12/04/2012. Models of the Atom. Quantum Physics versus Classical Physics The Thirty-Year War ( )

Transcription

1 Quantum Physics versus Classical Physics The Thirty-Year War ( ) Interactions between Matter and Radiation Models of the Atom Bohr s Model of the Atom Planck s Blackbody Radiation Models of the Atom Democritus, Dalton and Mendeleïev 1897 Thomson s Plum Pudding Model and the Discovery of the Electron 1911 Rutherford s Model and the Discovery of the Nucleus 1913 Bohr s Planetary Model and Spectral Lines 1926 Schrödinger s Cloud Model and the Probability Wave Function Einstein s Photoelectric Effect Compton s Effect De Broglie s Matter Waves Quantum Mechanics Democritus, Dalton, and Mendeleïev 1897 Thomson s Plum Pudding Model and the Discovery of the Electron Josepf James Thomson Nobel Prize 1906 Democritus ca. 460 BC ca. 370 BC John Dalton Mendeleïev Experimental Set-Up Plum Pudding Model 1

2 Ernest Rutherford Nobel Prize 1908 (Chemistry) 1911 Rutherford s Model and the Discovery of the Nucleus Bohr s Model of the Atom Classical Model of the Atom Atomic Spectra 1913 Bohr s Planetary Model Bohr s Atom Atomic Spectra Explained Bohr s Correspondence Principle Energy-Level Diagrams Classical Model of Atom Atomic Spectra absorption spectra dark lines emission spectra bright lines Why do atoms of a given element exhibit only certain spectral lines? Why do atoms absorb only the frequencies (wavelengths) that they were emitting? Why is the number of lines of the emission spectrum not always equal to the number of lines of the absorption spectrum? 1913 Bohr s Planetary Model Niels Bohr Nobel Prize 1922 What is the arrangement of the electrons around the nucleus? What keeps the electron from falling into a positive nucleus by electrical attraction? Why do elements exhibit different atomic spectra of discrete lines? 2

3 Bohr s Atom Atomic Spectra Explained Energy-Level Diagrams 1900 Planck s Blackbody Radiation Max Planck Nobel Prize 1918 Thermal Radiation Blackbody Blackbody Radiation The UV Catastrophe Planck s Quantum of Energy Blackbody Explained by Bohr s Atom Thermal Radiation Blackbody 3

4 Blackbody Radiation Formative Quiz Power (area under curve) increases with T Stefan s Law P = A e T Peak wavelength decreases as T increases 4 The temperature of your skin is approximately 35 0 C. (a) What is the peak wavelength of the radiation it emits? (b) What is the total power emitted by your skin. Assume that the area of your skin is 2.0 m 2. (c) Why don t you glow as bright as a lightbulb? Wien s Law max T = m K The UV Catastrophe Blackbody Explained by Bohr s Atom Our Universe is a Blackbody (Cosmology and Quantum Physics) 1905 Einstein s Photoelectric Effect Albert Einstein Nobel Prize 1921 Experiment Classical Physics Experimental Results Einstein s Interpretation Applications 4

5 Experiment Classical Physics vs Experimental Results Einstein s Interpretation K max = h f - K max maximum kinetic energy of elected electrons (photoelectrons) h Planck s constant f frequency of light work function of the metal Formative Quiz The maximum electron energy in a photoelectric experiment is 3.4 ev. When the wavelength of the illuminating radiation is increased by 25%, the maximum electron energy drops to 2.6 ev. (a) What is the original wavelength of the illuminating radiation? (b) What is the work function of the emitting surface? Applications Medicine Automatic Door Openers 5

6 Film Photomultiplier Tube Automatic Camera 1923 Compton s Effect Smoke Detector Arthur Compton Nobel Prize 1927 Experiment Classical Physics Experimental Results Compton s Interpretation Applications Experiment Classical Physics vs Experimental Results 6

7 Compton s Interpretation Photons have momentum p = h / Formative Quiz X-rays of wavelength = 22 pm are scattered from a carbon target and the scattered x-rays are detected at 85 0 to the incident beam. (a) What is the Compton shift of the scattered x-rays? (b) What percentage of the initial x-ray energy is transferred to an electron in such scattering? Compton Shift = - 0 = (h / (me c)) (1 cos ) Compton s Wavelength of Electron C = h / (me c) = nm Peak at 0 Photons interact with electrons tightly bound to the atom (effectively they collide with the atom itself) leading to a Compton shift too small to be detected. Application Photons and Electromagnetic Waves Dental X - Rays Louis De Broglie Nobel Prize 1929 Shortest doctoral thesis on record 1923 De Broglie s Matter Waves Matter Waves Davisson-Germer Experiment Matter Waves De Broglie s wavelength = h / p = h / (m v) frequency of a particle f = E/h Electron Diffraction and Interference Patterns Equations contain both particle (p and E) and wave ( and f) quantities. Application: The Electron Microscope 7

8 1927 Davisson-Germer Experiment Electron Diffraction Pattern First experimental evidence of the wave nature of particles (electrons). The Electron Microscope Quantum Mechanics 1925 Heisenberg s Uncertainty Principle Barrier Tunnelling Penguin Feather magnified 1500 times Applications 1926 Schrödinger s Cloud Model and the Probability Wave Function Probability Density and Electron s Orbitals Schrödinger's Cat 1928 Dirac s Equation Head of an Antarctic Mite magnified 1500 times 1925 Heisenberg s Uncertainty Principle Barrier Tunnelling Werner Heisenberg Nobel Prize 1932 x p x h / 4 E t h / 4 8

9 Alpha Decay Applications Scanning Tunnelling Microscope Nuclear Fusion Scanning Tunnelling Microscope Erwin Schrödinger Nobel Prize Schrödinger s Cloud Model and the Probability Wave Function Probability Density and Electron s Orbitals Schrödinger s Cat 1928 Dirac s Equation Paul Dirac Nobel Prize 1933 Dirac s equation describes the evolution of the wave function of a relativistic particle. It includes quantization of energy. Dirac predicted the existence of antimatter (positron). The positron was first observed in First positron track observed in a cloud chamber. 9

10 Summary 1897 Thomson Discovery of the Electron 1900 Planck Blackbody Radiation 1905 Einstein Photoelectric Effect 1911 Rutherford Discovery of Nucleus 1913 Bohr Quantum Model ofatom 1923 Compton Effect 1923 De Broglie Matter Waves 1925 Heisenberg Uncertainty Principle 1926 Shrödinger Wave Function 1927 Davisson-Germer Experiment 1928 Dirac Equation 10