Chap 9A - Chem 210 WANG Chapter 9A VSEPR Method for predicting shape Chapter 9.1 – 9.3 Molecular Geometry and Polarity 9.1 Molecular Shapes 9.2

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Unformatted text preview: Chem 210 / WANG / Chapter 9A VSEPR Method for predicting shape Chapter 9.1 – 9.3 Molecular Geometry and Polarity 9.1 Molecular Shapes 9.2 The VSEPR Method 9.3 Molecular Shape and Molecular Polarity http://www.up.ac.za/academic/chem/mol_geom/mol_geometry.htm http://www.chempractice.com/drills/java_G_structure.php ElectronElectron-group repulsions and the five basic molecular shapes. linear Shell Electron Pair Repulsion The single molecular shape of the linear electronlinear electrongroup arrangement. The VSEPR Theory ______ ______ _____ ______ _______ • Electron groups around the central atom repel central each other. • • They end up staying as far apart as possible. far as Both bonding and non-bonding pairs do this. bonding non- To predict shape around a central atom, use the following format: A - central atom X -surrounding atom AXmEn E -nonbonding valence electron-group integers Shape of CO2 or O=C=O is linear linear. Double bond at each side of the C atom counts counts as one electron group, as each group electron is shared with the same O atom. tetrahedral trigonal planar trigonal bipyramidal The Shape of Molecules Molecules Molecules have specific shapes. The shape of a molecule affects properties such as melting point, density and solubility. Shape can be predicted by the VSEPR theory. Valence The molecule is linear linear because repulsion among the two two sets of bonding bonding pairs causes them to stay as far apart as possible. Examples: CO2, HCN, BeF2 octahedral Note bond angles and names of each basic shape. angles names The The two molecular shapes of the trigonal trigonal planar electron-group arrangement. electronEx: SO3, BCl3, NO3-, CO32BCl Class Shape Class Shape Ex:SO2, O3, PbCl2, SnCl2 SnCl Fall 2009 / UM-SJTU JI Trigonal planar, BCl3 Note that all 4 atoms (B and 3 Cl) are on the same plane. The molecule is trigonal trigonal planar because the repulsion among the three three bonding bonding pairs makes them stay as far apart as possible. ACTUAL versus IDEAL Bond Angles Bond angles deviate from theoretical angles when deviate angles • Atoms attached to the central atom are different. • Sets of electron groups are different. are Effect of Double Bonds: Double bond has greater electron density stronger stronger repulsion. ideal H 1200 real 1200 1220 H C O C 1160 H H O 1220 Effect of Nonbonding(Lone) Pairs Lone pairs repel bonding pairs more strongly than bonding pairs repel each other. >1200 >1200 Sn Cl Cl <1200 ~ 950 1 Chem 210 / WANG / Chapter 9A VSEPR Method for predicting shape The three molecular shapes of the tetrahedral electrontetrahedral electron-group arrangement. Lewis structure 2D, 2D, does not tell positions of atoms. Both Lewis structures specify the same molecule! Molecular shape 3D. 3D. Examples: CCl4, CF2Cl2, CF SO42-, ClO4- Examples: NH3 , PF3, ClO3, H3O+ PF ClO H C H NonNon-bonding electron domain not connected to any atom invisible! invisible! Atoms are found at the end of bonding bonding electron domains only. Examples: H2O, OF2, SCl2 SCl Molecular Molecular Shape ↔ bonding pairs AX4 How 4 electron domains relate to various molecular shapes. 4-bonding pairs Tetrahedral Molecular shape AX2E2 H O H H H AX3E 3 bonding 2 bonding N pairs Pairs: Bent Bent Trigonal H H molecular shape Pyramidal H Molecular molecular shape Non-bonding pairs influence molecular shape. Tetrahedral shape: CCl4 Note that the 4 Cl atoms are on the Cl are atom corner of the tetrahedron. The C atom sits sits in the center. The molecule is tetrahedral tetrahedral because there are four four bonding pairs repelling each other. Trigonal pyramidal, NH3 Note that the N atom is NOT on atom the same plane as the 3 H atoms. atoms The molecule is Trigonal pyramidal, as