Lecture 17 VSEPR polarity 2 Read HW BLB 9 3 BLB 9 33 35 38 Sup 9 8 11 molecular geometry molecular polarity Exam 2 Monday March 2 6 30 review previous material so you UNDERSTAND what we ve done and what we are doing now and start preparing now Last day to sign up for conflict exam is Wed Feb 25 Final Exam Monday May 4 12 20 MUST register on elion for a final exam conflict or overload by March 8 See http www registrar psu edu exams exam overload cfm http www psu edu dus handbook exam html conflict Need help Get help TAs in CRC 211 Whitmore and Supplemental Instruction SI hours on Chem 110 website Sheets office hours Mon 12 30 2 Tue 10 30 12 in 324 or 326 Chem Bldg Sheets Page 1 Lecture 17 Calculating a dipole moment dipole moment an observable measure of the overall polarity of a molecule H F EN 4 0 2 1 1 9 is a it has magnitude direction debye D 3 33 10 30 C m for a diatomic molecule Qr r is separation distance Q is charge Q and Q are equal in magnitude for diatomic molecule units of Q 1 6 10 19 C charge of an e for two charges 1 and 1 separated by 1 4 8 D in polyatomic molecules depends on 1 bond polarities 2 molecular geometries Sheets Page 2 Lecture 17 Dipole trends in HX bond D D HX length EN expt ionic ionic r HF 0 92 1 9 1 82 4 42 41 HCl 1 27 0 9 1 08 6 10 18 HBr 1 41 0 7 0 82 6 77 12 HI 1 61 0 4 0 44 7 74 6 if full charge separation occurs 4 80 r D where r is bond length see prev page X halogen expt experimentally observed ionic expt ionic 100 consider HCl full charge separation ionic 6 10 D but HCl molecule has an actual 1 08 D thus the charge separation in HCl is only 1 08 D 6 10 D 0 18 of the full charge therefore it s polar covalent H X Sheets Page 3 Lecture 17 Oxidation numbers formal charges partial charges oxidation number a charge an atom would have if its bonds were completely ionic i e assign e to the more electronegative atom Chap 4 Lecture 34 FYI right now electronegativity is overemphasized Oxidation number FYI only for now 1 atoms in elemental form 0 2 monoatomic ions charge on the ion 3 nonmetals are usually negative O is 2 except in peroxides where it is 1 H is 1 when bonded to nonmetals and 1 when bonded to metals F is 1 other halogens 1 in binary compounds but positive in oxyanions 4 sum must equal the charge on the species formal charge a charge an atom would have if all bonding e were shared equally FC VE LSE review Chap 8 electronegativity is deemphasized partial charge real charges on atoms in molecules most accurate way of representing charges electronegativity is explicit Sheets Page 4 Lecture 17 Oxidation numbers formal charges partial charges cont consider HCl oxidation numbers formal charges H Cl H Cl partial charges real H Cl polar covalent 0 18 consider HCl full charge separation ionic 6 10 D but HCl molecule has an actual 1 08 D thus the charge separation in HCl is only 1 08 6 10 0 18 of the full charge dipole moment is one measure of the actual charges on atoms in a molecule Sheets Page 5 Lecture 17 Molecular geometry dipole moment Do not memorize this table think things out NOTE this table is presented in terms of molecular formula not in terms of ED geometry as we ve seen before Draw the generic structures think about them to verify NOTE X is the same atom on all positions A is the central atom formula AX AX2 AX3 AX4 AX5 AX6 Sheets molecular geometry linear linear bent trigonal planar trigonal pyramidal T shaped tetrahedral square planar seesaw trigonal bipyramidal square pyramidal octahedral Page 6 0 0 0 0 0 0 dipole moment HCl CO2 H2O BF3 NH3 CH4 Lecture 17 Dipole moments of cis trans isomers which molecule is polar and why cis Cl Cl C C H H trans Cl H C C H Cl this is an example of a geometric isomer atoms are connected in the same order with same bonds BUT spatial orientation is different this is NOT the same as structural isomers see Lecture 13 electronegativity H 2 1 C 2 5 Cl 3 0 bonds C H C Cl EN 2 5 2 1 0 4 EN 3 0 2 5 0 5 what about symmetry of the molecule NET dipole Sheets Page 7 Lecture 17 Example CO2 is not polar Lect 16 p 13 Is SO2 polar CF4 is not polar Lect 16 p 13 Is SF4 polar Sheets Page 8 Lecture 17 Yet another example Rank by increasing polarity A B Cl Cl Cl Cl C D Cl F F E F F F F F F F F Sheets Page 9 Lecture 17 Lewis structures accounts for e pairs involved in bonding lone pairs nonbonding VSEPR accounts for ED molecular geometry so how are bonds made anyway where are the electrons Sheets Page 10 Lecture 17 Molecular orbitals e distribution in atoms atomic orbitals s p d orbitals give us the probability of finding an electron review Chap 6 two models describe e distribution in molecules valence bond theory valence orbitals on one atom with valence orbitals on another atom resulting in a covalent bond what we re talking about now molecular orbital theory a better model that uses wave theory BUT not covered in Chem 110 will see in organic chemistry Sheets Page 11 Lecture 17 Covalent bonding covalent bonding overlap or combining of two singly occupied atomic orbitals to form a new doubly occupied molecular orbital allowing for sharing of electrons by the two nuclei e of opposite spin share the common space between nuclei two forces operating increased overlap of atomic orbitals better sharing closer distance between nuclei increases e density between nuclei thereby lowering energy see next page balance of forces bond length 0 74 for H2 Q1 Q 2 E recall d Sheets Page 12 Lecture 17 Sheets Page 13 Lecture 17 Two bond types 1 sigma bond results from overlap of orbitals e density is symmetric about the internuclear axis examples s s s p s spx p p spx spx internuclear radius where spx and spy are hybrid orbitals more in Lecture 18 Sheets Page 14 Lecture 17 Two bond types cont 2 bond results from overlap of orbitals e density is above below the internuclear axis example two p orbitals So a covalent bond results from orbital overlap understanding Lewis structures electron domain geometry molecular geometry is CRITICAL Sheets Page 15 Lecture 17 Before next class Read HW BLB 9 4 9 5 BLB 9 43 47 Sup 9 12 16 Sup Organic 5 6 Know molecular orbitals orbital hybridization sigma bond pi …
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