GenChem1Chap7 - 7 Quantum Theory and the Electronic...

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7. Quantum Theory and the Electronic Structure of Atoms microscopic viewpoint: atoms structure of atom – aspect relevant to chemistry: electrons around the nucleus information about the electronic structure: elec tro mag net ic ra - di a tion = wave of electric and magnetic fields wave: transport of energy ( but not matter ); periodic in space and time wavelength λ [m] ; amplitude A frequency ν = number of crests time [1/s] 1/s = Hz (hertz) 1cm = 10 - 2 m , 1 μ m = 10 - 6 m , 1nm = 10 - 9 m , 1 Å = 10 - 10 m νλ = c wave speed electromagnetic radiation: B E , B c ; generated by an accel- erating charge c = 2.997925 × 10 8 m s - 1 speed of light in vacuum GChem I 7.1
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visible light: narrow part of electromagnetic radiation spectrum ; λ = 390nm (violet) – 760nm (red) yellow-green : 550nm max: sun, eye sun light, incandescent light bulb: continuous range of frequen- cies (or wavelengths) = con tin u ous spec trum light emitted from certain gaseous substances has only a limited number of colored lines (frequencies or wavelengths) = dis con - tin u ous spec trum line spectrum , atomic spectrum hydrogen lamp: lines in the visible at λ = 656.3 nm , 468.1 nm , 434.0nm , 410.1nm H line spectrum Balmer series ν = 3.2881 × 10 15 1 s 1 2 2 - 1 n 2 , n = 3,4,... classical physics (pre-1900): lines cannot be explained blackbody radiation : black body = perfect absorber and emitter of radiation; spectrum of heated solids; also cannot be explained by classical physics Planck (1900): energy is discontinuous = energy is quantized GChem I 7.2
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there is a smallest, nonzero value for energy differences: quan- tum of energy energy of a system has only a discrete set of values: multiples of the quantum quantum of electromagnetic radiation : E = h ν h = 6.626 × 10 - 34 Js Planck’s constant photoelectric effect : metal e - electrons are ejected from the surface of certain metals exposed to light of at least a certain minimum frequency; if ν < ν th , no electrons are ejected number of electrons depends on the light intensity (amplitude), but the energy of an ejected electron does not Einstein (1905): light = stream of particles , photon h ν GChem I 7.3
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Classical physics: Rutherford atom is unstable ; electrons in cir- cular or elliptical orbits around the nucleus must emit radiation; consequently they loose energy and spiral into the nucleus
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