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Unformatted text preview: UM PHY126 Review Sheet 4 PHY126 Review Sheet 4 by Dr. Sa-Lin Cheng Bernstein 1. An atom : An atom contains a small, positively charged nucleus (radius r n 10- 15 m ), which is sur- rounded at relatively large distances (radius r e 10- 10 m ) by a number of electrons. r n /r e 10- 5 . An atom is electrically neutral. Thomsons plum-pudding model of the atom: There is no nucleus at the center of the plum-pudding model. Positive charge is spread uniformly throughout the atom. Negative electrons are dispersed uniformly within the positively charged pudding. The plum-pudding model was proven to be wrong in experiment by Ernest Rutherford. Rutherfords model (nuclear model or planetary model): Positive charge was concentrated in a small region called nucleus. The electrons were not stationary because they would be pulled inward by the attractive electric force. The electrons must be moving around the nucleus in some fashion, like planets revolving around the sun. planetary model. Difficulties for planetary model: the electron should spiral into the nucleus because it radiates electromagnetic waves. 2. The Bohr model of the hydrogen atom : Assumptions: (1) The total energies (KE+EPE) of the electron are discrete. The allowed energy levels correspond to different orbits for the electron as it moves around the nucleus. (2) Stationary orbit states: an electron in one of the orbits does not radiate electromagnetic waves. (3) A photon is emitted only when the electron changes orbits from a larger one with a higher energy to a smaller one with a lower energy. (4) E i- E f = hf. Each atom in the periodic table has unique set of spectral lines because each atom has a unique set of energy levels (see Figure 30.3 and 30.4). Prediction of the discrete wavelengths emitted by atomic hydrogen: Series Wavelength Region n Lyman 1 = R ( 1 1 2- 1 n 2 ) shorter wavelength n = 2 , 3 , 4 , Balmer 1 = R ( 1 2 2- 1 n 2 ) in the visible region n = 3 , 4 , 5 , Paschen 1 = R ( 1 3 2- 1 n 2 ) longer wavelength n = 4 , 5 , 6 , 1/6 April 15, 2007 UM PHY126 Review Sheet 4 Energies of the electron: (a) Centripetal force on the electron = electrical force exerted by the protons....
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- Spring '06