chem07 - KEV In a semiconductor, the filled valence band is...

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Unformatted text preview: KEV In a semiconductor, the filled valence band is separated from the conduction band by a band gap of energy, AE, (see diagrams below). Electrons that get excited/promoted across the band gap and into the conduction band are responsible for electrical conductivity in the semiconductor. At very low temperatures, the conduction band is devoid of electrons, and the conductivity, 6, 0f the semiconductor is essentially zero. As mom and more electrons are able to enter the conduction band, the conductivity (ability to flow of electricity) increases. There are two main ways to measure the band gap, AE, of a semiconductor. One of them is optical, and the other is thermal. In an experiment, one typically does something (A) [often as a function of something] to a sample (B) and then monitors an effect (C). In both the optical and the thermal methods for determining band gaps in semiconductors, the sample (B) is a piece of semiconductor material. Below, describe in detail an optical experiment that could be used to measure the band gap of a semiconductor as well as a thermal experiment that would give the same information. In the optical experiment, you will have to use your knowledge of spectroscopy (transitions between energy levels), while in the thermal method, you will need to use your knowledge of the Boltzmann distribution and concepts from (chemical) kinetics. In both cases, be sure to describe the experimental elements, (A) and (C) of your method as well as the guiding principles involved. [I Bung! Problem # 1 (worth 20 % credit) [two parts] (1) Optical method for determining AE. mm W‘ a» m PKWW D Lever on r it “jg KEV , When polished, a crystal of table salt (sodium chloride) is transparent to visible light, i.e., one can see through optical windows made of salt. Despite its transparency at visible wavelengths, however, a piece of (polished) salt is nevertheless opaque (absorbs electromagnetic radiation) at both infrared (IR) and ultraviolet (UV) wavelengths. Explain why salt is opaque in (l) the IR and (2) the UV and yet, (3) is transparent in the visible. Use an energy level diagram to illustrate all three aspects of your answer. Also, explain the nature of the transitions in (4) the IR. Lastly, sodium hydride is also an ionic molecule (a salt). (5) Would the main IR wavelengths at which a sodium hydride crystal absorbs be larger or smaller than those of sodium chloride? Why? (You need not consider the crystal lattice aspects of a salt to answer this question. For present purposes, you can simply think of the respective crystals as macroscopic collections of NaCl or NaH molecules.) Problem #2 (worth 20% credit) [five parts] “Jamar W’ pm) a» . _. , 97 T a) ‘T efiecrgm/e .4- e “1/ f/r‘ofiN [€me excl efl I; gar A #770” #07311! [2340M eke/'75? V/‘Xmfiamfl tel/6U M 631mm! 5W7? cht Mace ‘ /" Awqeffi‘aw 000mm “(5‘) I}? 7ZAAW770MJ exam Na ct I l/I‘BKAFONJ’ (flaw QJ’CJMAWoNJ’) EV Problem #3 (worth 25% credit) [five parts] (1) Write the ground state, electronic configuration for F‘. 9. z 6' [.9 2.9 if (2) Write the ground state, electronic configuration for K“. [A] bk [9" wig/6 3313/05 (3) Write the ground state, electronic configuration for 02. a” tags, fl,” 77,7, 77,7, (4) Write the ground state, electronic configuration for N2. , 3, 9‘2 6;. L 0;" Q3“ 72/7 fiat? (5) Consider the following MO energy level scheme for a hypothetical molecule. Its ground state, electronic configuration is 021t4. What is the electronic configuration for its first excited state? KEY (1) What is the molecular geometry of H20? Explain your reasoning using VSEPR theory. I 2 Z 7*- & z > ' q ', W M 9‘ ’# ‘ )wm 5‘0 Q t ’* ’* What‘fiw a, MMWW ‘ ' M?" M 047’ Jar-39510 W (2) What is the molecular geometry of SF4‘? Explain your reasoning using VSEPR theory. HI: — Z J 6 . 3,26, #759; 5 #J/\# Problem #4 (wOrth 20% credit) [four'parts] 05w 3) What is the molecular geometry of NagO'? In this case, you will find that VSEPR theory and Lewis dot structures are inadequate to help you predict its structure. Hint: Thinking in terms of F: atOmic orbitals and what they mean is a better route. H g Mfi—Fwozaw,mwkq”mfl W Problem #5 (worth 15% credit) [15 parts] (1) If you were to see a description of an atomic orbital with three electrons in it ( -H-I- ), would you find it plausible or not? If not, what principle would it have violated? N” FM W W (2) If you were to see a description of an atom in which the electrons were all stuck directly to the nucleus (held there by the Coulombic attraction between the negatively charged electrons and the positively charged nucleus), would you ind it plausible or not? If not, what principle would it have violated? N0 F . [p t t q a? , (3) If you were to see the following MO description for a hypothetical molecule, would you find it plausible or not? If not, what principle would it have violated? flMo #11400; M91 1+ ~— :12“ (4) The ionization energy of the hydrogen atom is 13.6 eV. If a hydrogen atom which has been excited to its n=2 level were to emit (radiate) a photon, what would be the energy of that photon? You may leave your answer as a fraction. “gab 'L (3 )LMEI/ MM” 56”“; by r Em’e'i “as? - 4 I [3‘6 M5 2—; E”3é' , , --————- 77 = / (5) In transition