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CHEMISTRY 161 Chapter 7 Quantum Theory and Electronic Structure of the Atom www.chem.hawaii.edu/Bil301/welcome.html

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REVISION 1. light can be described as a waves of a wavelength and frequency 2. light can be emitted or absorbed only in discrete quantities (quantum – package - photon) 3. duality of wave and corpuscle

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cathode (-) anode (+) focus anode (+) fluorescent screen Deflection of Cathode Rays particles are negatively charged; particles are defined as ‘electrons’ 2. Properties of Electrons

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de Broglie wavelength each particle can be described as a wave with a wavelength λ (interferences)

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out of phase wave add destructive interference

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in phase wave add constructive interference

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electrometer gold foil electron gun angle ( current interference patterns

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Diffraction of an electron beam (metal crystal) WAVE-PARTICLE DUALITY

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matter and energy show particle and wave-like properties WAVE-PARTICLE DUALITY MASS INCREASESWAVELENGTH GETS SHORTER MASS DECREASES WAVELENGTH GETS LONGER

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What are the wavelengths of a 0.10 kg ball moving at 35 m/s and an electron moving at 1.0 x 10 7 m/s? 1J = kg m 2 s -2 = 1.9 x 10 -34 m Ball: h = 6.626 x 10 -34 J s Solution:

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What are the wavelengths of a 0.10 kg ball moving at 35 m/s and an electron moving at 1.0 x 10 7 m/s? Solution:h = 6.626 x 10 -34 J s 1J=kg m 2 s -2 = 7.3 x 10 -11 m Electron:

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What are the wavelengths of a 0.10 kg ball moving at 35 m/s and an electron moving at 1.0 x 10 7 m/s? Solution:h = 6.626 x 10 -34 J s 1J=kg m 2 s -2 = 1.9 x 10 -34 m = 7.3 x 10 -11 m Electron: Ball: massive particles have immeasureably small wavelengths

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Wave-likeParticle-like BaseballProtonPhotonElectron WAVE-PARTICLE DUALITY large pieces of matter are mainly particle-like, with very short wavelengths small pieces of matter are mainly wave-like with longer wavelengths MASS

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1. light behaves like wave and particle 2. electron behaves like wave and particle 3. electrons are constituents of atoms 4. light is emitted/absorbed from atoms in discrete quantities (quanta)

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E initial E final EMISSION OF A PHOTON atoms and molecules emit discrete photons electrons in atoms and molecules have discrete energies

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EMISSION SPECTRA white light passing through a prism gives a continuous spectrum

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we can analyze the wavelengths of the light emitted HYDROGEN DISCHARGE

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EMISSION SPECTRA analyze the wavelengths of the light emitted only certain wavelengths observed

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white light (continuous spectrum) experimental evidence only certain energies are allowed in the hydrogen atom hydrogen gas (line spectrum)

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CHARACTERISTIC LINE SPECTRUM OF HYDROGEN Balmer found that these lines have frequencies related n=3 n=4 n=5

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Niels Bohr THE BOHR ATOM

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electrons move around the nucleus in only certain allowed circular orbits e-e-

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e-e- THE BOHR ATOM as long as an electron remains in a given orbit its energy remains constant and no light is emitted Bohr’s postulate electrons move around the nucleus in only certain allowed circular orbits

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WHY THE ELECTRON DOES NOT CRASH INTO THE NUCLEUS? Bohr postulated that the wavelength of the electron just fits the radius of the orbit. three wavelengths STABLE

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WHY THE ELECTRON DOES NOT CRASH INTO THE NUCLEUS? five wavelengths STABLE

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electrons move around the nucleus in only certain allowed circular orbits e-e- THE BOHR ATOM each orbit has a quantum number associated with it QUANTUM NUMBERS n is a QUANTUM NUMBER n= 1,2,3,4……... n = 4 n = 3 n = 2 n = 1

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n = 4 n = 3 n = 2 n = 1 THE BOHR ATOM QUANTUM NUMBERS and the ENERGY Z = atomic number of atom A = 2.178 x 10 -18 J = Ry THIS ONLY APPLIES TO ONE ELECTRON ATOMS OR IONS

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BOHR ATOM ENERGY LEVEL DIAGRAM Z=1 HYDROGEN ATOM!

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EnEn ENERGY n=1 -A BOHR ATOM ENERGY LEVEL DIAGRAM

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n=1 -A n=2 -A/4 EnEn ENERGY BOHR ATOM ENERGY LEVEL DIAGRAM

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n=1 -A n=2 -A/4 EnEn n=3 -A/9 n=4 ENERGY BOHR ATOM ENERGY LEVEL DIAGRAM

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n=1 -A n=2 -A/4 EnEn 0 n=3 -A/9 n=4 Energy -A/16 e-e-

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n=1 -A n=2 -A/4 EnEn 0 n=3 -A/9 n=4 Energy -A/16 e-e- ELECTRON EXCITATION excite electron to a higher energy level

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n=1 -A n=2 -A/4 EnEn 0 n=3 -A/9 n=4 Energy e-e- to excite the electron we need energy this can be in the form of a photon E photon = h

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n=1 -A n=2 -A/4 EnEn 0 n=3 -A/9 n=4 Energy e-e- ELECTRON DE-EXCITATION emission of energy as a photon e-e-

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nini nfnf only a photon of the correct energy will do ABSORPTION OF A PHOTON

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nini nfnf

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nini nfnf

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nini nfnf This means energy is absorbed!

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nfnf nini EMISSION OF A PHOTON This means energy is emitted!

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hydrogen emission spectrum n = 1 Ground state n = 2 n = 3 n = 4 n = Ion 8 Excited states... Energy For the Lyman series, n f = 1 and n i = 2,3,4… For the Balmer series, n f = 2 and n i = 3,4,5… For the Paschen series, n f = 3 and n i = 4,5,6…

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nini nfnf IONIZATION OF AN ATOM This means energy is absorbed!

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EE the ionization energy for one mole is IONIZATION ENERGY = 2.178x 10 -18 J atom -1 x 6.022x10 23 atoms mol -1 =13.12 x 10 5 J mol -1 = 1312 kJ mol -1 = 2.178 x 10 -18 J for one atom

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WAVELENGTH OF PHOTON IE = 2.178 x 10 -18 J for one H atom H H + + e –

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e-e- QUANTUM NUMBERS n = 4 n = 3 n = 2 n = 1 SUMMARY THE BOHR ATOM Z = atomic number of atom A = 2.178 x 10 -18 J = Ry

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Homework Chapter 7, pages 252-263 problems

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