particle? Warm Up Discuss light as a particle quantum, photoelectric effect, Use mathematical relationships to demonstrate the photoelectric effect. Compare and contrast a continuous spectrum to an atomic emission spectrum Test and Lab Return correction guidelines

22 Warm Up Find the energy and frequency of a ray with a wavelength of 24 nm. What is the energy and wavelength for a photon with a frequency of 5.21 x Hz?

23 Warm - Up is the symbol for is the symbol for The value of c is always The value of h is always The unit for wavelength is The unit for frequency is or c =, E = h / and E = h Find the energy and frequency of a ray with a wavelength of 24 nm. What is the energy and wavelength for a wave with a frequency of 3.24 x Hz?

24 Today s Agenda Does like always act like a wave? Photoelectric Effect Dual nature of light Photoelectric effect calculations Lab notebooks due today! Worksheets due tomorrow!

25 Light as a Wave Light transfers energy through a wave with a specific frequency and wavelength. c =, E = h E=hc HOWEVER, because science laughs at us for trying to figure out the universe, light doesn t ALWAYS act like a wave Science

26 Quantum Theory Light as a Particle Quantum concept: Matter can gain or lose energy in small specific amounts called quanta. Quanta: minimum amount of energy that can be gained or lost by an atom

27 Photoelectric Effect - Einstein Electrons (photoelectrons) are emitted from a metal s surface when a certain energy light shines on the surface. Photon is a massless particle that carries a quantum of energy. If light was a wave, photoelectric effect would NOT be observed.

28 Photoelectric Effect in Practice What is the value of Energy C? Energy C Energy = 2.4 x J Energy B Energy A

29 Building on PE Effect New Vocabulary and Notation: Incident Beam radiation that is used to eject an electron from a surface. (E i, i, i) Threshold energy, frequency, wavelength the required radiation to eject an electron dictated by a surface. (E t, t, t) Kinetic energy (KE) energy something possesses due to its motion.

30 Photoelectric Effect Problems Find the energies associated with each or. Use c= E=h or E =hc/ Is the incident energy more than the threshold energy? If so, electron is ejected! If the electron is ejected, find it s kinetic energy! Use E KE = E i E t

31 Photoelectric Effect Problems A photon in the IR region of the electromagnetic spectrum has a wavelength of 4.52 x 10-5 m. Will it have enough energy to eject a photon from a sodium surface which has a threshold frequency of 5.51 x Hz? Make a table!

32 Using the Photoelectric Effect A photon in the IR region of the electromagnetic spectrum has a wavelength of 7.23 x 10-9 m. Will it have enough energy to eject a photon from a sodium surface which has a threshold threshold frequency of sodium is 5.51 x Hz? E = hc/ E = (6.626x10-34 Js x 3x10 8 m/s) = 2.74 x J E = h Yes! 7.23x10-9 m E = 6.626x10-34 Js x 5.51x /s = 3.65 x J

33 Remember that E = h & E = hc/ Aluminum has a threshold wavelength of 295 nm. If a sample of aluminum foil is exposed to four types of radiation, which radiation source would eject a photon? Microwave? = 3.82 x /s Optical wave? = 5.18 x /s UV wave? = 1.02 x /s X ray wave? = 4.31 x /s 295 nm E =? J

34 Practice You have a beam of light with a wavelength of 424 nm. What is the energy of this light? A metal surface has a threshold frequency of 4.84 x Hz. What is the energy of this light? Can we eject electrons?

35 Photoelectric Effect Problems It requires a photon with a minimum energy of 4.41 x J to emit electrons from sodium metal. What is the threshold frequency? Will an incident photon of wavelength 325 nm eject an electron from the surface?

