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Chem121-Chapter-6-SH

Chem121-Chapter-6-SH - Chapter 6 Electronic Structure of...

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1 Chapter 6: Electronic Structure of Atoms © R. Spinney 2009 Electronic Structure The Chemistry of atoms is entirely dependent on the number and arrangement of electrons . This includes how and why chemical bonds form, and break, which is the fundamental principle of the science of chemistry. So why study light? The interaction of light with atoms and molecules has led to our current theory of atomic structure! Electromagnetic Radiation Electromagnetic radiation (EM) travels through space as waves which are characterized by: - wavelength ( λ ): distance (m) between successive peaks - frequency ( ν ) number of peaks that pass a given pt / sec (cycles/s or hertz or s -1 )
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2 Electromagnetic Radiation (cont’d) EM radiation is NOT associated with the movement of matter. It is only a periodic change in the electric and magnetic fields. Electromagnetic Radiation (cont’d) All EM radiation travels at the same speed (in a vacuum), the speed of light (c) c = 3.00 x 10 8 m/s Frequency and wavelength are inversely proportional, i.e. ν ∝ 1/ λ via c Electromagnetic Radiation (cont’d) There are many forms of EM radiation, differing only in frequency (or wavelength or energy) and how they are produced or detected (also see Table 6.1)
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3 Quantized Energy and Photons The development of electronic structure of atoms depended on 3 experiments which had to be explained: 1) blackbody radiation 2) the photoelectric effect 3) emission spectra of excited gas atoms Blackbody Radiation Heated objects emit light, where the ν (or λ ) is T dependent. ~1900 Max Planck suggested that the energy emitted by blackbody sources only occurs in fixed amounts. The smallest amt of E possible is a quantum whose E ∝ ν Blackbody Radiation (cont’d) High freq, ν , (short λ ) has a high E The proportionality constant is Planck’s constant, h, and E = h ν where h = 6.63 x 10 -34 Js
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4 Photoelectric Effect 1905 – Einstein, trying to explain how light shining on a metal surface can generate an electric current, extends this idea to include light, which is a stream of tiny packets called photons E photon = h ν Photoelectric Effect (cont’d) Problem : A laser emits a signal with a wavelength of 351 nm. Calculate the energy of a photon of this radiation. Solution : Emission (line) Spectra White light thru a prism Æ continuous spectrum (rainbow) Light is “bent” by prism, how much depends on λ Shorter λ more than longer λ
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