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Chemical Bonding: Molecular Shapes

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VSEPR Theory From a correct Lewis structure, we can get to the 3-D shape using this theory. VSEPR stands for valence shell electron pair repulsion. The theory is based on the idea that e- pairs want to get as far away from each other as possible!

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VSEPR Categories There are 5 categories from which all molecular shapes derive.

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Drawing w/ Perspective: Dash Diagrams We use the conventions below to depict a 3-D object on a 2-D surface.

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Determining 3-D Shape The 5 categories are a starting point. To determine the 3-D shape of a molecule, we consider the # of atoms and the # of e- pairs that are associated w/ the central atom. All the possibilities for molecular geometry can be listed in a classification chart.

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Steps to Determine Molecular Shape 1)Draw Lewis structure. 2)Count # of bonds and lone pair e-’s on the central atom. 3)Select geometric category. 4)Place e-’s and atoms that lead to most stable arrangement (minimize e- repulsions). 5)Determine 3-D shape.

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Linear/Trigonal Planar Geometries First, we have the linear and trigonal planar categories. CategoryBondsLone PairsGeometry Linear20linear Trigonal planar 30trigonal planar 21bent

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Like Charges Repel These clouds of electrons, areas of negative charge repel each other in such a way as to get as far apart from each other as possible. When there are only two electron clouds, the farthest they can get away from each other is 180 o, a straight line.

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Three Clouds With three electron clouds, the farthest the electron clouds can get away from each other is 120 o, the corners of an equilateral triangle. This shape is known as trigonal planer.

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Tetrahedral Geometries CategoryBondsLone PairsGeometry Tetrahedral40tetrahedral 31pyramidal 22bent 13linear

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Four clouds When four clouds are attached to a central atom, the farthest they can get away from each other is the four corners of a tetrahedron. The tetrahedral angle is 109.5 o.

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Non-bonding clouds also repel both bonding and non-bonding clouds. NH 3, ammonia, has a non-bonding pair of electrons. The central atom, N, is surrounded by 3 bonding and 1 non- bonding pair. These four clouds form roughly a tetrahedron, but because non-bonding pairs “spread out” more than bonding pairs, the angle between atoms is 107 o instead of 109.5 o. The atoms form a pyramidal shape.

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H 2 O, water, has 2 bonding and 2 non- bonding pairs of electrons. These four clouds form roughly a tetrahedron, but because non-bonding pairs “spread out” more than bonding pairs, the angle between atoms is 105 o instead of 109.5 o. The atoms form a bent shape.

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Distortion of Angles Lone pair e-’s take up a lot of room, and they distort the angles seen in the geometric categories.

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Trigonal Bipyramidal Geometries CategoryBondsLone PairsGeometry Trigonal Bipyramidal 50trigonal bipyramidal 41see-saw 32T-shaped 23linear 14

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Trig Bipy is Special In other categories, all positions are equivalent. In trig bipy, lone pairs always choose to go equatorial first. Why?

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Octahedral Geometries CategoryBondsLone PairsGeometry Octahedral60octahedral 51square pyramidal 42square planar 33T-shaped 24linear 15

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