36 Kinetic Energy of Ejected Electron Calculate the energy needed to eject a single photon of radiation from a metal with a t of 4.74 x 10-8 m? Would an incident photon, i = 7.78 x /s eject an electron? What is the kinetic energy of that electron? E KE = E i E t E KE = 5.15 x J 4.19 x J = 4.73 x J

37 Kinetic Energy Practice Problems When a metal surface is illuminated by radiation of frequency 6.4 x Hz, the maximum kinetic energy of the photoelectrons emitted is J. Calculate the t and E t for the metal. E KE = E i E t 3.8 x J = 5.15 x J E t E t = 5.11 x J E t = h t, 5.11 x J/6.626x10-34 Js = 7.72 x Hz

38 Kinetic Energy Calculations 1. The E t of a certain metal is 3 x J. If light of wavelength, i, 500 nm shines on the metal will electrons be ejected? What is the kinetic energy of the electrons emitted? 2. The energy required to remove an electron from a certain metal is 3.60 x J. Will an incident beam of 9.38 x Hz eject an electron? What is the kinetic energy of the electrons emitted?

39 Continuous Spectrum White light is separated into its visible components by the prism. Continuous because each color corresponds to a unique wavelength and frequency.

40 Atomic Emission Spectra Neon signs! Neon gas absorbs energy and becomes excited. Electrons return to their stable state by emitting that energy as light! Prism can separate light not continuous. Atomic Emission Spectra set of frequencies of waves given off by atoms of a specific element. Each element has it s own emission spectrum (because of energy steps).

41 Atomic Emission Spectra Each element s atomic emission spectrum is unique, and can be used to identify that element.

42 Today s Agenda Question of the Day: What is an atomic emission spectrum and how does is compare to a continuous spectrum? Review photoelectric effect calculations and summarize yesterday s lesson. Discuss atomic emission spectra and how they differ from a continuous spectrum.

43 Question of the (yester)day If light transfers energy like a wave, how does light also act like a particle?? Answer: Light acts as a particle because intensity of light does not affect the energy. The number of photons aimed at a surface does not change the # of photons emitted. Only the energy (wavelength, frequency) changes the photons ejected.

44 Today s Agenda Question of the Day: What is the kinetic energy of an ejected electron from a metal surface? Discuss (review) atomic emission spectra, and compare it to a continuous spectrum. Learn new vocabulary and notation for photoelectric effect calculations. Calculate the kinetic energy of an emitted electron.

45 Question of the (yester)day What is an atomic emission spectrum and how does it compare to a continuous spectrum??? Answer: Atomic emission spectra is of an element is the spectrum of frequencies emitted by the element's atoms. Each element has it s own signature spectrum and each spectrum contains only select wavelengths of light. A continuous spectrum is a rainbow and contains all wavelengths of light.

46 Atomic Emission Spectra Neon signs! Neon gas absorbs energy and becomes excited. Electrons return to their stable state by emitting that energy as light! Prism can separate light not continuous. Atomic Emission Spectra set of frequencies of waves given off by atoms of a specific element. Each element has it s own emission spectrum (because of energy steps).

47 Atomic Emission Spectra Each element s atomic emission spectrum is unique, and can be used to identify that element.

48 Question: What is the difference between a continuous spectrum and an atomic emission spectrum? A) Atomic emission gives off light, continuous does not. B) Continuous spectra contain all s light and atomic emission have selected s of light. C) A continuous spectrum requires a prism, atomic emission spectrum does not.

49 Warm Up! When radiation of wavelength 2.4 x 10-7 m falls on a metal surface, the maximum kinetic energy of the emitted electrons is found to be J What is the t and E t for the metal?

50 Today s Agenda Question of the Day: What is the Bohr model of the atom? Why is the atomic emission spectra of hydrogen discontinuous? Solar Cells Discuss the Bohr model of the atom. Investigate the atomic emission spectrum of hydrogen. Practice calculations with the DeBroglie equation.

51 Solar Cells The solar cell works in three steps: The photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. Electrons are knocked loose from their atoms, allowing them to flow through the material to produce electricity. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity.

52 Wave Particle Duality Light can act as a PARTICLE (transfer energy in discrete packets called QUANTA or PHOTONS of energy). When do we see this? Photoelectric Effect Light can act as a WAVE (transfer energy through an oscillating wave with a WAVELENGTH and FREQUENCY) When do we see this? Crazy quantum mechanics

53 Warm Up! Light can act as a and a. The photoelectric effect displays light acting as a. Incident radiation must overcome the energy dictated by a surface for an to be ejected. Find the threshold energy for an electron with 3.21 x J of kinetic energy ejected from a surface with an incident beam of 4.21 x J.

