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Molecular Geometry Lewis structures show the number and type of bonds between atoms in a molecule. –All atoms are drawn in the same plane (the paper). –Do not show the shape of the molecule. 1

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Molecular Shapes The shape of a molecule plays an important role in its reactivity. The shape of a molecule is determined by the bond angles and the bond lengths. By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule. 2

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Molecular Geometry Bond length: the distance between two atoms held together by a chemical bond –Bond length decreases as the number of bonds between two atoms increases. Single bond is the longest. Triple bond is the shortest. 3

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Molecular Geometry Bond angle: the angle made by the “lines” joining the nuclei of the atoms in a molecule H O H o 4

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Molecular Geometry Many of the molecules we have discussed have central atoms surrounded by 2 or more identical atoms: AB n where A = central atom B = outer atoms n = # of “B” atoms Examples: CO 2, H 2 O, BF 3, NH 3, CCl 4 5

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Molecular Geometry The shapes that AB n molecules can have depend, in part, on the value of n. For a specific value of n, only a few general shapes are observed. AB 2 molecules –linear –bent 6

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Molecular Geometry O C O H O H AB 2 molecules can either be linear or bent. CO 2 H2OH2O linear bent 7

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Molecular Geometry F B F F AB 3 molecules can either be trigonal planar, trigonal pyramidal, or T-shaped. Trigonal planar: “A” atom in the center and “B” atoms at each corner of an equilateral triangle. All atoms in the same plane. 8

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Molecular Geometry Trigonal pyramidal: “A” atom in the center with “B” atoms in the corners of an equilateral triangle. – “A” is above the plane of the triangle formed by “B” atoms N H H H 9

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Molecular Geometry Why are some AB 2 molecules linear while others are bent? Why are some AB 3 molecules trigonal planar while others are trigonal pyramidal or T-shaped? How can we accurately predict the shape of various AB n molecules? 10

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Molecular Geometry If “A” is a main group element, the valence-shell electron-pair repulsion model (VSEPR) can be used to predict the shape of an AB n molecule. VSEPR counts the number of regions around the central atom where electrons are likely to be found and uses this number to predict the shape. 11

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Molecular Geometry Electron domains: regions around the central atom where electrons are likely to be found. Two types of electron domains are considered: –bonding pairs of electrons –nonbonding (lone) pairs of electrons 12

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Molecular Geometry Bonding pairs of electrons: electrons that are shared between two atoms Cl ClCCl Cl Bonding pairs CCl 4 has 4 bonding pairs, C has 4 electron domains 13

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Nonbonding (lone) pairs of electrons: electrons that are found principally on one atom, not in between atoms = unshared electrons HNH H Molecular Geometry Nonbonding pair 14

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N in Ammonia (NH 3 ) has 4 electron domains: HNH H Molecular Geometry 3 bonding pairs 1 nonbonding pair 15

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Valence Shell Electron Pair Repulsion Theory (VSEPR) “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.” 16

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Molecular Geometry By considering the arrangement that minimizes repulsions between electron domains, we can determine the electron domain geometry –The arrangement of electron domains around the central atom 17

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Molecular Geometry 3 electron domains 2 electron domains Trigonal planar e - domain geometry Linear electron domain geometry 18

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Molecular Geometry 4 electron domains 5 electron domains Tetrahedral electron domain geometry Trigonal bipyramidal e - domain geometry 19

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Molecular Geometry 6 electron domains Octahedral electron domain geometry 20

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Electron-Domain Geometries All one must do is –draw the Lewis structure –count the total number of electron domains around the central atom double and triple bonds count as 1 electron domain The geometry will be that which corresponds to the number of electron domains. 21

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Molecular Geometry Determine the electron domain geometry of CO 2. Valence electrons: 16 OCOOCO Lewis structure: # of electron domains of C: 2 Electron domain geometry: linear 22

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Molecular Geometry Determine the electron domain geometry of PCl 3. Valence electrons: 26 Lewis structure: # of electron domains of P: 4 Electron domain geometry: tetrahedral Cl Cl PCl 23

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Molecular Geometries The electron-domain geometry is often not the shape of the molecule, however. The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs. Molecular geometry is a consequence of electron- domain geometry. 24

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Molecular Geometry H 2 O has 4 electron domains-- –electron-domain geometry = tetrahedral H H If you ignore the lone pairs of electrons, however, the atoms are arranged in a bent shape. O 25

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Molecular Geometry The molecular geometry is a consequence of electron domain geometry because the lone pairs of electrons take up space around the central atom. –This forces the atoms in the molecule to occupy positions around the central atom in a way that minimizes repulsion between the electron domains. 26

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Molecular Geometry Electron domain geometry and molecular geometry are the same only if there are no non- bonding electron domains. See tables 9.2 and 9.3 for the relationship between electron domain geometries and molecular geometries. 27

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Linear Electron Domain In the linear domain, there is only one molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is. 28

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Trigonal Planar Electron Domain There are two molecular geometries: –Trigonal planar, if all the electron domains are bonding, –Bent, if one of the domains is a nonbonding pair. 29

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Tetrahedral Electron Domain There are three molecular geometries: –Tetrahedral, if all are bonding pairs, –Trigonal pyramidal if one is a nonbonding pair, –Bent if there are two nonbonding pairs. 30

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Trigonal Bipyramidal Electron Domain There are four distinct molecular geometries in this domain: –Trigonal bipyramidal –Seesaw –T-shaped –Linear 31

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Octahedral Electron Domain All positions are equivalent in the octahedral domain. There are three molecular geometries: –Octahedral –Square pyramidal –Square planar 32

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Molecular Geometry Trigonal planar Tetrahedral 33

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Molecular Geometry Trigonal bipyramidal octahedral 34

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Molecular Geometry In order to determine the actual molecular geometry: –draw the Lewis structure –count the total # of electron domains multiple bonds = 1 electron domain –determine the electron-domain geometry –describe the molecular geometry in terms of the arrangement of the bonded atoms 35

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Molecular Geometry What is the molecular geometry of NH 3 ? Lewis Structure: # of electron domains = 4 36

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Molecular Geometry Electron domain geometry: tetrahedral Molecular geometry: trigonal pyramidal 37

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Molecular Geometry Example: Predict the molecular geometry of IF 5. Lewis structure: # electron domains: 38

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Molecular Geometry Electron domain geometry = octahedral Molecular geometry = square pyramidal 39

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Larger Molecules In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole. 40

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Polarity of Molecules Consider the carbon dioxide molecule: –contains two polar covalent bonds –nonpolar molecule Just because a molecule contains polar covalent bonds does not mean the molecule as a whole will be polar. 41

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Polarity of Molecules Polar Molecules –contain polar covalent bonds which are asymmetrically distributed within the molecule contain a “positive” end and a “negative”end –Examples: HCl H 2 O CH 3 OH -- ++ -- ++ ++ -- ++ ++ 42

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Polarity of Molecules Polar molecules have large dipole moments –A measure of the separation between the positive and negative charges in polar molecules. ++ -- H – F ++ -- 43

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Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule. The overall polarity of a molecule is determined by doing a vector addition of the individual bond dipoles: –add both the magnitude and direction of the dipole moments must consider the molecular geometry! 44

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Polarity of Molecules Examples: 45

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