1. Chapter 5 “Electrons in Atoms”

2. Section 5.3 Physics and the Quantum Mechanical Model l OBJECTIVES: Describe the relationship between the wavelength and frequency of light.

3. Section 5.3 Physics and the Quantum Mechanical Model l OBJECTIVES: Identify the source of atomic emission spectra.

4. Section 5.3 Physics and the Quantum Mechanical Model l OBJECTIVES: Explain how the frequencies of emitted light are related to changes in electron energies.

5. Section 5.3 Physics and the Quantum Mechanical Model l OBJECTIVES: Distinguish between quantum mechanics and classical mechanics.

6. Review l Bohr’s model was incomplete, described motion of larger objects like planets l Schrodinger used Bohr’s information and made a mathematical equation describing the behavior of electrons: The Quantum Mechanical Model.

7. Light l It was the study of light that led to the development of the quantum mechanical model. l Newton began describing light as a particle, but later evidence showed it also consisted of waves.

8. Parts of a wave Wavelength ( = lambda) Amplitude Origin Crest Trough

9. Light (cont.) l Light is a kind of electromagnetic radiation. l Electromagnetic radiation includes many types: gamma rays, x-rays, radio waves… l Speed of light = 2.998 x 10 8 m/s, and is abbreviated “c” = frequency: the number of wave cycles to pass a given point per unit of time l All electromagnetic radiation travels at this same rate when measured in a vacuum

10. - Page 139 “R O Y G B I V” Frequency Increases Wavelength Longer

11. Equation: c = c = speed of light, a constant (2.998 x 10 8 m/s) (nu) = frequency, in units of hertz (hz or sec -1 ) (lambda) = wavelength, in meters Electromagnetic radiation propagates through space as a wave moving at the speed of light.

12. Wavelength and Frequency l Are inversely related As one goes up the other goes down. l Different frequencies of light are different colors of light. l There is a wide variety of frequencies l The whole range is called a spectrum

13. - Page 140 Use Equation: c =

14. Radio waves Micro waves Infrared. Ultra- violet X- Rays Gamma Rays Low Frequency High Frequency Long Wavelength Short Wavelength Visible Light Low Energy High Energy

15. Long Wavelength = Low Frequency = Low ENERGY Short Wavelength = High Frequency = High ENERGY Wavelength Table

16. Atomic Spectra l White light is made up of all the colors of the visible spectrum. l Passing it through a prism separates it.

17. If the light is not white l By heating a gas with electricity we can get it to give off colors. (Neon Lights) l Passing this light through a prism does something different.

18. Atomic Spectrum l Each element gives off its own characteristic colors. l Can be used to identify the atom.

19. These are called the atomic emission spectrum Unique to each element, like fingerprints! Very useful for identifying elements When atoms absorb energy, electrons move into higher energy levels, and these electrons lose energy by emitting light when they return to lower energy levels.

20. Explanation of Atomic Spectra l When we write electron configurations, we are writing the lowest energy. l The energy level, and where the electron starts from, is called it’s ground state - the lowest energy level. l When an electron absorbs energy it is in an excited state. l Light emitted by an electron moving from a higher to a lower energy level has a frequency directly proportional to the energy change of the electron.

21. Changing the energy l Let’s look at a hydrogen atom, with only one electron, and in the first energy level.

22. Changing the energy l Heat, electricity, or light can move the electron up to different energy levels. The electron is now said to be “ excited ”

23. Changing the energy l As the electron falls back to the ground state, it gives the energy back as light

24. l They may fall down in specific steps l Each step has a different energy Changing the energy

25. { { { Lyman Series (Ultraviolet) Balmer Series (visible) Paschen Series (infrared)

26. l The further they fall, more energy is released and the higher the frequency. l This is a simplified explanation! l The orbitals also have different energies inside energy levels l All the electrons can move around. Ultraviolet Visible Infrared

27. Light is a Particle? l Energy is quantized. l Light is a form of energy. l Therefore, light must be quantized l These smallest pieces of light are called photons. l Energy & frequency: directly related.

28. Photoelectric Effect In the photoelectric effect, it was shown that metals eject electrons called photoelectrons when light shines on them. But not just any frequency of light will cause the photoelectric effect.

29. Equation: E = h E = Energy, in units of Joules (kg·m 2 /s 2 ) (Joule is the metric unit of energy) (Joule is the metric unit of energy) h = Planck’s constant (6.626 x 10 -34 J·s) = frequency, in units of hertz (hz, sec -1 ) = frequency, in units of hertz (hz, sec -1 ) The energy (E ) of electromagnetic radiation is directly proportional to the frequency ( ) of the radiation.

30. Examples 1) What is the wavelength of blue light with a frequency of 8.3 x 10 15 hz? 2) What is the frequency of red light with a wavelength of 4.2 x 10 -5 m? 3) What is the energy of a photon of each of the above?

31. SO, what is light? l Light is a particle - it comes in chunks. l Light is a wave - we can measure its wavelength and it behaves as a wave If we combine E=mc 2, c=, E = 1/2 mv 2 and E = h, then we can get: = h/mv (from Louis de Broglie) l called de Broglie’s equation l Calculates the wavelength of a particle.

32. Wave-Particle Duality J.J. Thomson won the Nobel prize for describing the electron as a particle. His son, George Thomson won the Nobel prize for describing the wave-like nature of the electron. The electron is a particle! The electron is an energy wave!

33. Confused? You’ve Got Company! “No familiar conceptions can be woven around the electron; something unknown is doing we don’t know what.” Physicist Sir Arthur Eddington The Nature of the Physical World 1934

34. The physics of the very small l Quantum mechanics explains how very small particles behave Quantum mechanics is an explanation for subatomic particles and atoms as waves l Classical mechanics describes the motions of bodies much larger than atoms

35. Heisenberg Uncertainty Principle l It is impossible to know exactly the location and velocity of a particle. l The better we know one, the less we know the other. l Measuring changes the properties. l True in quantum mechanics, but not classical mechanics

36. Heisenberg Uncertainty Principle You can find out where the electron is, but not where it is going. OR… You can find out where the electron is going, but not where it is! “One cannot simultaneously determine both the position and momentum of an electron.” Werner Heisenberg

37. It is more obvious with the very small objects l To measure where a electron is, we use light. l But the light energy moves the electron l And hitting the electron changes the frequency of the light.

38. Moving Electron Photon Before Electron velocity changes Photon wavelength changes After Fig. 5.16, p. 145

39. The Math in Chapter 5 l There are 2 equations: 1) c = 2) E = h Know these!