54 Today s Agenda Question of the Day: What is a DeBroglie wavelength? Discuss (review) Bohr Model of the Atom and why it failed. Introduce the DeBroglie wavelength and utilize the equation. HW: Quiz Review Worksheet due Wed

55 Continuous Spectrum White light is separated into its visible components by the prism. Continuous because each color corresponds to a unique wavelength and frequency.

56 Bohr Model of the Atom Excited State Ground State Principle Quantum Number : How far is the e - away from the nucleus?

57 Atomic Emission Spectra Atomic Emission Spectra set of frequencies of waves given off by atoms of a specific element. Each element has it s own emission spectrum (because of energy steps).

58 Atomic Emission Spectra Each element s atomic emission spectrum is unique, and can be used to identify that element.

59 Different Colors from Different Transitions (H atom)

60 Bohr Model : FAIL Bohr model did not describe more complex atoms. Scientific data supports that electrons do not travel in circular. orbits Why don t electrons spiral into nucleus? Maybe, if waves can have particle-like behavior particles can have wave-like behavior?

61 Louis DeBroglie s Waves All moving particles have wave characteristics. (If waves can act as particles, particles can act as waves!) This includes large particles, like cars H atom simulation

62 What s the Wavelength of a Moving DeBroglie equation: Car? h = Planks constant ( Js ) = h mv m = mass ( kg ) *** must be in kg! v = velocity ( m/s ) If a car is travelling at 25m/s with a mass of 900kg, what is the wavelength of the car?

63 What is the wavelength of a moving electron? m = 9.11 x kg v = 25 m/s = h mv car = 2.94 x m electron = 2.91 x 10-5 m We cannot even pretend to be able to measure the wavelength of a moving car, we can however measure the wavelength of a moving electron

64 What is the DeBroglie wavelength for: 1. A jogger with a mass of 50 kg moving at 5 m/s? 2. An GIVEN: electron moving at 2.2 x 10 6 m/s (mass of an electron = 9.1x10-31 kg)? 3. A DESIRED: 55 g rock flying through the air with a speed of EQUATION: 35 m/s? 4. You REARRANGE: in your car driving down the road at 60 mph (Assume that your car with you in it has a mass Solve: of about 1500 kg)? 5. What is the mass of a visible photon of 424 nm moving at 4 x 10 6 m/s?

65 Warm Up Complete as much as you can without notes! List the 8 variables from this unit List the 5 equations from this unit List the 2 constants from this unit What is the difference between a continuous spectrum and an atomic emission spectrum?

66 Today s Agenda REVIEW! In-class problems Homework problem solving Q&A Homework due tomorrow

67 Mixed Practice What is the frequency and energy for a wave with wavelength of 6.51 x 10-5 cm? Find the wavelength for an electron with a mass of 9.11 x g and a velocity of 2.6 x 10 5 m/s. What is the kinetic energy for an electron that is ejected from an aluminum surface that requires a frequency of 1 x Hz by a UV beam of 220 nm?

68 1. Arrange the following types of EM radiation in order of increasing wavelength: Ultraviolet, microwaves, radio waves, X-rays, visible waves 2. What are the units for,, and c? What are the units for mass in the de Broglie equation? 3. A microwave has a wavelength of 4 cm. How much energy does this microwave have? 4. A jogger has a mass of 65 kg and is moving at 7.5 m/s, what is the wavelength of the jogger? An electron is moving at a speed of 5.4 x 10 6 m/s with a mass of 9.11 x kg. Which wavelength is more meaningful and why?

69 Problem Solving Strategies Read the problem and create an INVENTORY. 1. List what is and GIVEN DESIRED 2. Write down the appropriate equation. 3. Rearrange the equation to solve for DESIRED. 4. Plug in numbers and solve!