there is a (invisible) nonbonding nonbonding pair of electrons pushing down the three three (visible) bonding pairs. bonding But, as “electron clouds”, they are “invisible”. Bent, Bent H2O Note the O atom is NOT in the atom is same line as the two H atoms. atoms The molecule is bent bent because there are two (invisible) two nonbonding nonbonding pairs of electrons repelling the two (visible) two bonding bonding pairs. Fall 2009 / UM-SJTU JI VSEPR method Effect of Nonbonding(Lone) Pairs Explain why the bond angles in H2O and NH3 should differ the bond angle in CH4. CH4 has 4 equal e groups. CH has H2O has two lone-pairs. lone-pairs. has NH3 has one lone pair. List List the above three compounds in order of increasing bond angles. H2O < NH3 < CH4 CH 104.5 < 107 < 109.5 from Molecules with Expanded Valence Shells 5 electron domains Trigonal Trigonal bipyramidal • Axial positions three three 90o repulsions. • Equatorial positions two two 90o repulsions, two two 120o repulsions. •Lone pairs occupy equatorial equatorial positions to minimize 90o repulsions. 2 Chem 210 / WANG / Chapter 9A VSEPR Method for predicting shape The four molecular shapes of the trigonal bipyramidal electron-group arrangement. electron- VSEPR method Molecules with Expanded Valence Shells The three molecular shapes of the octahedral electronelectron-group arrangement. 6 electron electron domains SF6 Octahedral Octahedral shape PF5, AsF5, SOF4 SF4, XeO2F2, IF4+, IO2F2- IOF5 •Axial positions four four 90o repulsions. •Equatorial positions four four 90o repulsions •All positions are equal! BrF5 A summary of common molecular shapes with two to six electron groups. ICl4- XeOF4 XeF2 , I3- , IF2- ClF3 , BrF3 XeF4 TeF5- Review the angles between bonding angles bonding electron domains in each shape. A molecule may have more than one central atoms. The rules regarding molecular geometry still hold. Tetrahedral around this C also. H Cl C H H C Cl Tetrahedral around this C H 1,2-dichloroethane List geometry for each central atom. H Tetrahedral around this C. C H C H H O Trigonal planar around this C H H C H N H Polarity of molecules A molecule with ONLY one polar bond will be polar. Example: HF is a polar HF molecule. δ+ δ- H Pyramidal around this N Fall 2009 / UM-SJTU JI H F •Slight positive side •Slight negative •Smaller electronegativity •Larger electronegativity A molecule with TWO OR MORE polar bonds may or may not be polar. Example: CO2 is nonpolar, H2O is polar Polarity of molecules Is CO a polar molecule? Answer: CO is a polar molecule. CO δ+ C •Slight positive side •Smaller electronegativity O δ- •Slight negative •Larger electronegativity 3 Chem 210 / WANG / Chapter 9A VSEPR Method for predicting shape Orientation of polar molecules in an electric field. Geometry and polarity of molecules Polarity of molecules For a molecule to be polar 1. It must have polar bonds polar 2. It must have the proper proper geometry geometry Is CO2 a polar molecule? CO2 is not a polar molecule. δ- δ+ O C •slight negative Electric field OFF Electric field ON Polar C C H H δ- δ+ F polar C H H H δ+ CH3F is polar. H H H ENH = 2.1 N H H bond dipoles H H N H H molecular dipole The dipoles reinforce each other, so the overall molecule is definitely polar. F CH2F2 is polar. non-polar polar polar polar non-polar Polarity of molecules depends on Geometries F F ends are more δthan H ends. H Non-Polar C F F .. δ+ H Is this one polar or non-polar? δ- C C-F bond is polar, but all ends are equally δ-. CF4 is nonpolar. nonpolar. F F Polar H end is more δ+ more than the F ends. CHF3 is polar. NH3 is polar. N-H bond is polar. H ends are more δ+ than the non-bonding e pair. more ENN = 3.0 δ- F Non-polar H N Molecular Polarity depends on Geometry H C-F bond is polar. F end is δCH3 end is δ+. •slight negative •Slight positive side Molecular Polarity depends on Geometry All C-H bonds are non-polar. CH4 is CH nonpolar H CH4 CH3F CH2F2 CHF3 CF4 δ- O F F Water Water