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04CHM114_0209

04CHM114_0209 - 'FnL-“OI-hhs‘ q" CM kg P.1...

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Unformatted text preview: 'FnL-“OI-hhs‘ q," CM kg P .1, ”HI” -h cat-hu- mm Mnflhugggb I'[email protected] +3 P8:»+‘.~.~l mauu 65439:; 5w? «crates... wolf-Jek-fig CHEM 114A Spring 2004 Mon—Wed Feb 9,111th 2004 Lecture No. 9-10 website \uwwpublicasuedubcaangell Fe' 9“ Y Em M 1; 5mm To “Jon. Chapter 6. Electronic Structure of Atoms. Developing a model for the atom STARTING POINT: Rutherford model of positively charged nucleus surrounded by negatively charged cloud of electrons. Key problem: to understand the arrangement and energy structure of the electrons. wru- kw mu 9 Wflvflrf ht! ‘Nrd'tfl. “63.4933? AMBITION to connect the electronic structure to the structure of the periodic table of elements (Rows of 8): em to.» tun-rue; We can a; e HM.“ wmdrv Is Jfiufi 1 _ tkifi“?ufi CLUBS: Special frequencies of radiation emitted from hot atoms and absorbed by cold atoms LINE SPECTRA what are they? Adi-town“: Fm; hm Imus m Inc-m... tel-mm ——3 Nuts.» c. euggg.’ Puumurs In MUCH? UNI: m TD PREPARE FOR AN UNDERSTANDING OF THIS Em WE NEED TO UNDERSTAND SOMETHING ABOUT LIGHT ‘53.}! Light is a wave motion in the vacuum of space or in whatever medium the light is being transmitted through. It is the electric field which is oscillating in amplitude. It carries energy which can be absorbed when it finds a way of exchanging energy with the surr'l‘izflgisfi 0. Full-h NF 8 ICH 1‘ The velocity of light: c mfigutVC'N‘) 3*.g‘MS“ macaw arm? (if F Rnuzmoy . White light: the solar spectrum Fig. 6.4 301' IT? ovum A M! Mr W 7H6 afomomflau6?~c «that? JPumuM. NMR 353*. "4". M6 (VH'>UV-KI¢M- A light wave is characterized by its wavelength (symbol 2.) or its frequency (symbol v). The two are related by the velocity of light (symbol c, value 3 X 108 mls. The speed of light is one of the key constants of nature Equation: 7W = c v C“ u.) Typical question: A light wave is known to have a frequency of 109 cycles per sec. . .- What is its wavelength? 0x335==m2mnzimau 3.53.9.3 2 .5033 03352.5 33 macaw Eb e 33 SN» 33 3; 33 S: 3; as Sm Ba 89. 3» 3° 3%: _ _ _ _ _ _ _ Ll 3L5} _ 31min 53:5 :< 5 Enqofimg 35 >2. roam. 3&0 Em<ow ma 0 .2 Eu _ . a .......... _ _ _ _ _ . - _ fl ._ 3-; 8-; 3-; 3-; a.“ aim 3; 3-» so ,5» 3“ so an is: .1 51o.q . min...N ax Ba «x 8L :3 138:: mqmnm W 00333 Next we need to tell you that light energy comes in packets called quanta (singular, quantum) Quantum Theory Max Planck, 1900 SOBQYIOH to we u UH‘Gfivnwf Cfibafi-hsu cA-rhcrade" Luau? £080.“, 64—... sum as ‘3' '"33 N t' {NW-(Ivy “Mr GUANVM Energy of quantum 1s d1rectly proportional to its frequency. We write the equation: I FLA-0| “a. gush, E: hv PH-o‘re w {I . 1TH; MA a»: (wear H IS Planck’s constant 1t IS another of E the constants of nature Value: 6.63 X 10‘34 1.3 Question: A photon of light has a frequency 680 x 106 cyclesfsec. What is its energy in Joules? Answer : NEXT STEP: A big insight due to Niels Bohr (Carlsburg brewery, around 1920). A quantum of light is emitted when an electron in an atom makes a jump between distinct energy levels in which it is permitted to exist Level 1 (high) i It: a -hv NM Level 2 (low) Conversely when a quantum of light energy is absorbed. . .. In; I T M '- These processes are both manifestations of the law of conservation of energy. It can be converted from one form to the other, but not destroyed or created. If Bohr is right we must study the line spectra of atoms to learn about their energy structure Line Spectra: Balmer Series. Astronomers had observed dark lines in the solar spectrum, and has sorted them into groups, one of which, the Balmer Series, was described by a simple mathematical relation. It was later found that the same series of lines was emitted from hot hydrogen. This emission spectrum of hydrogen is shown in Fig 6, 1203) The wavelengths of the lines, numbered all Where n = 3, 4, 5... starting With the lowest energy line (longest wavelength) which is found in the red part of the visible spectrum (see Eq. (6.4) p. 206) 0v IR. 6 Niels Bohr developed a model for the hydrogen atom based on the following related ideas: *e- move in orbits of “allowed” energy determined by quantum nos, 1,2,3... * orbits are distinguished from each other in energy and in distance from the nucleus. 0 the single electron of hydrogen can hop from one orbit to another, emitting or absorbing a light quantum depending on whether it decreases in quantum number, or increases. The energy Hn of the quantum depends on the orbits between which the electron is hopping. The model yielded an exam. prediction of the value of R in the Balmer series equation, and made predictions of other series of lines not yet discovered. Yet the model finally proved to be wrong!! 656.3 _ 700 Mum) Sun Hydrogen Fig. 3.10: Spectrum. (a) the sun and (b) hydrogen. Energy is on the vertical axis and a line is drawn for each photon energy observed. For the sun, there is a line at every energy — the spectrum is continuous. The hydrogen spectrum consists of discrete lines. In Bohr’s model, the atom is like a tiny solar system. The electron is held in place by electrostatic attraction e' for np”, balanced by cenw' ;. I ' u- on .; ‘. 9' Objects moving in a circular fashion (like a stone on the end of a string being swung around a boy’s head) have a Special type of momentum “angular momentum” = mvr The quantum numbers in Bohr’s model are determined by the condition (assumption) that this quantity, the orbital angular momentum, can only have values differing by integral nos. / p. o... alt-'1' (.a M 5.; mvr = n (h/21t) “:M‘LJ‘ “Ham (No explanation for this in the theory Remember it for later, when de Broglie will explain) When this model is solved a relation between energy and n is obtained 4" e 6' o 1‘ a s E=-const/n2 n=l,2,3..... (lowest energy for smallest n, E=O for n = infirm: _4h_ 11 = 1 is called ground state. Bohr’s theory gave the energy for this IR ground state in exact agreement “ . ‘ with experiment for the case of (“3) hydrogen atom. Also, the Bohr model gave, r (radius of orbit of quantum. no. 11) = const/ 112 ...
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