70 If a 1850 kg moving car has a wavelength of 1.2 x m, what is it s speed? GIVEN: wavelength = 1.2 x m mass m = 1850 kg DESIRED: velocity v =? m/s EQUATION: = h/mv REARRANGE: v = h/m! Plug in and Solve! = x Js = 30 m/s 1.2 x m/s x 1850 kg

71 Solving Photoelectric Effect Problems i t KE Hz 1 x m E J 1.35 x 10-18???? What is the incident wavelength?

72 Today s Agenda Question of the Day: What is the uncertainty principle? Discuss Heisenberg uncertainty principle. Schrodinger and quantum mechanical questions?? Electron configurations

73 Don t Solve Make Tables & Choose Equation! When a metal surface is illuminated by radiation of frequency 6.4 x Hz, the maximum kinetic energy of the photoelectrons emitted is J. Calculate the t and E t for the metal. Calculate the energy needed to eject a single photon of radiation from a metal requiring a wavelength of 4.74 x 10-7 m? What is the kinetic energy of an electron ejected with an energy of 7.3x J?

74 Warm Up 1. Why does H gas only emit selected colors of light? 2. Draw the Bohr Model of the atom. How can this help explain your answer to #1

75 Agenda What is the theory behind all these calculations and conversions? Dual nature of light Heisenberg and Schrodinger Atomic Emission!

76 VIB G YOR Light as a Wave EM Spectrum High E High Short

77 Light as a Particle Packets of energy called QUANTA Planks Constant and the photoelectric effect Bohr model of the atom Helped explain atomic emission spectra Doesn t explain complex atoms electrons don t travel in circular orbits! Quantum number = n = energy levels

78 Heisenberg Uncertainty Principle We cannot know the exact position and velocity of an electron at any one time. Simply by observing the electron we move it from it s position and possibly change it s speed. Young s double slit experiment

79 Schrodinger Equation Treats electrons as a wave in the hydrogen atom. Applied well to more complex molecules quantum model. Used a ridiculous analogy to a cat to explain unexplainable events of quantum mechanics. Schrodinger s cat!

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81 The Schrodinger Equation Highly complex equation that allow us to estimate where an electron probably is located Electrons have a probability of being in an orbital!

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83 Warm Up According to the Bohr model, what is n? What does it represent? Circle one option: If an electron moves from an n=1 level to an n=3 level energy is emitted/absorbed. If an electron moves from an n=5 level to an n=2 level energy is emitted/absorbed. Fill in the blanks: According to the principle we cannot know the precise and of an electron simultaneously.

84 Today s Agenda QOTD: How do we predict where an electron is located? Atomic Orbitals Placement rules Electron Configurations

85 Orbital: - area of high electron probability density. - Orbitals have: - Energy levels designated by principle quantum number (n) - Energy sublevels each n has sublevels where the e - are!

86 and up! Atom City Electron Hotel 4 th floor n = 4 4 sublevels 3 rd floor n = 3 3 sublevels 2 nd floor n = 2 2 sublevels 1 st floor n = 1 1 sublevel

87 Sublevels and Orbitals l Sublevels have letter names and are organized by shape! Sublevel s p d f Shape of orbital

88 and up! 4 th floor n = 4 Atom City Electron Hotel 4 sublevels s, p, d, and f 3 rd floor n = 3 3 sublevels s, p, and d 2 nd floor n = 2 2 sublevels s and p 1 st floor n = 1 1 sublevel s

89 Sublevels and Orbitals l Sublevels have letter names and are organized by shape! Sublevel s p d f Shape of orbital # of orbitals 1 # of e How many electrons fit in an orbital?? 2 e - per orbital

90 and up! 4 th floor n = 4 Atom City Electron Hotel 4 sublevels s, p, d, and f 3 rd floor n = 3 3 sublevels s, p, and d 2 nd floor n = 2 2 sublevels s and p 1 st floor n = 1 1 sublevel s

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92 We need to assign addresses for electrons! Electron configurations: Assignment of the electrons in an atom (which level, which sublevel?)

