Quantum 7 September 2009

Quantum 7 September 2009 - Georgia Institute of Technology...

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Georgia Institute of Technology | CHEM 1310 | Recitation | Fall 2009 The Development of Quantum Mechanics Early Physicists used the properties of electromagnetic radiation to develop the fundamental ideas about the structure of the atom. The fundamental assumption for their work was that electromagnetic (EM) radiation behaved as waves with a continual dispersal of energy. Frequency, wavelength, and the speed of light were the fundamental measures used for characterization. c = speed of light = 2.99 x 10 8 m/s λ = wavelength (in m) ν = frequency (in s -1 or hertz) c = λν The wavelength represents the distance from crest to crest (or peak to peak). The amplitude represents the height of the wave. The frequency is representative of the number of cycles over a given period of time. For example, if the plot shown above was obtained over a 1 second period of time, the frequency would be 5 cycles/second because 5 complete waves are shown on the plot. The following table represents the divisions within the electromagnetic spectrum: λ (m) 10 -12 10 -10 10 -8 4 x 10 -7 to 7 x 10 -7 10 -4 10 -2 10 2 EM Radiation Gamma Rays X-Rays UV Visible IR Micro- waves Radio Waves Commonly the wavelength is expressed in units of nm or Å instead of m. 10 9 nm = 1 m or 10 -9 m = 1 nm 1 Å = 10 -10 m or 10 -10 m = 1 Å Because of the rapid developments and progress made in physics toward the end of the 19 th century, most physicists were confident that the field had reached maturity and additional developments were unlikely. However, Planck’s study of blackbody radiation (any object can emit blackbody raditation) yielded a shocking result. Before this experiment, scientists had assumed that energy behaved as waves with a continuous emission. “Blackbody” radiation is the energy emitted by an object that has been heated to incandescence. One easy example is an iron file that has been heated extensively. Planck studied the energy emitted after heating and observed that discrete amounts of energy
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Georgia Institute of Technology | CHEM 1310 | Recitation | Fall 2009 were emitted. These discrete energy packets were identified as quanta . The following expression represents the relationship between the energy observed and the frequency of the quanta of radiation emitted. E = nh ν Ε = energy ν = frequency (s -1 or Hz) h = 6.626 x 10 -34 Js (Planck’s Constant) n=1,2,3… In general, we omit the n term and use E = h ν . The expression can be rearranged using ν = c/ λ . Therefore, E = hc/ λ . Thus, radiation with longer wavelengths is less energetic. What is the most energetic radiation from the table above? The discovery of the discrete emission of energy was a turning point in the physics era, and physicists started to realize that some species have particulate characteristics. Therefore, the idea that physics had matured as a subject started to fade and more
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This note was uploaded on 11/30/2009 for the course CHEM 1101 taught by Professor Bottomley during the Fall '08 term at Georgia Tech.

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Quantum 7 September 2009 - Georgia Institute of Technology...

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