Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 10. The Shapes of Molecules 10-1

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1 Chapter 10 The Shapes of Molecules 10-1

2 The Shapes of Molecules 10.1 Depicting Molecules and Ions with Lewis Structures 10.2 Valence-Shell Electron-Pair Repulsion (VSEPR) Theory and Molecular Shape 10.3 Molecular Shape and Molecular Polarity 10-2

3 Figure 10.1 The steps in converting a molecular formula into a Lewis structure. Molecular formula Step 1 Atom placement Place atom with lowest EN in center Step 2 Add A-group numbers Sum of valence e - Step 3 Remaining valence e - Draw single bonds. Subtract 2e - for each bond. Step 4 Lewis structure Give each atom 8e - (2e - for H) 10-3

4 Molecular formula Atom placement Sum of valence e - Remaining valence e - Lewis structure 10-4

5 SAMPLE PRBLEM 10.1 PRBLEM: Writing Lewis Structures for Molecules with ne Central Atom Write a Lewis structure for CCl 2 F 2, one of the compounds responsible for the depletion of stratospheric ozone. PLAN: Follow the steps outlined in Figure

6 SAMPLE PRBLEM 10.2 PRBLEM: Writing Lewis Structure for Molecules with More than ne Central Atom Write the Lewis structure for methanol (molecular formula CH 4 ), an important industrial alcohol that is being used as a gasoline alternative in car engines. 10-6

7 SAMPLE PRBLEM 10.3 PRBLEM: Writing Lewis Structures for Molecules with Multiple Bonds Write Lewis structures for the following: (a) Ethylene (C 2 H 4 ), the most important reactant in the manufacture of polymers (b) Nitrogen (N 2 ), the most abundant atmospheric gas 10-7

8 Resonance: Delocalized Electron-Pair Bonding 3 can be drawn in 2 ways - Neither structure is actually correct but can be drawn to represent a structure which is a hybrid of the two - a resonance structure. B B A C A C Resonance structures have the same relative atom placement but a difference in the locations of bonding and nonbonding electron pairs. is used to indicate that resonance occurs. 10-8

9 SAMPLE PRBLEM 10.4 Writing Resonance Structures PRBLEM: Write resonance structures for the nitrate ion, N 3-. PLAN: After Steps 1-4, go to 5 and then see if other structures can be drawn in which the electrons can be delocalized over more than two atoms. 10-9

10 Formal Charge: Selecting the Best Resonance Structure An atom owns all of its nonbonding electrons and half of its bonding electrons. Formal charge is the charge an atom would have if the bonding electrons were shared equally. Formal charge of atom = # valence e - - (# unshared electrons + 1/2 # shared electrons) B For A # valence e - = 6 # nonbonding e - = 4 # bonding e - = 4 X 1/2 = 2 Formal charge = 0 A C For B # valence e - = 6 # nonbonding e - = 2 For C # valence e - = 6 # nonbonding e - = 6 # bonding e - = 2 X 1/2 = 1 Formal charge = -1 # bonding e - = 6 X 1/2 = Formal charge = +1

11 Resonance (continued) Three criteria for choosing the more important resonance structure: Smaller formal charges (either positive or negative) are preferable to larger charges. Avoid like charges (+ + or - - ) on adjacent atoms. A more negative formal charge should exist on an atom with a larger EN value

12 Resonance (continued) EXAMPLE: NC - has 3 possible resonance forms - N C N C N C A B C formal charges N C N C N C Forms B and C have negative formal charges on N and ; this makes them more preferred than form A. Form C has a negative charge on which is the more electronegative element, therefore C contributes the most to the resonance hybrid

13 SAMPLE PRBLEM 10.5 Writing Lewis Structures for ctet Rule Exceptions PRBLEM: Write Lewis structures for (a) H 3 P 4 (pick the most likely structure); (b) BFCl

14 VSEPR - Valence Shell Electron Pair Repulsion Theory 10-14

15 Figure 10.3 Electron-group repulsions and the five basic molecular shapes. linear trigonal planar tetrahedral trigonal bipyramidal octahedral 10-15

16 Figure 10.4 The single molecular shape of the linear electron-group arrangement. Examples: CS 2, HCN, BeF

17 Figure 10.5 The two molecular shapes of the trigonal planar electrongroup arrangement. Class Examples: Shape S 2, 3, PbCl 2, SnBr 2 Examples: S 3, BF 3, N 3-, C

18 Factors Affecting Actual Bond Angles Bond angles are consistent with theoretical angles when the atoms attached to the central atom are the same and when all electrons are bonding electrons of the same order. Effect of Double Bonds ideal Effect of Nonbonding(Lone) Pairs H H C larger EN greater electron density H C H real Lone pairs repel bonding pairs more strongly than bonding pairs repel each other. Cl Sn Cl

19 Figure 10.6 The three molecular shapes of the tetrahedral electrongroup arrangement. Examples: CH 4, SiCl 4, S 4 2-, Cl 4 - NH 3 PF 3 Cl 3 H 3 + H 2 F 2 SCl

20 Figure 10.7 Lewis structures and molecular shapes

21 Figure 10.8 The four molecular shapes of the trigonal bipyramidal electron-group arrangement. PF 5 AsF 5 SF 4 SF 4 Xe 2 F 2 IF 4 + I 2 F 2 - ClF 3 BrF 3 XeF 2 I 3 - IF

22 Figure 10.9 The three molecular shapes of the octahedral electrongroup arrangement. SF 6 IF 5 BrF 5 TeF 5 - XeF 4 XeF 4 ICl

23 Figure The steps in determining a molecular shape. Molecular formula Step 1 See Figure 10.1 Lewis structure Step 2 Count all e - groups around central atom (A) Electron-group arrangement Step 3 Bond angles Note lone pairs and double bonds Step 4 Molecular shape (AX m E n ) Count bonding and nonbonding e - groups separately

24 SAMPLE PRBLEM 10.6 Predicting Molecular Shapes with Two, Three, or Four Electron Groups PRBLEM: Draw the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) PF 3 and (b) CCl

25 SAMPLE PRBLEM 10.6 continued Predicting Molecular Shapes with Two, Three, or Four Electron Groups 10-25

26 SAMPLE PRBLEM 10.7 PRBLEM: Predicting Molecular Shapes with Five or Six Electron Groups Determine the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) SbF 5 and (b) BrF

27 SAMPLE PRBLEM 10.8 Predicting Molecular Shapes with More Than ne Central Atom PRBLEM: Determine the shape around each of the central atoms in acetone, (CH 3 ) 2 C=

28 Figure The tetrahedral centers of ethane and ethanol. ethane CH 3 CH 3 ethanol CH 3 CH 2 H 10-28

29 Figure The orientation of polar molecules in an electric field. Electric field FF Electric field N 10-29

30 SAMPLE PRBLEM 10.9 Predicting the Polarity of Molecules PRBLEM: From electronegativity (EN) values (button) and their periodic trends, predict whether each of the following molecules is polar and show the direction of bond dipoles and the overall molecular dipole when applicable: (a) Ammonia, NH 3 (b) Boron trifluoride, BF 3 (c) Carbonyl sulfide, CS (atom sequence SC) 10-30

31 SAMPLE PRBLEM 10.9 Predicting the Polarity of Molecules continued 10-31