CHEMISTRY 161 Chapter 7 Quantum Theory and Electronic Structure of the Atom
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
cathode (-) anode (+) focus anode (+) fluorescent screen Deflection of Cathode Rays particles are negatively charged; particles are defined as ‘electrons’ 2. Properties of Electrons
de Broglie wavelength each particle can be described as a wave with a wavelength λ (interferences)
out of phase wave add destructive interference
in phase wave add constructive interference
electrometer gold foil electron gun angle ( current interference patterns
Diffraction of an electron beam (metal crystal) WAVE-PARTICLE DUALITY
matter and energy show particle and wave-like properties WAVE-PARTICLE DUALITY MASS INCREASESWAVELENGTH GETS SHORTER MASS DECREASES WAVELENGTH GETS LONGER
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 m Ball: h = x J s Solution:
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 = x J s 1J=kg m 2 s -2 = 7.3 x m Electron:
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 = x J s 1J=kg m 2 s -2 = 1.9 x m = 7.3 x m Electron: Ball: massive particles have immeasureably small wavelengths
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
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)
E initial E final EMISSION OF A PHOTON atoms and molecules emit discrete photons electrons in atoms and molecules have discrete energies
EMISSION SPECTRA white light passing through a prism gives a continuous spectrum
we can analyze the wavelengths of the light emitted HYDROGEN DISCHARGE
EMISSION SPECTRA analyze the wavelengths of the light emitted only certain wavelengths observed
white light (continuous spectrum) experimental evidence only certain energies are allowed in the hydrogen atom hydrogen gas (line spectrum)
CHARACTERISTIC LINE SPECTRUM OF HYDROGEN Balmer found that these lines have frequencies related n=3 n=4 n=5
Niels Bohr THE BOHR ATOM
electrons move around the nucleus in only certain allowed circular orbits e-e-
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
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
WHY THE ELECTRON DOES NOT CRASH INTO THE NUCLEUS? five wavelengths STABLE
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
n = 4 n = 3 n = 2 n = 1 THE BOHR ATOM QUANTUM NUMBERS and the ENERGY Z = atomic number of atom A = x J = Ry THIS ONLY APPLIES TO ONE ELECTRON ATOMS OR IONS
BOHR ATOM ENERGY LEVEL DIAGRAM Z=1 HYDROGEN ATOM!
EnEn ENERGY n=1 -A BOHR ATOM ENERGY LEVEL DIAGRAM
n=1 -A n=2 -A/4 EnEn ENERGY BOHR ATOM ENERGY LEVEL DIAGRAM
n=1 -A n=2 -A/4 EnEn n=3 -A/9 n=4 ENERGY BOHR ATOM ENERGY LEVEL DIAGRAM
n=1 -A n=2 -A/4 EnEn 0 n=3 -A/9 n=4 Energy -A/16 e-e-
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
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
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-
nini nfnf only a photon of the correct energy will do ABSORPTION OF A PHOTON
nini nfnf
nini nfnf
nini nfnf This means energy is absorbed!
nfnf nini EMISSION OF A PHOTON This means energy is emitted!
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…
nini nfnf IONIZATION OF AN ATOM This means energy is absorbed!
EE the ionization energy for one mole is IONIZATION ENERGY = 2.178x J atom -1 x 6.022x10 23 atoms mol -1 =13.12 x 10 5 J mol -1 = 1312 kJ mol -1 = x J for one atom
WAVELENGTH OF PHOTON IE = x J for one H atom H H + + e –
e-e- QUANTUM NUMBERS n = 4 n = 3 n = 2 n = 1 SUMMARY THE BOHR ATOM Z = atomic number of atom A = x J = Ry
Homework Chapter 7, pages problems