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Chapter 7

Chapter 7 Quantum Theory and Atomic Structure.pdf

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Chapter 7 Quantum Theory and Atomic Structure 1. Quantum Theory and Atomic Structure 1. 1890-1930: Science revolution in how we view make-up of the universe 2. Before that: 1. John Dalton's atomic theory: 1. Individual units of matter 2. Rutherford's nuclear model 1. replaced "plum pudding" theory of atom 3. Problem: Nucleus and electron attract each and if they are to remain apart, Kinetic Energy of electron must balance Potential Energy (attraction) 1. Classical Physics Law: 1. A negative particle moving in a curved path around a tve 1. One must emit radiation and thus lose Energy 2. Why didn't the electron lose energy and spiral into the nucleus? 1. Breakthroughs that followed Rutherford's mode forced a rethinking of classical picture 2. The Nature of Light 1. Visible light is one type of electromagnetic radiation 2. X-rays, microwaves are others 3. Electromagnetic Radiation: 1. Energy propagated by electric and magnetic fields that alternately increase and decrease in intensity as they move through space 4. Classical Wave Model 1. Distinguished between waves and particles 2. Explains how rainbows form, why objects look distorted in H2O, etc. 3. Cannot explain many observations on the atomic scale i.e. Energy behaves as if it consists of particles 3. The Wave Nature of Light 1. Wave properties of electromagnetic radiation described by two interdependent variables 1. Frequency (ω): the number of cycles the wave undergoes per second 1. Units: S^-1 or hertz (Hz) 2. Wavelength (λ): the distance between any point on a wave and the corresponding point on the next crest (or trough) of the wave 1. i.e. the distance the wave travels during one cycle 2. units: m, nm (10^-9m), picometers (pm, 10^-12m), or non-SI unit angstrom (A, 10^-10) 3. Speed of a Wave: Distance traveled per unit of time (m/s) is the product of its frequency (cycles per second) and its wavelength (meters per cycle) 1. speed (units) = m/s 4. Light travels at: 1. 2.99792458x1068 m/s 2. 3.00x10^8 5. Speed of Light 1. C = frequency (ω) x wavelength (λ) 6. Amplitude: the height of the crest 1. measure of the strength of the electric and magnetic fields 2. Amplitude related to intensity of the radiation 1. Brightness in the case of visible light 2. Electromagnetic Spectrum 1. Visible light only a small portion of electromagnetic spectrum 2. All the waves in the spectrum travel at the same speed but differ in frequency and wavelength 3. Some regions of spectrum utilized in particular devices 1. microwaves: ovens 2. radio waves: radios 3. X-rays: medicine 4. Spectrum consists of continuous frequency and wavelength 4. Classical Distinction Between Energy and Matter 1. Energy and matter behave differently everyday 2. Light of a given wavelength travels at different speeds through different transparent media-- vacuum, air, water, quartz, etc. 3. When light passes from one medium to another (e.g. from air to H2O), the speed of the waves change 4. If the wave strikes the boundary between air and H2O at an angle other than 90 degrees, the change in speed causes a change in direction, and the wave continues at a different angle 1. Angle of refraction depends on the materials on either side of the boundary and the wavelength of the light 5. Dispersion: white light separates (disperses) into its component colors as when it passes through a prism,because each incoming wave is refracted at a slightly different angle 1. Rainbows result when sunlight is dispersed through H2O droplets 6. In contrast, a particle (e.g. pebble) does not undergo refraction when passing from one medium to another (Fig 7.4B). Its speed changes abruptly and then continues to slow down in a curved pattern 7. Diffraction of Waves 1. waves strike the edge of an object and bend around it 2. If the wave passes through a slit about as wide as its wavelength, it bends around both edges of the slit and forms a semi-circular wave on the other side of the opening 8. Particles Behave Differently 1. If you throw sand at a slit, some sand particles hit the edge, while others go through the opening and continue linearly in a narrower group 9. If waves pass through two adjacent slits, the emerging waves interact with each other through a process called interference 1. If the crests of the waves coincide (in phase), they interfere constructively and the amplitudes add together 2. If the crests coincide with troughs (out of phase) they interfere destructively and the amplitudes cancel 1. Result: diffraction patterns of brighter and darker regions 10. Particles pass through the slits in straight paths 5. Atomic Spectra 1. 3rd Key observation about matter and energy 2. Light emitted by an element vaporized and thermally excited, as you see in a neon sign 3. Light emitted by excited hydrogen atom is passed through a narrow slit and is then refracted by a prism 4. Light does not create a continuous spectrum or rainbow, as sunlight does 5. Line spectrum-a series of fine lines of individual colors separated by colorless (black) spaces 6. The wavelength of these spectral lines are characteristic of the element producing them 7. Spectroscopists studying the atomic spectrum of hydrogen have identified several series of such lines in different regions of the electromagnetic spectrum 8. Equations of the following general form, call the Rydberg equation, were formed to predict the position and wavelength of any line in a series 1. Rydbergh equation 1. 1/wavelength = R(1/n1-1/n2) 2. n1 and n2- positive integer 1. n2>n1 3. R-Rydbergh Constant 1. = 1.096776x10^7m^-1 2. The Rydberg equation and the value of the constant are based on data rather than theory 3. No one knew why the spectral lines of H appear in this pattern 4. Line spectra did not correlate with classical theory 5. An electron spirals closer to nucleus shown to emit continuous radiation 1. continuous spectrum, not line spectrum 2. Rutherford's nuclear model seemed at odds with atomic line
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