S117Lecture82011

S117Lecture82011 - Understanding Chemical Bonding Lewis dot...

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Understanding Chemical Bonding • Lewis dot structures – electron counting scheme – Based on octet rule and some exceptions • We will now study shapes of molecules: – VSEPR – predicting molecular shapes from Lewis dot structures – Valence Bond Theory • Provides a localized picture of chemical bonding (as opposed to MO theory) • bonding through orbital overlap • Hybridization as a way of explaining molecular shapes • Molecular orbital (MO) theory – Molecular orbital are made by combining atomic orbitals – Molecular orbitals are solutions to the molecular Schrodinger equation. – Covalent bonding through delocalized molecular orbitals – Provides an explanation for paramagnetic O 2 – Natural explanation of resonance structures
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CH 3 -CH 2 -OH and CH 3 -O-CH 3 are examples of molecules which have the same formula but different structures and different properties (ISOMERS). Here the atoms are connected in different ways. For larger molecules it is possible to have exactly the same connectivity between the atoms but dramatically different structures. We already saw this when talking about alkanes: the C-C single bond is allowed to rotate freely, which yields the situation Here is a much more complicated example We will study the reasons behind molecular shapes
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Balloons can help us understand molecular shapes!! The following pictures show the arrangement when two, three… balloons are tied together And balloons have such arrangements because they are spatially constrained by the other balloons It turns out that electron clouds act much the same way as balloons. Electron clouds (orbitals) repel each other (negative charge-negative charge repulsion, or electrostatic repulsion) leading to the same result as balloons. This concept is called Valence Shell Electron Pair Repulsion (VSEPR)
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VSEPR theory • Simple idea: Electron pairs repel each other so move them as far apart as possible. • WHY? Electrostatic repulsion!! O C O electrons in bonds O O C more repulsion This is why CO 2 is linear H H H H C 90 H C H H H 109.5 A tetrahedron maximizes the distance between the bonds.
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Linear Molecules But what about H 2 O? H 2 O is bent! Why?
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S117Lecture82011 - Understanding Chemical Bonding Lewis dot...

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