93 Electron Configurations s block p block d block f block

94 Warm Up Write the electron configurations: Ca Te Se P Kr

95 Agenda What are the valence shell electrons and how do we identify them? Electron Configs past f block Exceptions and Principles Orbital Diagrams

96 Exceptions Cr predicted : [Ar]4s 2 3d 4 buuuuuut 3d 4 is SO CLOSE to 3d 5 so the config is [Ar]4s 1 3d 5 Cu predicted: [Ar]4s 2 3d 9 buuuuuut 3d 9 is SO CLOSE to 3d 10 so the config is [Ar]4s 1 3d 10 What about Ti, Au, and Hg, are these exceptions?

97 Rules of Electron Placement Aufbau Principle every electron occupies the lowest energy orbital possible. Pauli exclusion principle each orbital can hold a maximum of 2 electrons of opposite spins. Hund s rule each orbital holds one electron until all orbitals at that energy are filled, then they are paired.

98 Noble Gas Notation Short cut!! The noble gases: He down to Rn Noble gases have FULL p shells and are EXTREMELY STABLE.

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100 Exceptions The aufbau diagram predicts a small energy difference between s and d. For some transition metals if they can have a HALF FILLED or FULL FILLED d shell, they will STEAL an electron from the s. (The d shell really likes to be d 5 or d 10 ).

101 Warm Up Write the electron configurations for: Cr Mo Au How many orbitals are contained in an s subshell? p? d? f?

102 Today s Agenda How do you graphically represent the placement of electrons? Orbital and Lewis Diagrams Valence electrons Review!

103 Orbital Diagrams Write out the appropriate orbitals for each shell and label them. Fill in the electrons according to the Aufbau Principle, Pauli Exclusion Principle, and Hund s rule. Carbon: 1s 2 2s 2 2p 2 1s 2s 2p

104 He Be

105 C N O

106 Practice: Write the electron configurations for the following: Mg Si Kr Cr Draw the orbital diagrams

107 Valence Electrons Valence electron: the electrons in the outermost shell (the highest n) of an atom These do all the bonding!! Important to know how many are there!!

108 Lewis Structures G. N. Lewis born in Weymouth, MA in Lewis dot diagrams show the valence electrons in each atom and help us to predict bonding, molecular geometry and ionic charge. Nitrogen: What is electron config? How many valence electrons? 1s 2 2s 2 2p 3 N What is the highest n? How many e -?

109 Warm Up What are the three principles of electron placement? What are the subshells in an n=3 energy level? How many electrons can be contained in all the orbitals related to an Argon s third energy level?

110 Today s Agenda Review and Lab! Lewis and orbital diagram practice! Collect more data Review sheet due Tuesday, review day on Monday, have questions ready! Test Ch 5 Tuesday

111 Lewis Structures Write electron configuration. Count the electrons in the highest n shells Place electrons (dots) around element symbol. Na Xe Se

112 Practice Write electron config (noble gas notation is acceptable). Draw Orbital Diagrams. Draw Lewis Structure. S Ge Cr Cl K

113 Things to know about e - configs Just follow the periodic table and each element adds one more electron. The structure of configurations is: n block number of electrons i.e. 2s 2 or 3p 6. The n for the d block is one less than the s and p for that row (n-1). The f block is two less (n-2). If n = 6 for s and p, n = 5 for d and n = 4 for f

114 Noble Gas Notation Find the noble gas immediately before the element in question. (If asked for e- config for Sr, start with Kr) For Sr (strontium) you write: [Kr] 5s 2. This notation implies that the configuration up to Kr is EXACTLY the same for Sr.

115 Review According to Bohr s model why do atomic emission spectra contain only certain frequencies of light? Why is it reasonable to measure the wavelength of an electron and not to measure the wavelength of a moving car? An electron is in the state until it s energy and becomes excited. To relax back to the state a energy must be.

116 Warm - Up Calculate the energy needed to eject a single photon of radiation from a metal requiring a wavelength of 4.74 x 10-7 cm? What is the kinetic energy of an electron ejected with an incident energy of 7.3x J?

117 Today s Agenda Question of the day: Can you make a PERFECT homework solution set??? Create a class solution set to the homework problems. Electron configurations and the periodic table. Homework DUE FRIDAY: Read 5.3 and do problems