2_BOHR - Structure of the Atom Part I(continued What IS the...

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Mary J. Bojan FA 2010 1 Structure of the Atom – Part I (continued) What IS the structure of an atom? What are the properties of atoms? REMEMBER: structure affects function! Important questions: Where are the electrons? What is the energy of an electron? Dual Nature of Light λν =c E = h ν Line Spectrum Bohr Model of Hydrogen Atom
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Mary J. Bojan FA 2010 2 Quantized Energy Energy comes in discrete packets, or “ quanta Energy of a quantum ε is ε = h ν ν = frequency of light h = Planck’s constant = 6.63 × 10 -34 J · s Total energy in light beam is n h ν (n = 1, 2, 3, …) Dual Nature of Light Wave λν = c Particle E = h ν Experimental support: black-body radiation (Planck, 1900) photoelectric effect (Einstein, 1905) line spectra of hydrogen (Bohr, 1913)
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Mary J. Bojan BUT No e ‘s emitted until ν = ν 0 No waiting time Kinetic energy of e = h( ν ν 0 ) = ½ mv 2 where v = speed of electron emitted E photon = h ν = E b + ½ mv 2 Classically, expect metal to “soak” up energy of light until e binding energy (E b ) is overcome. Metal atoms absorb only 1 quantum of energy: it happens all at once as if struck by a particle photon 3 FA 2010
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Mary J. Bojan FA 2010 When light of wavelength = 420nm is focused on a metal surface, electrons are ejected with a speed of 7.50 x10 -5 m/s. What is the binding energy of the electron in the metal? 4
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Mary J. Bojan FA 2010 5 Line Spectra of Atoms Spectrum Type ν s Examples Monochromatic Continuous Discrete (or line) DEMONSTRATIONS: How can we explain all of this? Spectroscopy : study of light interacting with matter Spectrum : distribution of ν in emitted radiation
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Mary J. Bojan Line spectra Hydrogen Neon 6 FA 2010
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Mary J. Bojan Observation of line spectra implies that atoms have discrete (quantized) energy levels. 7 FA 2010
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Mary J. Bojan n = 1, 2, 3, ….. principal quantum number R H = Rydberg constant = 2.18x10 -18 J 8 FA 2010
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Mary J. Bojan FA 2010 9 Energy of an Electron (Bohr Model) Put electron into the orbital: attractive interaction Energy will be negative (means energy is given off) Reverse the process: try to remove the electron Energy will be positive (energy is absorbed) Energy is given off when an electron is put into orbital. Coulomb’s Law helps where Q 1 = charge of electron (negative) Q 2 = charge of proton (positive) d = orbit radius (distance between nucleus and electron) Note: Orbit energy in Bohr Model is negative, so it must correspond to energy needed to put electron into the orbit.
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Mary J. Bojan where n i and n f are integers.
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