Chapter7 - 1 Chapter Seven Atomic Structure 2 History: The...

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1 Atomic Structure Chapter Seven
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2 History: The Classic View of Atomic Structure
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3 Properties of Cathode Rays 1. Cathode rays are emitted from the cathode when electricity is passed through an evacuated tube. 2. The rays are emitted in a straight line, perpendicular to the cathode surface. 3. The rays cause glass and other materials to fluoresce. 4. The rays are deflected by a magnet in the direction expected for negatively charged particles. 5. The properties of cathode rays do not depend on the composition of the cathode. For example, the cathode rays from an aluminum cathode are the same as those from a silver cathode.
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4 Cathode Ray Tube
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5 J. J. Thomson used the deflection of cathode rays and the magnetic field strength together, to find the cathode ray particle’s mass-to-charge ratio : m e /e = –5.686 × 10 –12 kg/C Investigating Cathode Rays
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6 Mass-to-Charge Ratio of Cathode Rays The ratio m e / e for cathode rays is about 2000 times smaller than the smallest previously known m e / e (for hydrogen ions). 1. If the charge on a cathode ray particle is comparable to that on a H + ion, the mass of a cathode ray particle is much smaller than the mass of H + ; or 2. If the mass of a cathode ray particle is comparable to that of a H + ion, the charge of a cathode ray particle is much larger than the charge on H + ; or 3. The situation is somewhere between the extremes described in the first two statements. To resolve the situation we must know either the mass or the charge of the cathode ray particle.
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7 George Stoney: names the cathode-ray particle the electron. Robert Millikan: determines a value for the electron’s charge: e = –1.602 × 10 –19 C Millikan’s Oil Drop Experiment Charged droplet can move either up or down, depending on the charge on the plates. Magnitude of charge on the plates lets us calculate the charge on the droplet. Radiation ionizes a droplet of oil.
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8 Thomson determined the mass-to-charge ratio; Millikan found the charge; we can now find the mass of an electron: m e = 9.109 × 10 –31 kg/electron This is almost 2000 times less than the mass of a hydrogen atom (1.79 × 10 –27 kg) Some investigators thought that cathode rays (electrons) were negatively charged ions . But the mass of an electron is shown to be much smaller than even a hydrogen atom, so an electron cannot be an ion. Since electrons are the same regardless of the cathode material, these tiny particles must be a negative part of all matter. Properties of the Electron
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Thomson proposed an atom with a positively charged sphere containing equally spaced electrons inside. He applied this model to atoms with up to 100 electrons. J. J. Thomson’s
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Chapter7 - 1 Chapter Seven Atomic Structure 2 History: The...

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