VSEPR

VSEPRValence Shell Electron Pair Repulsion Theory

Vocabulary:

““lone pairlone pair”” = = ““non-bonding pairnon-bonding pair”” = = ““unshared pairunshared pair””= any electron pair that is not involved in bonding= any electron pair that is not involved in bonding

““bonding pairbonding pair”” = = ““shared pairshared pair””= any electron = any electron pairpair that is involved in bonding that is involved in bonding

“domain” = any electron pair, bond, or any double or triple bond is considered one domain.

Example: BeCl2

1) Central Atom? Be (only 1 atom)

2 things on central atom

VSEPR THEORY: Example: BeCl2

2) Electron Dot?

2) Bar Diagram?

Cl Be Cl

Cl—Be—Cl

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


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

Cl ClBe


4) Shape? • LINEAR – 180o

Cl ClBe


LINEAR

• 2 bonds• both are bonding pairs

• They push each other to opposite sides of center (180 apart).

BeCl2


• Does not matter if double or triple bonds!

• Still 2 things on central atom• Examples:

~ CO2

~ HCN

Example: BF3

1) Central Atom? B (only 1 atom)


VSEPR THEORY: Example: BF3

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: BF3

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

BF

F

F

VSEPR THEORY: Example: BF34) Shape? • trigonal planar

BF

F

F


TRIGONAL PLANAR

• 3 bonds• all are bonding pairs

• They push each other apart equally at 120 degrees.

BF3

•Again double, triple bonds do not matter and do not change shape• Still three bonds to central

atom• Example

~ CH2O



BENT

• 3 things:• 2 are bonding pairs• 1 is a lone pair

• The 2 bonding pairs are pushed apart by 3rd pair (not seen)• Lone pair pushes little harder

and bonds are < 120o (~116o)

SnF2

NOTE:

• The geometry around the central atom is trigonal planar.

• The molecular shape is bent.

SnF2

Example: CH4

1) Central Atom? C (only 1 atom)


VSEPR THEORY: Example: CH42) Electron Dot?

2) Bar Diagram?

H C H

H

H

H—C—H

H

H

VSEPR THEORY: Example: CH4

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

CHH

H

H

VSEPR THEORY: Example: CH4

4) Shape?

tetrahedral

CHH

H

H


TETRAHEDRAL

• 4 bonds

• Each repels the other equally - 109.5 - not the expected 90.• Think in 3D.

CH4

Example: NH3

1) Central Atom? N (only 1 atom)


VSEPR THEORY: Example: NH32) Electron Dot?

2) Bar Diagram?

H N H H

H—N—H H

VSEPR THEORY: Example: NH3

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!

HN

HH

~109.5o

VSEPR THEORY: Example: NH34) Shape?

trigonalpyramidal

HN

HH

< 109.5 (~107o)


TRIGONAL PYRAMIDAL

• 4 domains• 3 bonding pairs• 1 lone pair

• The thicker, lone pair forces the others a little bit closer together (~107)

NH3

4 domains on central atom

BENT

• 4 domains• 2 bonding pairs• 2 lone pairs

• The bonds are forced together still closer (104.5) by the 2 thick unshared pairs.

H2O

Comparing the 2 “bents”…

Both bent molecules are affected by unshared pairs – 1 pair on the left, 2 on the

right.

Other Molecular Geometry

Just for fun, lets look at some others that we will not study in detail in this course…

Note that if there are more than five domains around the Note that if there are more than five domains around the central atom, it must be an exception to the octet rule!central atom, it must be an exception to the octet rule!

5 e- pairs on central atom

TRIGONAL BIPYRAMIDAL

• 5 shared pairs

• Three pairs are found in one plane (“equator”) 120 apart; the other two pairs are at the “poles,” 180 apart, 90 from the “equator.”

PCl5


SEE-SAW

• 4 shared pairs & 1 unshared pair

• One of the equator pairs is unshared & pushes the other 2 together.• The 2 poles are pushed

slightly together.

SF4


T-SHAPED

• 3 shared & 2 unshared pairs

• 2 of the 3 equator pairs are unshared.• All 3 remaining pairs are

pushed together.

ClF3


LINEAR


• All 3 equator pairs are unshared. The 2 remaining pairs are forced to the poles.

XeF2

5 e5 e-- pairs on central atom pairs on central atom

5 shared, 0 unshared5 shared, 0 unshared 4 shared, 1 unshared4 shared, 1 unshared



OCTAHEDRAL

• 6 shared pairs

• Each pair repels the others equally.• All angles = 90

Now, if one of these Now, if one of these pairs was unshared pairs was unshared …… SF6

6 e- pairs on central atomSQUARE PYRAMIDAL

• 5 shared pairs & 1 unshared pair

• 4 shared pairs in one plane; the 5th pair at the pyramid’s top.

If the pair at the top If the pair at the top was unshared …was unshared … IF5


SQUARE PLANAR


• The 4 shared pairs are in the same plane; the 2 unshared pairs are 90 from them.

XeF4

6 e6 e-- pairs on central atom pairs on central atom


4 shared, 2 unshared4 shared, 2 unshared

Steps for using VSEPR:

1. Draw a Lewis Dot Structure.2. Predict the geometry around the central atom.3. Predict the molecular shape.

… … also, we can try and predict the angles between atoms.also, we can try and predict the angles between atoms.

All e- pairs push each other as far apart as possible.

• Shared (bonding) pairs are “stretched” between two atoms that want them.• “Longer & Thinner”

• Unshared (non-bonding) pairs are not “stretched.”• “Shorter & Thicker”

Electron Pair Repulsion• 2 lone pairs require the most space & repel each other the

most, resulting in the greatest distance (angle).

• 1 lone pair (thick) & 1 bonding pair (thin) require less space

• 2 bonding pairs (both thin) require the least space & repel each other the smallest distance (angle).

Let try some…1. How many things around central atom?2. What structure is the molecule based on?3. Are any atoms replaced by lone electron pairs?4. What is the final structure? Bond angles?

• PF3

• NO2-

• NO3-

• OF2

• CO2

• NH4+

Documents

VSEPR