The Shapes of Molecules Or in Big Scientific Words “MOLECULAR GEOMETRY”

Simulate Electron Pair Arrangements Use Balloons to represent an Electron Pair Use Balloons to represent an Electron Pair What is a pair of electrons going to do to another pair of electrons? What is a pair of electrons going to do to another pair of electrons? THEY REPEL EACH OTHER So if we want to mimic what happens in an atom, we need to get the balloons as far apart as possible. So if we want to mimic what happens in an atom, we need to get the balloons as far apart as possible.

First Some Terminology Bonding Pair of Electrons Nonbonding Pair of Electrons Also called “Lone Pair”, “Unshared Pair” Also called “Shared Pair”

2 Electron Pairs on Central Atom How can we get these electron pairs (balloons) as far apart as possible? How can we get these electron pairs (balloons) as far apart as possible? What does the shape look like? What does the shape look like?LINEAR 180º Apart 180º Apart This is a Lewis Structure and a 3D drawing Shared Pairs Unshared Pairs 2020 Beryllium fluoride

3 Electron Pairs on Central Atom Get three balloons as far apart from each other. Get three balloons as far apart from each other. How would you describe the shape? How would you describe the shape? TRIGONAL PLANAR 120º Apart 120º Apart This is a 3D drawing of BH 3 Bonding Pairs Nonbonding Pairs 3030 Boron trihydride

What would it look like if? We had a Double Bond or a Triple Bond? We had a Double Bond or a Triple Bond? Does a double count as two pairs regardless? Does a double count as two pairs regardless? Since the two pairs are in the same place do they count as only one? Since the two pairs are in the same place do they count as only one? Because they are forced into the same place they only count once. Because they are forced into the same place they only count once. What else new do you notice? Sulfur dioxide

3 Pairs Around Central Atom The lone pair is still there but we can’t see it. The lone pair is still there but we can’t see it. The molecule actually looks like The molecule actually looks like Shared Pairs Adjusted Lone Pair 2121 BENT Roughly 120º Apart 3D Shape Lewis Structure

4 Pairs Around Central Atom Think 3D not 2D… Think 3D not 2D… How might you describe shape? How might you describe shape? Shared Pairs Lone Pairs 4040 TETRAHEDRAL 109.5º Apart Lewis Structure How do you draw it? Silicon tetrafluoride

How to Draw a Tetrahedral Shape Lewis Structure Bold Line Means its Coming At You. Dashed Line Means its Going Away From You 3D Shape Regular Lines Mean its in the Plane of the Paper

4 Electron Pairs Around Center What does it look like? What does it look like? TRIGONAL PYRAMIDAL Roughly 109.5º Apart How do you draw it? Bonding Pairs Nonbonding Pair 3131 Lewis Structure Ammonia

How to Draw Trigonal Pyramidal

4 Electron Pairs around Center What does this one look like? What does this one look like?BENT Roughly 109.5º Apart Roughly 109.5º Apart How do you draw it? Shared Pairs Unshared Pairs 2222 Lewis Structure 3D Shape Water

5 Electron Pairs! Expanded Octets Expanded Octets Phosphorus pentafluoride Phosphorus pentafluoride What does it look like? What does it look like? Shared Pairs Unshared Pairs 5050 Lewis Structure TRIGONAL BIPYRAMIDAL Some are 120º Apart. Others are 90º

Try to Draw It

Six Pairs How do the balloons arrange themselves? How do the balloons arrange themselves?OCTAHEDRAL Why “octa”? Shared Pairs Lone Pairs 6060 Lewis Structure Octahedron All 90º apart How do you draw it? Sulfur hexafluoride

VSEPR Theory Valence Shell Electron Pair Repulsion Valence Shell Electron Pair Repulsion Electron Pairs try to get as far away from each other as possible. Electron Pairs try to get as far away from each other as possible.