Chapter 8 - Electrons in atoms

Chapter 8 - Electrons in atoms - ELECTRONIC STRUCTURE OF...

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26 ELECTRONIC STRUCTURE OF ATOMS Light & other electromagnetic radiation have both wave-like & particle-like properties. Atomic spectra show that electrons also have wave-like properties - in addition to being particles. Atoms have discrete energy levels energy is quantized QUANTUM MECHANICS describes this quantization & explains the … ELECTRONIC STRUCTURE OF ATOMS Atomic energy levels & electron configuration Atomic orbitals & Pauli Exclusion & resulting properties of atoms & ions … Ionization energy & ionic (& atomic) radii Electron affinity
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27 Light & Electromagnetic Radiation Light is wavelike: Oscillating electric & magnetic fields, propagating through space – direction of propagation is orthogonal to electric & magnetic fields Waves have frequency, wavelength & amplitude: Wavelength , λ = crest to crest (or trough to trough) distance Energy of wave amplitude 2 Frequency , ν = (# of crests passing a reference point ) (elapsed time) Units of ν are cycles per second or Hertz , Hz 1 Hz = 1 cycle per second In S.I. units, we omit “cycle” 1 Hz = 1 s 1
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28 ν waves move past reference point each second each wave covers a distance of λ ν λ = distance covered per second i.e. the speed, v , of traveling wave All electromagnetic waves travel with the same, constant , speed: v = c = 3.00 × 10 8 m s 1 c = ν λ ν = c/ λ or λ = c/ ν Electromagnetic spectrum ν = 10 20 10 17 10 14 10 11 10 8 10 5 10 3 Hz λ = 10 12 10 9 10 6 10 3 1 10 3 10 6 m gamma X rays UV IR microwaves FM AM visible Visible light wavelengths range from 400 nm (violet light) to 700 nm (red light) 1 nm = 10 9 m ( one nanometer) red orange λ increases yellow green blue ν increases white light violet prism screen
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29 Light is particle-like (all EM radiation): Other experiments show -2that light behaves like a stream of particles: photons Energy of photon proportional to ν , the frequency. E photon = h ν h = Planck’s constant 6.626 × 10 34 J s eg. E blue photon = (6.626 × 10 34 J s) × (6.4 × 10 14 s 1 ) = 4.2 × 10 19 J NOTE: brighter light = greater # of photons Photoelectric effect Einstein 1905 Light can eject e s from a metal h ν e metal
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30 Experimental observations: 1. ν must be greater than a minimum ( threshold ) frequency , characteristic of metal 2. Kinetic energy of emitted e increases linearly with ν , for ν > ν thres 3. # of e s emitted is proportional to light intensity Energy of Photon = Energy needed to eject e from metal + Kinetic energy of e h ν = E thres + mv 2 2 where v is the velocity of the emitted e K.E. of electron Cesium Sodium ν 5.2 × 10 14 s 1 5.6 × 10 14 s 1 ν thres (Cs) ν thres (Na)
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31 These observations are at odds with the wave description of light. eg. high intensity waves should have enough energy to eject electrons not observed if ν < ν thres light is both wave-like & particle-like explained by Quantum Mechanics PHOTOCHEMICAL REACTIONS Chemical reaction induced by absorption of light h ν eg. H 2 (g) + Cl 2 (g) 2 HCl(g) The light must be blue - green (or have even larger ν ) to initiate the reaction - i.e. there is a threshold frequency.
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