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Chapter 10 Molecular Geometry and Chemical Bonding Theory Dr. Peter Warburton

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1 Chapter 10 Molecular Geometry and Chemical Bonding Theory Dr. Peter Warburton

2 © Peter Warburton 2008 All media copyright of their respective owners2 Lewis dot structures Lewis dot structures only give an idea of the electron distribution in the species. There is NO INFORMATION about the molecular geometry, which depends on the relative position of nuclei around the central atom.

3 © Peter Warburton 2008 All media copyright of their respective owners3 VSEPR Model We try and connect the information of electron distribution in a Lewis dot structure to molecular geometry by using the Valence-Shell Electron-Pair Repulsion (VSEPR) theory. GROUPS of electrons repel each other, ending up as far from each other as physically possible.

4 © Peter Warburton 2008 All media copyright of their respective owners4 The textbook talks about repulsion of ELECTRON PAIRS. I prefer the term repulsion of ELECTRON GROUPS because multiple bonds are treated as ONE PAIR of electrons in VSEPR theory even though there are more than one pair of electrons present.

5 © Peter Warburton 2008 All media copyright of their respective owners5 A lone pair is ONE GROUP of electrons A single bond is ONE GROUP of electrons A double bond is ONE GROUP of electrons A triple bond is ONE GROUP of electrons

6 © Peter Warburton 2008 All media copyright of their respective owners6 Electron distribution vs. geometry Electron distribution Molecular geometry Looks at “shape” of electron group distribution Looks at “shape” of nuclear positions around the central atom INCLUDES lone pairs No terminal nuclei on lone pairs means we IGNORE (“can’t see”) all lone pairs.

7 © Peter Warburton 2008 All media copyright of their respective owners7 Electron distribution vs. geometry If the central atom has NO lone pairs on it, then the electron group distribution and the molecular geometry ARE THE SAME!

8 © Peter Warburton 2008 All media copyright of their respective owners8 Modified Figure 10.2 (GROUPS)

9 © Peter Warburton 2008 All media copyright of their respective owners9 AX n notation The central atom A is bonded to n atoms or functional groups, denoted as X. This notation ignores lone pairs, so it is suited for categorizing molecular geometries, which also ignore lone pairs.

10 © Peter Warburton 2008 All media copyright of their respective owners10 Modified Fig 10.4 (2 to 4 GROUPS)

11 © Peter Warburton 2008 All media copyright of their respective owners11 Modified Fig 10.7 (5 GROUPS)

12 © Peter Warburton 2008 All media copyright of their respective owners12 Modified Fig 10.7 (6 GROUPS)

13 © Peter Warburton 2008 All media copyright of their respective owners13 Notice! With experience, we tend to start drawing Lewis dot structures with molecular geometry information included… instead of

14 © Peter Warburton 2008 All media copyright of their respective owners14 Getting geometry information 1.Draw the Lewis dot structure 2.Determine the number of electron groups on the central atom to get electron group arrangement (Figure 10.2) 3.Use the number of lone pairs and the arrangement (Figures 10.5 and 10.7) to determine molecular geometry

15 © Peter Warburton 2008 All media copyright of their respective owners15 Advanced geometry considerations Lone pairs are the “biggest” electron group (better at repelling other electron groups). Triple bonds are the next “biggest” group. Double bonds are “smaller”. Single bonds are the “smallest” electron group.

16 © Peter Warburton 2008 All media copyright of their respective owners16 Tetrahedral arrangement (advanced)

17 © Peter Warburton 2008 All media copyright of their respective owners17 Trigonal planar arrangement (advanced)

18 © Peter Warburton 2008 All media copyright of their respective owners18 Problem What is the arrangement of electron groups, and geometry around the central atom for the following molecules? SF 2 XeO 4 H 3 O + AsF 5

19 © Peter Warburton 2008 All media copyright of their respective owners19 Molecular dipole moments If there are polar covalent bonds (partial charge separations) in a molecule, the molecule MAY OR MAY NOT have a permanent dipole moment. A permanent dipole moment means there are regions of the entire molecule that are permanently partially negative and permanently partially positive.

20 © Peter Warburton 2008 All media copyright of their respective owners20 Permanent dipole moments To determine if a molecule has a permanent dipole moment, we add together the vectors that describe the charge separation of polar covalent bonds.

21 © Peter Warburton 2008 All media copyright of their respective owners21 Permanent dipole moments To add vectors, we chain vectors by putting the tail of the next vector on the head of the previous vector. The resultant vector is then drawn from the tail of the first vector to the head of the last vector in the chain. This resultant vector is the permanent dipole moment.

22 © Peter Warburton 2008 All media copyright of their respective owners22 Recall HCl We saw earlier that the diatomic molecule HCl has a polar covalent bond. Since there is only one bond, this one vector of charge separation ALSO describes the permanent dipole moment of HCl.

