8.1AtomicPhysics

8.1AtomicPhysics - Atomic spectra and atomic structure. 8.1...

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1 8.1 Atomic Physics Atomic Spectra Bohr Model Extensions of the Bohr model X-ray emission Electrons in Atoms Quantum numbers Pauli Exclusion Principle Atomic spectra and atomic structure. The spectra of atoms provide information about the energies of the electron in the atom. Sharp peaks at discrete wavelengths indicate that only specified energies are allowed in the atom. For the Hydrogen atom the Bohr theory explains the energies in a simple manner based on a quantization of angular momentum . The quantization is explained by the de Broglie theory in terms of standing waves for the electron . Atomic structure The scattering of alpha particles (He 2+ ) nuclei from a thin gold foil. The back scattering of a few alpha particles showed that the nucleus is a small compact object. Ernest Rutherford 1911 Geiger and Marsden Planetary model of the atom Alpha particle Scattering from a small compact nucleus Atomic spectra Emission high voltage gas of atom A e - A hf A -> e - (slow) + A * e - (fast) + A * -> A + hf Excitation Emission spectrometer Atomic Spectra Absorption A light source (white light) gas of atoms light minus absorbed wavelengths spectrometer
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2 Atomic Spectra Emission Absorption (dark lines) Discrete spectral lines are observed. Balmer series for Hydrogen ultraviolet visible Lowest λ Highest λ A series of peaks closer together (continuum) at low λ Rydberg Constant The Balmer series could be analyzed mathematically in terms of an empirical equation. H 22 11 1 R 2n ⎛⎞ =− ⎜⎟ λ ⎝⎠ Rydberg Constant R H = 1.0973732x10 7 m -1 n = 3,4, 5 . ......... Integers larger than 2. Disagreement with classical theory Classical physics for the planetary model of the atom predicts that the energy of the electron can have any value - cannot explain discrete spectral lines. The classical theory could not explain the stability of the atom, why the electron does not fall into the nucleus radiating energy. z+ e - Planetary Model of the atom hf Bohr Theory 1. Electrons move in circular orbits. 2. Only specified atomic energy levels are allowed. 3. Energy is emitted when electron go from one energy level to another. 4. The orbital angular momentum of the electron is “quantized” in units of h/2 π = = (called h bar) L = mvr = n = m v r n=1, 2, 3 . ...... h has units of angular momentum s m kg ) m ( s m ) kg ( mvr 2 s m kg s s kgm s J h 2 2 2
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3 Angular momentum of a tennis ball r m v r= 0.5 m m = 0.1 kg v= 2 m/s L =mvr 2 (0.1 )(2 / )(0.5 ) 0.1 0.1 kgm kg m s m J s s == = 34 34 6.6 10 1.05 10 22 hx J s xJ s ππ = h 33 34 0.1 10 1.0 10 LJ s n s = h Ln = h What is n for the ball?
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8.1AtomicPhysics - Atomic spectra and atomic structure. 8.1...

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