Chapter 10 : Chemical Bonding II : Molecular Shapes

1. Chapter 10 : Chemical Bonding II : Molecular Shapes • Outline • VSEPR Theory • Molecular Shape and Polarity

2. The Five Basic Shapes Molecular geometry is determined by the electrostatic repulsions between valence shell electron pairs. The electron pairs come in two forms i) pairs and ii) pairs. We will first consider molecular geometries associated with 2 to 6 bonding pairs around the central atom VSEPR Theory

3. .. .. Be .. Cl Cl .. .. .. .. .. 2 Electron Groups: Linear Geometry BeCl2 The repulsion between the electron pairs is minimized if they are as far apart as possible. VSEPR Theory

4. How to treat double bonds ? According to VSEPR theory, the electron pairs in two double bonds repel each other just as they do in the case of two single bonds. In general, when using VSEPR treat a double bond as a single bonding pair. VSEPR Theory

5. 3 Electron Groups: Trigonal Planar Geometry The repulsion between the electron pairs is minimized if they are as far apart as possible. VSEPR Theory

6. Trigonal Planar Geometry with Double Bonds VSEPR Theory

7. 4 Electron Groups : Tetrahedral Geometry The repulsion between the electron pairs is minimized if they are as far apart as possible. The electron pair repulsion squeezes the molecule into three dimensions. VSEPR Theory

8. Why is it called tetrahedral ? VSEPR Theory

9. 5 Electron Groups : Trigonal Bipyramidal Geometry Note that in the trig. bipyram. Geometry there are two different bond angles. VSEPR Theory

10. 6 Electron Groups : Octahedral Geometry VSEPR Theory

11. N The Effect of Lone Pairs : Four Electron Groups All our examples so far have not had any lone pairs on the central atom. What happens when there are lone pairs ? The repulsion between the electron pairs is minimized if they are as far apart as possible. VSEPR Theory

12. Incorrect 2D representation If we do not distinguish between bonding electron pairs and lone pairs, then there are four electron pairs around the central atom. The electron pairs must adopt a tetrahedral geometry to minimize the repulsive forces. However, in doing so, the atoms of the NH3 molecule do not take a tetrahedral geometry VSEPR Theory

13. Lone pair Bond angles are 107o (less than the ideal 109.50 for tetrahedral molecule). Why ? VSEPR Theory

14. ●● H-O-H ●● VSEPR Theory

15. Lone pair occupies the equatorial rather than the axial position. Why ? Five Electron Groups : 4 Bonding Pairs and 1 Lone Pair VSEPR Theory

16. 3 Bonding and 2 Lone Pairs VSEPR Theory

17. 2 Bonding and 3 Lone Pairs VSEPR Theory

18. Six Electron Groups : 5 Bonding Pairs and 1 Lone Pair VSEPR Theory

19. Six Electron Groups : 4 Bonding Pairs and 2 Lone Pairs VSEPR Theory

20. Examples Use VSEPR theory to predict the electron group and molecular geometries of the following species. HCN :SO2 : CF4 : IF4-: SF4 :CO32- : NH3 : XeF4 VSEPR Theory

21. Using VSEPR to Predict Polarity Recall the concept of polar bonds. H Cl The chlorine atom is more electronegative so it attracts electrons more strongly. This is represented by a dipole arrow. We can use molecular geometry to help predict molecular polarity. But first….a math review. Polarity

22. Adding vectors. (The tail to tip method). A man drives south from Saskatoon for an hour at 100 km/hr and then east for 1 hour at 50 km/hour . How far from Saskatoon is he ? Review of vectors A vector is a quantity that has both magnitude and direction Driving at 80 km/hour. Driving east at 80 km/hour. Polarity

23. F F B F Who cares ? What does it have to do with Chem112 ? Dipoles are vectors. To determine if a molecule is polar or not….add up the vectors. Polarity

24. •• O H H •• Another example : H2O Polarity

25. H H B F Be careful when there are different atoms attached to the central atom. Then the vector dipoles do not have equal magnitude ! Polarity

26. A molecule is polar if it has an overall net dipole moment. SO2 is polar but CO2 is not. Why ? Polarity

27. Are the following molecules polar ? HCN :SO2 : CF4 : IF4-: SF4 :CO32- : NH3 : XeF4 Polarity