# Chapter7 - CHAPTER 7 Atomic Structure and Periodicity 7.1...

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7 - 1 CHAPTER 7: Atomic Structure and Periodicity 7.1 Electromagnetic Radiation wave - a periodic disturbance through a medium frequency n cycles per second velocity of propagation v = ln wavelength l amplitude for electromagnetic radiation, velocity of propagation = c = 3.0 x 10 8 m/s or c = ln (see fig. 7.2, pg 281) Figure on pg. 294 shows pickle emitting light at 589 nm due to excited Na atoms n = c l = 3.0 x 10 8 m/c 589 x 10 -9 m = 5.09 x 10 14 Hz 7.2 The Nature of Matter By 1900, physicists believed they knew most about matter and energy, and that there was little yet to learn. Max Planck - 1900 - attempted to explain the absorption and emission of light by hot bodies. Theories based on classical ideas of the continuous wave nature of light failed to give equations which fit the data. Planck had to assume that energy could be lost or gained only in whole number multiples of hn, where h = 6.626 x 10 -34 Jsec E = nh n Energy is quantized; it can be absorbed or emitted in discrete bundles called quanta. What is the energy of the quanta emitted by the pickle light? E = h n = (6.626 x 10 -34 Jsec)(5.09 x 10 14 Hz) = 3.37 x 10 -19 J Photoelectric Effect - Albert Einstein - 1905 light shining on electrode causes electrons to be ejected, causing a photocurrent. It was found that the current increased with the intensity of the light, but that if the light were below a certain frequency, no current was produced regardless of the intensity of the light Einstein proposed that light consisted of quanta of energy E = h n = hc l light behaves as if it is made up of discrete bundles called photons.

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Chapter 7 Atomic Structure and Periodicity 7 - 2 Einstein also derived E = mc 2 or m = E c 2 If E = mc 2 and E = h n , then mc 2 = h n . Since n = c/l, mc = h/ l . This equation relates the momentum of a photon (a particle property) with its wavelength (a wave property). Since energy and matter are the same thing, Louis deBroglie proposed (1923) that, for a real particle with velocity v, mv = h/ l . or l = h/mv For a 80 kg person running at 6.7 m/sec (a 4-min mile) l = h/mv = (6.626 x 10 -34 Jsec)/(80 kg)(6.7 m/sec) = 1.24 x 10 -36 m compared to nucleus, 1 x 10 -15 m, this number is so small as to be meaningless. For an electron (9.11 x 10 -31 kg) with a velocity of 6.7 x 10 6 m/sec l = h/mv = (6.626 x 10 -34 Jsec)/(9.11 x 10 -31 kg)(6.7 x 10 6 m/sec) = 1.1 x 10 -10 m = 0.11 nm This is in the range of x-rays. Atoms are 1Å to 2 Å (0.1 - 0.2 nm) in diameter, and a regular crystal (such as NaCl) acts as a diffraction grating for X-rays, causing a diffraction pattern. At Bell labs, in 1927, an electron beam was diffracted through a Ni crystal and found to exhibit a diffraction pattern nearly identical to an X-ray diffraction pattern. This shows that electrons display both particle and wave properties. 7.3 The Atomic Spectrum of Hydrogen A rainbow, or the light passing through a diffraction grating or prism is a continuous spectrum of light.
Chapter 7 Atomic Structure and Periodicity 7 - 3 Under some conditions, atoms and molecules are seen to absorb or emit discrete wavelengths rather than continuous spectra of light. This is called a line spectrum. (fig. 7.6, pg. 288). This means that there are only certain energy levels that the H atom can have, the energy of H is quantized.

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