1 Molecular Shapes lCO 2 is linear, SO 2 is bent. Why are they different? lThe shape of a molecule is determined by its bond angles, the angles made by lines connecting the nuclei of the atoms in a molecule. lThe bond angle in CO 2 is 180 o ; the bond angle in SO 2 is 120 o.

2 Molecular Shapes lHow can we predict the shape of a molecule? lExamine the shapes of collections of objects with a common center: balloons, plastic eggs or rubber balls or acorns. Why do they have these shapes?

3 The VSEPR Model lValence-Shell Electron-Pair Repulsion lElectron pairs (or groups of pairs) try to avoid one another because of repulsions between like-charged particles lRegions where electrons are likely to be found will be called electron domains. lA double or triple bond comprises a single electron domain. lElectron domains occur as far apart as possible

4 Repulsions of Atoms lPairs of electrons, being negatively charged, repel one another just like the objects just examined.

5 VSEPR Theory lCan predict the angles between electron domains, whether bonding or nonbonding (unshared): l2 domains - linear (180 o ) l3 domains - trigonal planar (120 o ) l4 domains - tetrahedral (109.5 o ) l5 domains - trigonal bipyramidal (90 o & 120 o ) l6 domains - octahedral (90 o ) VSEPR09m15an1

6 VSEPR Theory Shapes of some simple AB n molecules

7 What is wrong with Jason’s model of water?

8 VSEPR Theory lElectron domains can be occupied by atoms (bonding electron pairs) or by a nonbonding pair of electrons, giving various structures lWater has a tetrahedral arrangement of electron pairs, but a bent arrangement of atoms

9 Electron-Domain Geometry This is not the same as molecular geometry!

10 Molecular Geometry lMolecular geometry describes the shape of only those electron domains that contain bonding electrons. lMolecular geometry is predicted by writing a Lewis structure, then determining the electron-domain geometry, and finally determining the molecular geometry.

11 Molecular Geometry

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14 Five Electron Groups

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16 Molecular Geometry

17 Molecular Shapes lWhy do CO 2 and SO 2 have different shapes?

18 What are the structures of the following molecules and ions? BeF 2 (g)CO 2 HCNH 2 O XeF 2 SO 2 NO 2 - SF 2 BH 3 SO 3 NH 3 ClF 3 SiH 4 SF 4 XeF 4 SO PF 5 IF 5 NO 3 - SF 6 lVisualizations and models may be found online.

19 Possible Shapes lABlinear lAB 2 linear, bent lAB 3 trigonal planar, trigonal pyramidal, T-shaped lAB 4 tetrahedral, see-saw (distorted tetrahedral), square planar lAB 5 trigonal bipyramidal, square pyramidal lAB 6 octahedral

20 What are the structures of the following molecules and ions? lGroup Work: HCNXeF 2 NO 2 - SF 2 NH 3 ClF 3 NO 3 - SF 4 XeF 4 SO 4 2 -

21 The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles lBy experiment, the H-X-H bond angle decreases on moving from C to N to O: lSince electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. lTherefore, the bond angle decreases as the number of lone pairs increases.

22 lSimilarly, electrons in multiple bonds repel more than electrons in single bonds. The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles

23 Molecules with More than One Central Atom lIn acetic acid, CH 3 COOH, there are three central atoms. lWe assign the geometry about each central atom separately.

24 Polarity of Polyatomic Molecules lBonds can be polar if electrons are shared unequally lIf molecules are polar, they have a separation of positive and negative charge within the molecule, and they exhibit a dipole moment, which is the product of the charge and the distance between centers of positive and negative charge

25 lFor diatomic molecules, the bond is more polar (greater dipole moment), the greater the difference in electronegativity between the two atoms.  +  - H : F Polarity is in the direction of the bond  +  - H : F Polarity is in the direction of the bond Polar Bonds 4:51

26 lYes, if they are diatomic. lMaybe, if they are polyatomic. Bond polarity of two or more bonds can cancel one another.   -   +   +   - Cl : Be : Cl   -   +   +   - Cl : Be : Cl Are molecules polar if they have polar bonds?

27 Polarity of Molecules lCan a molecule be polar if its bonds are all nonpolar? lWhat structures will give rise to nonpolarity even when bonds are polar? lAny completely symmetrical molecule (a symmetrical structure with identical outside atoms) is always nonpolar. lMany properties of molecules depend on whether they are polar or nonpolar.

28 Always NonPolar lABlinear lAB 2 linear, bent lAB 3 trigonal planar, trigonal pyramidal, T-shaped lAB 4 tetrahedral, see-saw (distorted tetrahedral), square planar lAB 5 trigonal bipyramidal, square pyramidal lAB 6 octahedral

29 Polarity of Molecules

30 Which of these molecules is polar? lH 2 O lCCl 4 CH 4 CH 2 Cl 2 lCO 2 SO 2 lSF 2 SF 4 SF 6 lXeF 2 lPCl 3 BCl 3 Electronegativities F4.0 O3.5 Cl, N3.0 Br2.8 C, I, S2.5 H, P2.1 B2.0

31 Partner Quiz lFor each of the following: lDetermine the molecular geometry lIdentify the molecule as polar or nonpolar H 2 S CH 3 Cl SO 3 PF 5 NBr 3 Electronegativities F4.0 O3.5 Cl, N3.0 Br2.8 C, I, S2.5 H, P2.1 B2.0

32 Please don’t groan Which bear would be most likely to dissolve in water – one from California, or one from Alaska? The one from Alaska, of course, since it is a polar bear.