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Chapter 9 Molecular Geometries and Bonding Theories

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## Chapter 9 Molecular Geometries and Bonding Theories

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**Chemistry, The Central Science, 10th edition**Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten Chapter 9Molecular Geometriesand Bonding Theories John D. Bookstaver St. Charles Community College St. Peters, MO 2006, Prentice-Hall, Inc.**November 23**ONLINE PRACTICE QUESTIONS UNIT 8 – BY NEXT SUNDAY!!! WATCH PODCASTS 8-3 is this lesson 8-1 and 8-2 review of chapter 8 • Molecular Shapes • VSEPR theory • Electron Domain Geometry • Molecular Geometry • Hw for section 9.1 and 9.2 : 1,2,3,11 to 23 odd**What Determines the Shape of a Molecule?**• The Lewis structure is drawn with the atoms all in the same plane. • The overall shape of the molecule will be determined by its bond angles.**What Determines the Shape of a Molecule?**• Electron pairs, whether they be bonding or nonbonding, repel each other. • By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule.**Electron Domains**• We can refer to the electron pairs as electrondomains. • Each pair of electrons count as an electron domain, whether they are in a lone pair, in a single, double or triple bond. • This molecule has four electron domains.**Electron-Domain Geometries**• All one must do is count the number of electron domains in the Lewis structure. • The geometry will be that which corresponds to that number of electron domains.**In order to predict molecular shape, we assume the valence**electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimizes this repulsion. • We call this process ValenceShellElectronPair Repulsion (VSEPR) theory. • There are simple shapes for AB2 to AB6 molecules.**Electron domain geometry: When considering the geometry**about the central atom, we consider all electrons (lone pairs and bonding pairs). • When naming the molecular geometry, we focus only on the positions of the atoms.**VSEPR Model**• To determine the shape of a molecule, we distinguish between lone pairs (or non-bonding pairs, those not in a bond) of electrons and bonding pairs (those found between two atoms). • We define the electron domain geometry (or orbital geometry) by the positions in 3D space of ALL electron pairs (bonding or non-bonding). • The electrons adopt an arrangement in space to minimize e--e- repulsion.**Examples – Draw the Lewis structures, and then determine**the orbital geometry of each. Indicate the number of electron domains first: • H2S • CO2 • PCl3 • CH4 • SO2**Examples – Draw the Lewis structures, and then determine**the orbital geometry of each: • e- domain orbital geometry • or e- domain geom • H2S 4 tetrahedral • CO2 2 linear • PCl3 4 tetrahedral • CH4 4 tetrahedral • SO2 3 trigonal planar**To determine the electron pair ( electron domain) geometry:**• draw the Lewis structure, • count the total number of electron pairs around the central atom, • arrange the electron pairs in one of the above geometries to minimize e--e- repulsion, and count multiple bonds as one bonding pair. • But then we have to account for the shape of the molecule**Molecular Geometries**Within each electron domain, then, there might be more than one molecular geometry.**Linear Electron Domain**• In this 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.**Nonbonding Pairs and Bond Angle**• Nonbonding pairs are physically larger than bonding pairs. • Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.**By experiment, the H-X-H bond angle decreases on moving from**C to N to O: • Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. • Therefore, the bond angle decreases as the number of lone pairs increase.**Similarly, electrons in multiple bonds repel more than**electrons in single bonds.**Multiple Bonds and Bond Angles**• Double and triple bonds place greater electron density on one side of the central atom than do single bonds. • Therefore, they also affect bond angles.**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.**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**Examples – Same examples as before, now determine the the**molecular geometry of each, including shapes and bond angles: • H2S • CO2 • PCl3 • CH4 • SO2**Examples – Determine the molecular geometry of each,**including shapes and bond angles: • Shape Angles • H2S bent <109° • CO2linear 180° • PCl3trigonal pyramid <109° • CH4tetrahedral 109.5° • SO2bent <120°**Molecules with Expanded Valence Shells**• For elements of the 3rd shell and below, some atoms can have expanded octets. • AB5 (trigonal bipyramidal) or AB6 (octahedral) electron pair geometries.**Trigonal Bipyramidal Electron Domain (5 e domains)**• There are two distinct positions in this geometry: • Axial • Equatorial**Molecules with Expanded Valence Shells**• To minimize e--e- repulsion, lone pairs are always placed in equatorial positions.**Trigonal Bipyramidal(e- domain)**• There are four distinct molecular geometries in this domain: • *Trigonal bipyramidal • *Seesaw • *T-shaped • *Linear**Trigonal Bipyramidal Electron Domain**Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.**Octahedral electron domain**• 6 electron pairs • All positions are equivalent in the octahedral domain. • There are three molecular geometries: • *Octahedral • *Square pyramidal • *Square planar**Examples – Determine the Shape of each, indicate the**electron domain, molecular geometry and angles. • PF5 • XeF4 • SF6 • SCl4**Examples – Determine the Shape of each:**• PF5trigonal bipyramid 90°, 120° • XeF4square planar 90° • SF6octahedral 90° • SCl4see-saw <90°,< 120° See moving chart**November 28**• Large molecules • Molecular shape and Polarity • Hybridization • Multiple bonding • HW • 25, 26, 35, 47, 51, 55**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.**Larger Molecules**This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.**Shapes of Larger Molecules**• In acetic acid, CH3COOH, there are three central atoms. • We assign the geometry about each central atom separately.**Examples – Determine the shape and angles about each atom:**• 1 • 2**Trigonal planar linear**• Examples – Determine the shape and angles about each atom: • 1 • 2 Bent trigonal pyramid Trigonal planar**Molecular Shape and Molecular Polarity**• When there is a difference in electronegativity between two atoms, then the bond between them is polar. • It is possible for a molecule to contain polar bonds, but not be polar. • For example, the bond dipoles in CO2 cancel each other because CO2 is linear.**In water, the molecule is not linear and the bond dipoles do**not cancel each other. • Therefore, water is a polar molecule. • The overall polarity of a molecule depends on its molecular geometry.**Polarity**By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.