chem lecture 5

chem lecture 5 - One photon of ultraviolet light (300 nm)...

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Chemistry 6A 1 One photon of ultraviolet light (300 nm) can dislodge an electron from the surface of a metal. Two photons of red light with the same total energy as the one photon of UV light do not produce photoelectrons. Why not? A. Because light is behaving as a wave in this situation. C. The ultraviolet light has more photons than the red light. D. The amount of energy delivered from red light is not enough to overcome the threshold energy. D. The amount of energy delivered from the ultraviolet light supplies more kinetic energy to get the electron to come off.
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Chemistry 6A 2 What is the ionization energy (in J) for a hydrogen atom in its first excited state (n=2)? a. 2.18 x 10 -18 J b. 2.42 x 10 -19 J c. 5.45 x 10 -19 J Is light of wavelength of 545 nm able to induce this transition? (yes, no) E n = Z 2 e 4 m e 8 ε 0 2 h 2 = Z n 2 2 R H
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Chemistry 6A Key Concepts & Questions Lecture 5 Let’s Recap: • Planck Equation E = h ν : From blackbody radiation to the photoelectric effect, for which the KE of the electron is independent of intensity, proportional to frequency • Bohr’s quantization postulate for one-electron atoms : allowed orbits, allowed energy levels (states); transitions between allowed states Now, let’s consider an electron as a wave . ..
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Chemistry 6A 4
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Chemistry 6A 5 Proposal: If matter behaves as a wave, then both light and matter obey the equation: λ and p are inversely related! Any particle, no matter how large or small, moving with a linear momentum, p = mv, has wavelike properties and a wavelength associated with its momentum. Recall that the momentum indicates the ability of a moving body to exert impact on another body upon collision. De Broglie also helped to shed light on why the energy of a particle must be quantized : “Matter waves” must be standing waves! λ = h m e v λ = h p or where p = linear momentum
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Chemistry 6A 6 Usually characterized by boundaries Crests and troughs occur at fixed positions and the amplitude of the wave at certain points is zero (no displacement): nodes Waves with inner nodes at equal intervals are termed harmonics General formula: λ = 2L/n where n = 1, 2, 3, 4 … beginning with the fundamental or first harmonic
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Chemistry 6A 7 De Broglie recognized that standing waves are examples of quantization and suggested that the electron in a Bohr orbit may be associated with a circular standing wave.
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Chemistry 6A 8 Interference in standing waves explains why only certain wavelengths exist for each orbit of a Bohr atom: If fractional wavelengths occur, they interfere with each other on successive cycles and destroy the wave. # wavelengths = circumference
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Chemistry 6A Why don’t Baseballs have Measurable Wavelengths? The
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This note was uploaded on 08/07/2009 for the course CHEM 6A taught by Professor Pomeroy during the Spring '08 term at UCSD.

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chem lecture 5 - One photon of ultraviolet light (300 nm)...

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