molecule is polar because of its: 1. polar bonds 2. bent geometry. N H H H YES Is this a polar molecule? YES More positively δ+ charged. H H C H If so, which part is δ+? Which part is δ-? H N H More negatively charged δ- If there is a way to approach the molecule from two ends, of which one is δ+, the other δ-, the molecule is POLAR! Fall 2009 / UM-SJTU JI 4 Chem 210 / WANG / Chapter 9A VSEPR Method for predicting shape Using VSEPR to predict molecular geometry Is this a polar molecule? YES H H C More positively δ+ charged. H FIRST determine electron domain geometry: electron H N H More negatively charged δ- A second way to approach this molecule to find δ+, vs. δ-. It is POLAR! Lewis Structure and Shape • Draw Lewis structure of HC2H and use VSEPR theory to predict the shape around C. • Draw the Lewis structure • Count the total number of electron pairs around the central atom (multiple bonds count as one bonding pair). • Arrange electron domains to minimize repulsion THEN determine molecular geometry: molecular • Lone pairs (in equatorial positions if applicable) are “invisible”! SAMPLE PROBLEM 1: ethane and ethanol. Predict the various bond angles in both molecules. Lewis Structure and Shape • Draw Lewis structure of H2CO and use VSEPR theory to predict the shape around C. H δ+ HCO H C H Trigonal planar O δ- •Is this molecule polar? YES •If so, indicate the + and – ends of the molecule. Solution (a) BF3 has 24 valence e- and all electrons around BF the B will be involved in bonds. The shape is AX3, trigonal planar. F HCCH B F0 120 F ethane CH3CH3 H C C ethanol CH3CH2OH H linear Nonpolar molecule – no polar bonds at all!! Solution continued The steps in determining a molecular shape. (b) COS is linear. 1. C and S have the same EN (2.0) 2. O has higher EN (3.5). C=O C=O bond is quite polar. 3. The molecule is polar overall. polar S C O Molecular formula Step 2 Electron-group arrangement e- Count all groups around central atom (A) Step 3 Bond angles P Step 4 Count bonding & nonbonding egroups separately. (c) For PF3 - there are 26 valence electrons, 1 nonbonding pair F The shape is based upon the tetrahedral arrangement. F Note lone pairs & double bonds Molecular shape (AXm En) Fall 2009 / UM-SJTU JI PROBLEM 2 Answer F Step 1 Lewis structure 1. F (EN 4.0) is more electronegative than B (EN 2.0). 2. All dipoles directed from B to F. 3. Because all are at the same angle and of the same magnitude, the nonpolar molecule is nonpolar. The F-P-F bond angles should be <109.50 due to the repulsion of the nonbonding electron pair. P F F F <109.50 The final shape is trigonal pyramidal. The type of shape is AX3E 5 Chem 210 / WANG / Chapter 9A VSEPR Method for predicting shape PROBLEM 2 Answer Cl (d) For COCl2, C has the lowest EN and will be the center atom. C Cl O There are 24 valence e-, 3 atoms attached to the center atom. C does not have an octet; a pair of nonbonding electrons will move in from the O to make a double bond. The shape for an atom with three Type AX3 atom attachments and no C nonbonding pairs on the central Cl Cl atom is trigonal planar. O 124.50 The Cl-C-Cl bond angle will be less C 0 due to the electron than 120 Cl Cl density of the C=O. 1110 O Fall 2009 / UM-SJTU JI SAMPLE PROBLEM 4 F PROBLEM 3 ANSWER F F Sb F F F F F Sb F Determine the shape around each of the central atoms in acetone, (CH3)2C=O. F tetrahedral (a) SbF5 • 40 valence e• all electrons in bonding pairs; bipyramidal. • shape is AX5 - trigonal bipyramidal. O H H C C H (b) BrF5 • 42 valence e• 5 bonding pairs 1 nonbonding pair on central atom. • Shape is AX5E, square pyramidal. square C HH H H HC H O tetrahedral H CCH H trigonal planar F F F Br F >1200 F <1200 6 ...
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This note was uploaded on 07/30/2011 for the course CHEM 210 taught by Professor Zhang during the Spring '09 term at Shanghai Jiao Tong University.

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