Chapter31[1]

Chapter31[1] - CHAPTER 31 > The smallest bit of an...

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CHAPTER 31 > The smallest bit of an element that still retains its elemental characteristics is called an atom (a-tom, meaning without division). Thomson’s Plum Pudding Model: Thomson discovered the electron . The Rutherford Model: A miniature solar system. 1911- Rutherford, Geiger, Marsden fired α -particles (He nuclei) at thin gold foil. Since α - particles are positive, they should be deflected by the positive “pudding”. The deflection should be small because α -particles are massive and uniform. Oops! Most of the α -particles passed right through as if the foil weren’t even there! SOME OF THEM , however, were deflected SO MUCH, they almost reversed direction! ER: “It was almost as incredible as if you fired a 15” shell at a piece of tissue paper and it came back and hit you”. ER proposed the solar system model. The “Sun” is the positively-charged nucleus and the electrons are like the “planets” in orbit around it.
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WE ARE MOSTLY EMPTY SPACE!! If the nucleus were a fly on the 50-yd line of the Orange Bowl, the nearest electron would orbit up in the furthest bleachers. atom nucleus r 10,000! r When the α -particle “fly” happens to hit the nucleus, it will bounce back. Problems: 1) If electron is in orbit, it undergoes centripetal acceleration. Accelerating charges radiate energy. The electron would eventually spiral into the nucleus and we wouldn’t exist. 2) Maxwell’s Eqns. say frequency of radiation coming from atom should be same as frequency of orbit, constantly changing, continuously increasing as electron spirals in to nucleus. THIS IS NOT OBSERVED! (Lucky for us, or we wouldn’t be here!) Light coming from atoms has only discrete (quantized) frequencies. Recall the experiment we did with the funky glasses: you saw distinct colored lines when the gas was heated. Each chemical element has a unique “fingerprint” of lines. CSI people often mention “spectroscopic analysis”. That’s what this is! This is called a line spectrum . Emission spectrum shows the bright colors. The absorption spectrum shows the black lines (like a photo- negative). White light is passed through the hydrogen gas and only certain colors are absorbed. 22 71 11 1 Balmer Series: R n 3,4,5,. .. 2n R is the Rydberg constant: R = 1.097x10 m     Each value of n corresponds to the wavelength of a different color line. Ex: Let n = 5
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THE HYDROGEN SPECTRUM  71 22 11 1 1.097x10 m 25 434.1nm The blue line.      1 In general: R n' 1,2,3,4,5,. .. n n' 1,n' 2,n' 3,. (n') n For n’ =: 1- Lyman, 2-Balmer, 3-Paschen, 4-Brackett
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THE BOHR MODEL 1. Electrons move in circular orbits like little planets around the Sun. 2. Only certain orbits are allowed . Angular momentum, L n = n h/2 π (n = 1, 2, 3, …). This is “h – bar”.
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Chapter31[1] - CHAPTER 31 > The smallest bit of an...

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