metal coordination complexes, what is a typical coordination number? (multiple choice) (a) 1, (b)@ (C) 8, (d) 12, (e) 16, (f) 40 (6) In the case of radioactive decay, what order is the differential rate law? M l “a, c. at 7 Which isoto e is the fissionable isoto e of uranium? ( ) _ p p t 23; WW (8).What fissionable material can be made from the other prominen isotope of uranium? r w (9) What happens to the spacings between the energy levels in the 1D particle-in—a—box problem when the width of the box is shortened? wwwm (10) If one wanted to know the relationship between the equilibrium constant of a given chemical reaction and the individual rate constants involved in its mechanism, what law oflciple that we studied should one look to? [(2 £631.; W‘ .0; W'Wfl C “r “'9” (11) What is the energy spacrng between v=3 and v=4 in a 1D harmonic oscillator? bar ' KEY (12) If one wanted to know how to write a differential equation for the rate of a given elementary step in the mechanism of a reaction, what law or principle that we studied should one look to? (13) During the nuclear decay process known as electron capture, what must the orbital angular momentum of the captured electron be? /Q 0 (l4) Recall the relationship between force and potential energy. If the force were equal to -cx, where c is a constant here, wlfl; would be the corresponding potential energy? . fl 5—. CK Q N. \ Fobv =- F' w 4t (15) Use the steady—state approximation to write the differential rate law for the Lindemann mechanism. j, 7L A M :7; A r % fl$W flxjfymclwfi A L743 Z = A Mflrw + 12 51417 $34 2 44,04‘7’ i? £75/43 7"22 end of exam pfin Problem #1 Hydrogenic Atoms (20 points total) (a) Sketch to scale the energy levels of the hydrogen atom up through n = 3. Label ation. To set a quantitative scale, start by indicating the n = infinity Then, fill-in the [ 10 points] each level by its orbital design level at the top and the n = 1 level at the bottom of your energy level diagram. proper locations for n == 2 and n = 3 levels and orbital designations. [ 10 points] (b) On the same energy scale, off to the side of the diagram in part (a), draw the energy level (with proper orbital designations) for n = 2 in He‘”. This should be quantitatively correct, i.e., on the same scale as part (a). Show your reasoning in detail. a if, 22* 3 W 8 Me W M 5* “ “ W (a) 0 71:95 ,. a: 3/0" 349i...” 7” W M55” Cs”) W 71:2. 2'9 fl 5% fl ‘3 Problem #2 Electronic configurations for atoms (15 pts.) k/ (a) State Hund’s Rules. , ’ MeflrmwtazW¢w Oct payee” W M ( W‘WL‘Z 6‘— Q (b) Write out the ground state electronic configuration for C. / _ 2 I / 2 2 i Z i3 £3 2% #7 M [‘5 £5 2/ 3/0 (0) Write out the ground state electronic configuration for 0”. Is it still paramagnetic? Exp] 'n. l9 ,29 2,9 2/7 y MP, (d) Ne+ is isoelectronic with what neutral atom? F * ) (6) Write out the ground state, valenCC electronic configuration for I. w” 2. .5' 5 8 ‘5]? Problem #3 Periodic properties and the periodic table _ ( 16 pts) (b) Which has the-larger diameter, Sc or C0? Why? ‘ ‘ 3 ka— (’0 (ON 015: My” flcfi‘m @- 71AM)???” #197264’ , (c) 'Which has the larger first ionization potential, Ne or F? Why? Me (gawep 3% ((1) Which has the larger second ionization potential, Ne or Na?%\’h::c£ W7 if filial/€40 Na... 0 Na?- ;emay egarefl W (e) Which family (Group) of elements has the highest electron affinities? ##lofiw (t) Which element has the highest (first) ionization potential? He (a) Which has the larger diameter, Ba+ or. Sr‘“? ., (g) Which family (Group) has the valen’Ce electronic configuration, RPS? 0mg (h) What is the family (Group) of elements that contains sodium called? Alwaé ; flmw OVaT MKMW e. gflflfi/ 5“” M67” 49) - Problem #4 Chemical Bonding Concepts (7 14 pts.) K E y (a) What is the bond order of Mgz? Show your work. *1”. §'*‘ 0 i 92C?” ' 5,47%» (b) What is the most electronegative element in the periodic table? (c) Which has the stronger chemical bond, HP or HBr? HF q ((1) There are two electronegativity scales, one attributed to Pauling and the other to z fall] (e) What is the definition of a dipole moment? What are its units called? s9..__ methane, carbon dioxide, rogen fluorid? ou must get all of ese correct to get any fl W' the following has a 4 , sulfur hexafluoride, h ~v- or this question , i.e., (f). f? What is the coordination complex bonding theory named that utilizes molecular o itals as opposed to the theory that is based strictly on sical electrostatic interactions? 3 ~ Problem #5 VSEPR Theory and Structures ( 20 pts.) y (a) [10 pts.] Using VSEPR theory, predict the structure of IFS. Indicate your reasoning. ' ; r. ‘ '2 . J \ ‘ .. f :r . A ' . s”. M . r -- '(b) Use VSEPR theory to predict the structure of ammonia and of water. Show approximate bond angles. ‘ e \ 1‘ W (a) [4 pts.] Construct molecular orbital correlation diagrams for H2, He}, Hez, and Hf and in Problem # 6 Chemical Bonding: Molecular Orbital Theory ( 15 pts.) each case, determine the nd order. /6’ End of exam ...
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This note was uploaded on 11/11/2009 for the course CHEMISTRY 030.204 taught by Professor K.h.brown during the Fall '03 term at Johns Hopkins.

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chem07 - KEV In a semiconductor, the filled valence band is...

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