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Chemistry 121 Chapter 6 THE ELECTRONIC STRUCTURE OF ATOMS 6.1 The Wave Nature of Light The electronic structure of an atom refers to the arrangement of electrons Light is a form of electromagnetic radiation Radiation carries energy through space Electromagnetic radiation is characterized by its wavelength All wave have a characteristic wavelength, λ, and an amplitude, A The frequency of a wave, ν, is the number of cycles that pass a point in 1 second. The units of ν are hertz (1 Hz = 1 s -1 ) The speed of a waved is given by its frequency multiplied by its wavelength c = νλ for light Modern atomic theory arose out of studies of the interaction of radiation with matter Electromagnetic waves go through a vacuum at a speed of c= 2.99792458 x 10 8 m/s Electromagnetic waves have characteristic wavelengths and frequencies Example: Visible radiation has wavelengths between 400 nm (violet) and 750 nm (red). Sample Exercise 6.1: Light near the middle of the ultraviolet region of the spectrum has a frequency of 2.73 x 10 16 s -1 . Yellow light near the middle of the visible spectrum has a frequency of 5.45 x 10 14 s -1 . Calculate the wavelength that corresponds to each of these two frequencies of light. (uv light) λ = c/ν = 3.00 x 10 8 ms -1 = 1.10 x 10 -8 m 2.73 x 10 16 s -1 (yellow light) λ = c/ν = 3.00 x 10 8 ms -1 = 5.50 x 10 -7 m 5.45 x 10 14 s -1 Homework Problems: 6.7 and 6.10 1 Study material for this chapter: Textbook sections 6.1 through 6.9
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6.2 Quantized Energy and Photons Planck stated that energy can be absorbed or released from atoms only in certain amounts These amounts are known as quanta o A quantum is the smallest amount of energy that can be absorbed or emitted as electromagnetic radiation The relationship between energy and frequency is: E = hν, where h is Planck’s constant (6.626 x 10 -34 Js) To understand quantization, consider the notes by a violin (continuous) and piano (quantized) o A violin can produce any notes depending on where the fingers are placed on the bridge o A piano can only produce the notes corresponding to the keys on the keyboard The photoelectric effect provides evidence for the particle nature of light It also provides evidence of quantization Light shining on a metal’s surface can cause electrons to be ejected from the metal The electrons will only be ejected once the threshold frequency has been reached Below the threshold frequency, no electrons are ejected Above the threshold frequency, the number of electrons ejected depends on the intensity of the light Einstein assumed that light travels in packets called photons The energy of one photon is E = hν Homework Problems: 6.21, 6.24 and 6.27 6.3 Bohr’s Model of the Hydrogen Atom Line Spectra Radiation composed of only one wavelength is called monochromatic Radiation that spans a whole array of wavelengths is called continuous When radiation from a light bulb is separated into its different wavelength
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This note was uploaded on 05/14/2009 for the course CHEM 121 taught by Professor Wyzlouzil during the Fall '07 term at Ohio State.

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