Structure and Properties of Atoms

Structure and Properties of Atoms - 12.1 12. Structure and...

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12.1 12. Structure and Properties of Atoms (text Ch 6) A. Wave-Particle Duality of Light Light travels as an electromagnetic wave with a wavelength ( λ ). The amplitude is the intensity (brightness) The number of cycles that pass a given point per unit time is the frequency ( υ ). One Hertz (Hz) is one cycle per second. The speed ( c ) at which light moves through space is a constant, 2.998 × 10 8 m s 1 . Therefore, c = υλ Electromagnetic waves span a spectrum , but our eyes can only see around 400 – 700 nm. White light from the sun consists of all wavelengths between this range (and more).
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12.2 Work by Max Planck and Albert Einstein in the early 1900’s demonstrated that light can also be considered as a stream of particles known as photons , which have the energy: λ υ hc h = = E where h = Planck’s constant, 6.626 × 10 34 J s Note that the energy of the photon is directly proportional to frequency, yet inversely proportional to wavelength. o Shorter wavelength = higher frequency = higher energy o Exposure to ultraviolet light and x-rays can cause cancer! B. Atomic Spectra Recall that white light consists a continuum of all wavelengths between 400 – 700 nm (the visible region). However, the spectra given off by atoms of gaseous elements consist of lines that are at specific wavelengths, so we obtain a line spectrum (not a continuum). Atomic spectra are characteristic to the element in question, and the spectra can be used for identification purposes.
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12.3 For example, the atomic spectrum of hydrogen can be produced by striking electric discharge through H 2 . This energy breaks the bond and forms gaseous H atoms. The emission is then passed through a prism, which splits the light into its component wavelengths, giving a line spectrum. Photons are produced when an electron moves from one energy level to another, and the energy difference between the levels is released as light energy (photons). Energy levels for electrons in atoms are limited to specific values, i.e. they are quantized. With the H atom, four lines of specific wavelength were produced. Since wavelength is related to photon energy, it follows that the H atom emits only those four energies.
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12.4 C. The Bohr Model In the 1910’s, Neils Bohr developed a Nobel-prize-winning theory to explain the hydrogen spectrum. (While the theory was completely incorrect, it opened doors to the development of the current and accepted quantum mechanical model). Bohr’s assumption was that the electron particle moved in a circular orbit around a central proton and that there were orbits at distinct radial distances (levels) from the proton. Energy would then be released or
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This note was uploaded on 04/17/2008 for the course CHEM 020 taught by Professor Griffith during the Fall '07 term at UWO.

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Structure and Properties of Atoms - 12.1 12. Structure and...

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