Understanding VSEPR Theory in Molecules

Module 9: Molecules General Chemistry Chem2331

Module 9 Outline Module 9 Session 1 VSEPR model 1. Shapes of molecules: perfect geometries AX2, AX3, AX4, AX5 and AX6 2. Shapes of molecules with lone pairs: AXnEm 3. Predicting angles: effect of lone pairs and double bonds 4. Molecular dipole Chem 2331: General Chemistry 2

VSEPR: Valence Shell Electron Pair Repulsion Theory linear tetrahedral trigonal planar 109.5 is the angle of a tetrahedron Examples: Examples: Examples: SO3, BF3, NO3 , CO32 CS2, HCN, BeF2 CH4, SiCl4, SO42-, ClO4- Chem 2331: General Chemistry 3

VSEPR: Valence Shell Electron Pair Repulsion Theory Less important structures that you may need for homework (not for the test) trigonal bipyramidal octahedral SF6 PF5 Chem 2331: General Chemistry 4

VSEPR: Valence Shell Electron Pair Repulsion Theory Atoms and lone pairs in the space A - central atom A - central atom X -surrounding atom X -surrounding atom E -nonbonding valence electron-group E -nonbonding valence electron-group AXmEn The textbook will cover more complicated structures that are not relevant in Health Sciences integers The lone pairs will behave like a bulky surrounding atom. e.g. SO2 Chem 2331: General Chemistry 5

VSEPR: Valence Shell Electron Pair Repulsion Theory Examples: SO2, O3, PbCl2, SnBr2 What geometry would you obtain for AX1E2? Chem 2331: General Chemistry 6

VSEPR: Valence Shell Electron Pair Repulsion Theory Examples: CH4, SiCl4, SO42-, ClO4- NH3 H2O PF3 OF2 ClO3 SCl2 H3O+ Chem 2331: General Chemistry 7

VSEPR: Valence Shell Electron Pair Repulsion Theory Less important structures that you may need for homework (not for the test) PF5 AsF5 SOF4 SF4 XeO2F2 IF4+ IO2F2- XeF2 I3- IF2- ClF3 BrF3 Chem 2331: General Chemistry 8

VSEPR: Valence Shell Electron Pair Repulsion Theory Less important structures that you may need for homework (not for the test) SF6 IOF5 BrF5 TeF5- XeOF4 XeF4 ICl4- Chem 2331: General Chemistry 9

VSEPR: Valence Shell Electron Pair Repulsion Theory Draw the Lewis structures and identify the structure of the following compounds. Give an approximation to the angles in the molecule a) SCN(-) b) CHCl3 c) PO4(3-) d) SO3 e) CH2O f) BH3 g) XeO2 h) NCl3 i) XeF4 Chem 2331: General Chemistry 10

VSEPR: Valence Shell Electron Pair Repulsion Theory 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. 120o 122o Effect of Double Bonds H H 120o C O 116o C O ideal H H greater electron density real Effect of Nonbonding (Lone) Pairs Both lone pairs and multiple bonds will use more space than regular bonds Lone pairs repel bonding pairs more strongly than bonding pairs repel each other. Sn Cl Cl 95o Chem 2331: General Chemistry 11

VSEPR: Valence Shell Electron Pair Repulsion Theory Chem 2331: General Chemistry 12

VSEPR: Valence Shell Electron Pair Repulsion Theory What is the approximate angle of the O C O bond in the citric acid molecule below? O A. 120 B. 109.5 C. < 120 D. < 109.5 E. > 120 O OH OH O HO HO Chem 2331: General Chemistry 13

Molecular Dipole Polarity of a molecule The boiling point of NH3is 33.34 C while the boiling point of BF3 is much lower -100.3 C WHY? The dipoles reinforce each other, so the overall molecule is definitely polar. ENN = 3.0 ENH = 2.1 N N N HH H HH HH H H molecular dipole bond dipoles F BF3 is non-polar because the dipole of the bonds cancel each other. B F F (EN 4.0) All bonds in the same plane F B (EN 2.0) 1200 Chem 2331: General Chemistry 14

Molecular dipole Which of the following molecules is the most polar molecule? Br Cl F Cl Br Br Cl F Br Cl Te Te Te Te Te Br Cl F Br Cl Br Cl F Cl Br a) b) c) d) e) Chem 2331: General Chemistry 15

Molecular dipole Activity: Are the following molecules polar? PF3 COCl2 CCl4 CS2 SF2 Hint: Draw first its Lewis Structure. Identify the central atom and the surrounding atoms Consider if the geometry should be: Tetrahedral, Trigonal, Linear Leave room for lone pairs. Do lone pairs or multiple bonds distort the regular geometry? Indicate which angles are smaller or bigger than the regular ones. Bond polarity: Is there a electronegativity difference? Is the bond polarity adding up to a molecular dipole? Which of these molecules is nonpolar? Choose one answer. a. ClF3 b. SF6 c. CH3NH2 d. CH2O Chem 2331: General Chemistry 16