Chapter09-MRC-NEW

Chapter09-MRC-NEW - Chapter 9 Molecular Geometries and...

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Chapter 9 Molecular Geometries and Bonding Theories
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VSEPR “vesper” THEORY V alence S hell E lectron P air R epulsion T heory
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Molecular Shapes The shape of a molecule plays an important role in its reactivity. By noting the number of bonding and non- bonding electron pairs we can easily predict the shape of the molecule.
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What Determines the Shape of a Molecule? Electron pairs, whether they be bonding or non-bonding, 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.
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Electron Domains (Regions of Electron Density) We can predict the shape of a molecule by looking at the electron pairs (bonds and lone pairs), or electron domains around the central atom. Double and triple bonds count as one electron domain since they define a single “direction”. This molecule has four “ electron domains ” around A. Bonding Pairs (BP) Lone Pair (LP)
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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.” VSEPR is based purely on electrostatic interactions. Order of repulsions: BP-BP < BP-LP < LP-LP
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How Many Lone Pairs Are There? (1) How many valence electrons ( VE ) does the central atom have when neutral? (Which group of the periodic table is it in?) ( 2) What is oxidation state ( OS ) of central atom? (3) VE - OS = UE = number of unshared electrons on the central atom (4) ½ UE = LP Number of “lone pairs” or “Non- Bonding Domains” since 2 e - are required for each “lone pair” Overall: LP = ½ (VE – OS)
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Electron- Domain Geometries These are the electron-domain geometries for two- through six- electron domains around a central atom.
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Electron-Domain Geometries All one must do is count the number of electron domains in the Lewis structure. The geometry of the electron domains will be that which corresponds to that number of electron domains.
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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 the atoms in the molecules (not including the non-bonding pairs).
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Molecular Geometries Two molecules can have the same number of electron domains, but a different molecular geometry. See iso-electronic molecules on right!
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Linear Electron Domains In molecules where there are 2 electron domains, there is only one molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter how many electron domains there are.
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Trigonal-Planar Electron Domains If a molecule has three electron domains, there are two possible molecular geometries: Trigonal planar, if all the electron domains are bonding Bent, if one of the domains is a nonbonding pair.
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Angles Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are
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Chapter09-MRC-NEW - Chapter 9 Molecular Geometries and...

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