ch30_print - The Nature of the Atom 1 Early Models of the...

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The Nature of the Atom 1 – Early Models of the Atom Thompson’s model: The atom is a volume of positive charge, with electrons embedded throughout the volume. In 1911, Geiger and Marsden (under supervision of Rutherford) di- rected a beam of alpha particles (nuclei of helium atoms) against a thin metal foil. Most of the alpha particles passed through the foil as if it were empty space. Some were deflected at very large angles, some even backward. Dr.D.Wackeroth Spring 2005 PHY102A The Nature of the Atom Rutherford explained these observations as follows: the positive charge in an atom is concentrated in a very small nucleus sitting at the center. The electrons were assumed to be outside the nucleus, orbiting the nucleus like planets orbiting the sun. However, Rutherford’s model had two problems: 1) It could not explain the discrete characteristic frequencies of elec- tromagnetic radiation sent out by atoms. 2) Electrons orbiting a positively charged nucleus are continuously accelerated by the centripetal force provided by the electrostatic force between the electron and the nucleus. Accelerated electrons radiate electromagnetic waves. As they radiate, the electrons loose energy and should plunge into the nucleus. Dr.D.Wackeroth Spring 2005 PHY102A
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The Nature of the Atom 2 – Atomic Spectra Suppose an evacuated glass tube is filled with hydrogen (=atom made of 1 proton and 1 electron) at low pressure. If a sufficiently highly voltage is applied between metal electrodes in the tube, an electric current flows. The gas then emits light. Analyzing the emitted light with a spectrograph, a series of discrete lines is observed (so-called emission spectrum ), each corresponding to a different color (wavelength). The wavelengths are characteristic of the element: Dr.D.Wackeroth Spring 2005 PHY102A The Nature of the Atom The emission spectrum of hydrogen in the visible region can be de- scribed by the following equation ( Balmer series ): 1 λ = R µ 1 2 2 - 1 n 2 where n = 3 , 4 , 5 ,... is an integer. R is the so-called Rydberg con- stant : R = 1 . 097 × 10 7 m - 1 The Lyman and Paschen series can be obtained by replacing 1 / 2 2 in the Balmer series by 1 / 1 2 and 1 / 3 2 , respectively. An element can also absorb light at specific wavelengths. The corre-
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This note was uploaded on 02/12/2012 for the course PHYSICS 104 taught by Professor Staff during the Fall '10 term at Rutgers.

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ch30_print - The Nature of the Atom 1 Early Models of the...

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