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 THEORYVSEPR THEORY

What does VSEPR stand What does VSEPR stand for?for?

VValence alence SShell hell EElectron lectron PPair air RRepulsionepulsion

Why is this important to Why is this important to know?know?

It explains how molecules It explains how molecules and ions behave.and ions behave.

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

Another example: Another example:

It explains why It explains why part of a part of a soap molecule attracts soap molecule attracts waterwater while while the other part the other part attracts grease and oils.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 90o, much weaker at 120o, and very weak at 180o.

B) What are the ideal arrangements of electron pairs to minimize repulsions?

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

VSEPR THEORY: VSEPR THEORY: Basic procedureBasic procedure

1) 1) Determine the central Determine the central atom atom (usually the atom (usually the atom with the lowest subscript with the lowest subscript and/or the atom capable of and/or the atom capable of forming the most bonds).forming the most bonds).

2) 2) Draw the electron dot Draw the electron dot structure and bar diagramstructure and bar diagram

3) 3) Determine the molecular Determine the molecular geometry using ALL geometry using ALL electron pairs AND atoms electron pairs AND atoms around the central atom.around the central atom.

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

VSEPR THEORY: VSEPR THEORY: Example: BeHExample: BeH22

1) Central Atom?1) Central Atom?

Be Be (only 1 atom)(only 1 atom)

2) Electron Dot?2) Electron Dot?

2) Bar Diagram?2) Bar Diagram?

H Be H

H—Be—H

Note that Be violates the octet rule—this is an exception!

3) Geometry? Hint: What is 3) Geometry? Hint: What is the furthest apart you can the furthest apart you can spread two atoms attached spread two atoms attached to a central atom?to a central atom?

4) Shape? Ignore any 4) Shape? Ignore any unbonded pairs of electrons unbonded pairs of electrons —not necessary in this case.—not necessary in this case.

LINEARLINEAR

VSEPR THEORY: VSEPR THEORY: Example: BFExample: BF33

B B (only 1 atom)(only 1 atom)

2) Electron Dot?2) Electron Dot?

2) Bar Diagram?2) Bar Diagram?F—B—F

Note that B violates the octet rule—this is an exception!

F B F F

3) Geometry? Hint: What is 3) Geometry? Hint: What is the furthest apart you can the furthest apart you can spread three atoms attached spread three atoms attached to a central atom?to a central atom?

VSEPR THEORY: VSEPR THEORY: Example: BFExample: BF334) Shape? Ignore any 4) Shape? Ignore any

unbonded pairs of electrons unbonded pairs of electrons —not necessary in this case.—not necessary in this case.

trigonal planartrigonal planar

VSEPR THEORY: VSEPR THEORY: Example: CHExample: CH44

C C (only 1 atom)(only 1 atom)

VSEPR THEORY: VSEPR THEORY: Example: CHExample: CH442) Electron Dot?2) Electron Dot?

H—C—H

VSEPR THEORY: VSEPR THEORY: Example: CHExample: CH44

3) Geometry? Hint: What is the 3) Geometry? Hint: What is the furthest apart you can spread four furthest apart you can spread four atoms attached to a central atom? atoms attached to a central atom? Think in 3D!Think in 3D!

VSEPR THEORY: VSEPR THEORY: Example: CHExample: CH44 4) Shape? Ignore any unbonded 4) Shape? Ignore any unbonded

pairs of electrons —not necessary pairs of electrons —not necessary in this case.in this case.

tetrahedraltetrahedral

VSEPR THEORY: VSEPR THEORY: Example: NHExample: NH33

N N (only 1 atom)(only 1 atom)

VSEPR THEORY: VSEPR THEORY: Example: NHExample: NH332) Electron Dot?2) Electron Dot?

H N H H

H—N—H H

VSEPR THEORY: VSEPR THEORY: Example: NHExample: NH33

3) Geometry? Hint: What is the 3) Geometry? Hint: What is the furthest apart you can spread three furthest apart you can spread three atoms plus one unbonded pair of atoms plus one unbonded pair of electrons attached to a central atom? electrons attached to a central atom? Think in 3D!Think in 3D!

~109.5o

VSEPR THEORY: VSEPR THEORY: Example: NHExample: NH33 4) Shape? Ignore any unbonded 4) Shape? Ignore any unbonded

pairs of electrons —it IS pairs of electrons —it IS necessary in this case.necessary in this case.

trigonaltrigonal

pyramidalpyramidalH

~109.5o

VSEPR THEORY: VSEPR THEORY: Example: HExample: H22OO

O O (only 1 atom)(only 1 atom)

VSEPR THEORY: VSEPR THEORY: Example: HExample: H22OO2) Electron Dot?2) Electron Dot?

O—H H

VSEPR THEORY: VSEPR THEORY: Example: HExample: H22OO

3) Geometry? Hint: What is the 3) Geometry? Hint: What is the furthest apart you can spread two furthest apart you can spread two atoms plus two unbonded pairs of atoms plus two unbonded pairs of electrons attached to a central atom? electrons attached to a central atom? Think in 3D!Think in 3D!

~109.5o

VSEPR THEORY: VSEPR THEORY: Example: HExample: H22OO 4) Shape? Ignore any unbonded 4) Shape? Ignore any unbonded

pairs of electrons —it IS pairs of electrons —it IS necessary in this case.necessary in this case.

bentbent

~109.5o

In conclusion:In conclusion:

Since water (also called the Since water (also called the universal solvent) is bent it universal solvent) is bent it is able to dissolve ionic is able to dissolve ionic substances:substances:

O side tends to be – (the electron pairs hybridize into one group)

H sides tend to be +

This negative side tends to attract positive ions

These positive ends tend to attract negative ions

VSEPR THEORY:VSEPR THEORY:

How do we determine How do we determine the shapes of the shapes of

molecules and ions?molecules and ions?

INTRODUCTION

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