INTRODUCTION A)Lewis structures do not indicate the three dimensional shape of a molecule do not show the arrangement space of the atoms, what we call the molecular geometry or molecular structure.

Molecules have definite shapes – the shape of a molecule controls some of its chemical and physical properties

VSEPR THEORY  What does VSEPR stand for?  Valence Shell Electron Pair Repulsion

VSEPR THEORY  Why is this important to know?  It explains how molecules and ions behave.

VSEPR THEORY  For example: It explains why water molecules are so good at dissolving ionic substances even though water does not have an ionic bond. It explains why water molecules are so good at dissolving ionic substances even though water does not have an ionic bond.

VSEPR THEORY  Another example: It explains why part of a soap molecule attracts water while the other part attracts grease and oils. It explains why part of a soap molecule attracts water while the other part attracts grease and oils.

. A) Assumptions of VSEPR Theory 1)Electron pairs in the valence shell of an atom tend to orient themselves so that the total energy is minimized. This means that: the electrons will approach the nucleus as close as possible yet take positions as far away from each other as possible to minimize _______________.

2) Because lone pairs of electrons are spread out more broadly than bond pairs: –repulsions are greatest between two lone pairs –intermediate between a lone pair and a bond pair –weakest between two bonding pairs of electrons.

3) Repulsive forces decrease rapidly with increasing interpair angle - greatest at 90 o, much weaker at 120 o, and very weak at 180 o. B) What are the ideal arrangements of electron pairs to minimize repulsions?

VSEPR THEORY  Another example: Use it to clean up greasy hands from working on your car or sprucing up your nails between manicures!

VSEPR THEORY: Basic procedure  1) Determine the central atom (usually the atom with the lowest subscript and/or the atom capable of forming the most bonds).

VSEPR THEORY: Basic procedure  2) Draw the electron dot structure and bar diagram

VSEPR THEORY: Basic procedure  3) Determine the molecular geometry using ALL electron pairs AND atoms around the central atom.

VSEPR THEORY: Basic procedure  4) Modify the geometry to determine the molecular shape if non-bonding electron pairs exist by ignoring them, BUT LEAVE THE ATOMS OF BONDED PAIRS WHERE THEY ARE. This is done because even if the electrons have no atom attached, these unbonded electron pairs still affect the shape of the structure.

VSEPR THEORY: Example: BeH 2  1) Central Atom?  Be (only 1 atom)  Be (only 1 atom)

VSEPR THEORY: Example: BeH 2  2) Electron Dot?  2) Bar Diagram? H Be H H—Be—H Note that Be violates the octet rule—this is an exception!

VSEPR THEORY: Example: BeH 2  3) Geometry? Hint: What is the furthest apart you can spread two atoms attached to a central atom? H H Be

VSEPR THEORY: Example: BeH 2  4) Shape? Ignore any unbonded pairs of electrons —not necessary in this case.  LINEAR  LINEAR H H Be

VSEPR THEORY: Example: BF 3  1) Central Atom?  B (only 1 atom)  B (only 1 atom)

VSEPR THEORY: Example: BF 3  2) Electron Dot?  2) Bar Diagram? F—B—F F Note that B violates the octet rule— this is an exception! F B F F

VSEPR THEORY: Example: BF 3  3) Geometry? Hint: What is the furthest apart you can spread three atoms attached to a central atom? B F F F

VSEPR THEORY: Example: BF 3  4) Shape? Ignore any unbonded pairs of electrons —not necessary in this case.  trigonal planar  trigonal planar B F F F

VSEPR THEORY: Example: CH 4  1) Central Atom?  C (only 1 atom)  C (only 1 atom)

VSEPR THEORY: Example: CH 4  2) Electron Dot?  2) Bar Diagram? H C H H H H—C—H H H

VSEPR THEORY: Example: CH 4  3) Geometry? Hint: What is the furthest apart you can spread four atoms attached to a central atom? Think in 3D! C H H H H

VSEPR THEORY: Example: CH 4  4) Shape? Ignore any unbonded pairs of electrons —not necessary in this case.  tetrahedral  tetrahedral C H H H H

VSEPR THEORY: Example: NH 3  1) Central Atom?  N (only 1 atom)  N (only 1 atom)

VSEPR THEORY: Example: NH 3  2) Electron Dot?  2) Bar Diagram? H N H H H—N—H H

VSEPR THEORY: Example: NH 3  3) Geometry? Hint: What is the furthest apart you can spread three atoms plus one unbonded pair of electrons attached to a central atom? Think in 3D! H N H H ~109.5 o

VSEPR THEORY: Example: NH 3  4) Shape? Ignore any unbonded pairs of electrons —it IS necessary in this case.  trigonal  trigonalpyramidal H N H H ~109.5 o

VSEPR THEORY: Example: H 2 O  1) Central Atom?  O (only 1 atom)  O (only 1 atom)

VSEPR THEORY: Example: H 2 O  2) Electron Dot?  2) Bar Diagram? O H H O—H H

VSEPR THEORY: Example: H 2 O  3) Geometry? Hint: What is the furthest apart you can spread two atoms plus two unbonded pairs of electrons attached to a central atom? Think in 3D! H O H ~109.5 o

VSEPR THEORY: Example: H 2 O  4) Shape? Ignore any unbonded pairs of electrons —it IS necessary in this case.  bent  bent H O H ~109.5 o

VSEPR THEORY  In conclusion: Since water (also called the universal solvent) is bent it is able to dissolve ionic substances:

O side tends to be – (the electron pairs hybridize into one group) H sides tend to be + H O H

H O H This negative side tends to attract positive ions These positive ends tend to attract negative ions

VSEPR THEORY: How do we determine the shapes of molecules and ions?