AtomicTheoryC3.pdf - Atomic Theory and the Periodic Table...

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•1 CHEM 1000A 3.0 Atomic theory and the Periodic table I 1 Atomic Theory and the Periodic Table Petrucci, Harwood and Herring: Chapters 9 and 10 • Aims: – To examine the Quantum Theory, – to understand the electronic structure of elements, – To explain the periodic table – To explain atomic properties CHEM 1000A 3.0 Atomic theory and the Periodic table I 2 Atomic Theory and the Periodic Table I will assume you understand: – Radiation (frequency, wavelength, … PHH 9-1) – Introductory atomic structure (PHH 2-1 to 2-6) – Bohr atom (although I will do some of this) CHEM 1000A 3.0 Atomic theory and the Periodic table I 3 Bohr Atom Observations led to the development of the Bohr model for the atom: In particular: The spectra of atomic absorption and emission are line spectra, not continua. Atoms can only emit and absorb certain frequencies Understanding black body radiation needed the ideas of quantization of energy levels (Planck) Understanding some of the properties of light needed a quantum theory. (e.g. photoelectric effect)
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•2 CHEM 1000A 3.0 Atomic theory and the Periodic table I 4 Emission from an atomic lamp only gives discrete wavelengths The energy released comes from electrons changing energy states This suggests the electrons can only have particular energy states CHEM 1000A 3.0 Atomic theory and the Periodic table I 5 Bohr Atom The fundamental principles of the Bohr model of the atom are: – The electron moves in circular orbits around the nucleus – The electron has fixed orbits. The electron has constant energy and no energy is emitted – An electron can only go from one allowed orbit to another CHEM 1000A 3.0 Atomic theory and the Periodic table I 6 Bohr Atom Quantitatively The radius of an orbit is r n =n 2 a 0 Where n= 1,2,3,… and a 0 = 53pm (determined by assuming the angular momentum is quantized in units of h/2 π ) The electron energy is an orbit is less than if it was free and E=-R H /n 2 Where R H = 2.179 x 10 -18 J
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•3 CHEM 1000A 3.0 Atomic theory and the Periodic table I 7 Bohr Atom in space CHEM 1000A 3.0 Atomic theory and the Periodic table I 8 Bohr atom in energy CHEM 1000A 3.0 Atomic theory and the Periodic table I 9 Bohr atom This was very successful as it predicted the energy of the transitions in a hydrogen atom = = 2 i 2 f H 2 H n 1 n 1 -R E Hence n R - E
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•4 CHEM 1000A 3.0 Atomic theory and the Periodic table I 10 So what’s wrong with the Bohr Model? Cannot explain emission spectra from multi-electron atoms Cannot explain the effect of magnetic fields on spectra No reason for the fixed, quantized orbits CHEM 1000A 3.0 Atomic theory and the Periodic table I 11 Quantum Mechanics The most important idea is that of wave- particle duality i.e. small particles display wave-like properties – Einstein had said light was a wave but to explain some results it has particle like properties.
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