23 © Peter Warburton 2008 All media copyright of their respective owners23 Water The permanent dipole moment in water can be seen by adding together the charge separation vectors of the two polar covalent O-H bonds. Lewis structure Adding vectors Permanent dipole moment

24 © Peter Warburton 2008 All media copyright of their respective owners24 Symmetry and dipole moments A molecule with more than one polar bond MIGHT NOT have a permanent dipole moment when the charge separations are symmetrically distributed so that the resultant vector sums up to to zero.

25 © Peter Warburton 2008 All media copyright of their respective owners25 Geometry and dipole moments

26 © Peter Warburton 2008 All media copyright of their respective owners26 Permanent dipole moments and molecular properties Ionic bonds are generally strong because of the strong electrostatic attraction between positive and negative charges. Molecules with permanent dipole moments have regions with partial positive and negative charges that attract the opposite regions on other molecules of the same type. Such intermolecular forces affect the bulk properties of collections of molecules.

27 © Peter Warburton 2008 All media copyright of their respective owners27 Quantifying dipole moments Dipole moments measure the amount of charge separation (in Coulombs) that occurs over the bond length (in meters) in a derived unit called a debye (D) 1 D = 3.34 x C  m Dipole moment for HCl is 1.08 D

28 © Peter Warburton 2008 All media copyright of their respective owners28 Problem a) The molecule BrF 3 has a dipole moment of 1.19 D. Which of the following geometries are possible: trigonal planar, trigonal pyramidal, or T- shaped? b) The molecule TeCl 4 has a dipole moment of 2.54 D. Is the geometry tetrahedral, seesaw, or square planar?

29 © Peter Warburton 2008 All media copyright of their respective owners29 Valence bond theory Bonds form between atoms when: 1.Orbitals (the “allowed” electron distributions) in the atoms overlap to create molecular bonding orbitals. 2.Each molecular bonding orbital has NO MORE THAN 2 electrons in it.

30 © Peter Warburton 2008 All media copyright of their respective owners30 Bond strength Covalent bonds are strongest when there is maximum orbital overlap between atomic orbitals. This maximum overlap occurs in the same direction as the atomic orbitals point.

31 © Peter Warburton 2008 All media copyright of their respective owners31 Hybrids Hybrids occur when we mix two or more different types of the same thing. The resultant hybrid has similarities to the original things, but is distinctly different from both.

32 © Peter Warburton 2008 All media copyright of their respective owners32 Labradoodle A Labradoodle is a hybrid dog breed that mixes Labrador retrievers with poodles. +

33 © Peter Warburton 2008 All media copyright of their respective owners33 Hybrid orbitals The atomic orbitals of atoms can be mixed together (WHEN REQUIRED!) to form hybrid atomic orbitals that are different from the source orbitals. Such hybrid orbitals are used to better explain molecular geometry and bonding.

34 © Peter Warburton 2008 All media copyright of their respective owners34 Oxygen atom orbital diagram We would expect water to have a 90  angle based on the atomic orbitals on oxygen.

35 © Peter Warburton 2008 All media copyright of their respective owners35 Oxygen hybrid orbitals plus gives

36 © Peter Warburton 2008 All media copyright of their respective owners36 Oxygen hybrid orbitals plus gives One s and three p orbitals combine to give four sp 3 hybrid orbitals

37 © Peter Warburton 2008 All media copyright of their respective owners37 In general A total of n atomic orbitals combine to give n hybrid orbitals of a given kind.

38 © Peter Warburton 2008 All media copyright of their respective owners38 Hybrid orbitals

39 © Peter Warburton 2008 All media copyright of their respective owners39 Oxygen hybrid orbital diagram We would expect water to have a ~109.5  angle based on the hybrid sp 3 orbitals on oxygen.

40 © Peter Warburton 2008 All media copyright of their respective owners40 Determining hybrid orbitals diagrams 1. Draw the Lewis dot structure 2.Use VSEPR theory to predict electron group arrangement 3.Use Table 10.2 to determine what hybrid orbitals have the same arrangement 4.Create the hybrid orbital diagram based on changing the ground state diagram

41 © Peter Warburton 2008 All media copyright of their respective owners41 Problem Describe the bonding of I 3 - in terms of valence bond theory.

42 © Peter Warburton 2008 All media copyright of their respective owners42 Multiple bonding Multiple bonds (double or triple bonds) are possible when more than one set of orbitals can overlap between two atoms. The first bond is the sigma (  ) bond, which occurs from orbital overlap on the axis between the atoms. The second and third bonds are pi (  ) bonds that occur from orbital overlap both above and below the axis between the two atoms.

43 © Peter Warburton 2008 All media copyright of their respective owners43 Ethene has a double bond Notice we’ve chosen to create sp 2 hybrid orbitals No orbital overlap between these p orbitals There is orbital overlap between these p orbitals

44 © Peter Warburton 2008 All media copyright of their respective owners44 Ethyne has a triple bond Notice we’ve chosen to create sp hybrid orbitals and not sp 3 or sp 2 There is orbital overlap between two different sets of p orbitals


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