VSEPR

ValenceShellElectronPairRepulsion

model

Is based on the number of regions of high electron density around the central atomElectron density: The number of electrons in a unit volume.

Can be used to predict the structure of a moleculeDoes fail in some cases; models are oversimplifications

Concepts that must be understood:

Valence electrons: the electrons in the last shell or energy level of an atom. These are responsible for the interactions made with other atoms the formation of chemical bonds).

Lewis structure: A structural formula in which electrons are represented by dots; two dots between atoms represent a single bond, four dots between atoms represent a double bond, and six dots represent a triple bond. › A single bond is represented by one line, a

double bond by two lines, and a triple bond by three lines each line represents two electrons.

Electron pair: those electrons that are in the same orbital. Bonds are also electron pairs. › Both the electron pairs

in chemical bonds and lone pairs are considered to determine the structure of a molecule.

Foundations of VSEPR Model

The structure around a given atom is determined principally by minimizing electron pair repulsions.

You may recall that electrons repel each other because of their negative charge.

Electrons in lone pairs repel electrons involved in bonding.

These repulsions determine the bond angles.

Steps to Predict a VSEPR structure Draw the Lewis Structure

Count how many atoms have bonded to the central atom.› Name the central atom “A” and the surrounding atoms

“B” Count how many lone pairs the central atom has

› Name these “E” Consult the VSEPR chart.

› When you consult the chart, remember that the number of domains is given by adding the number of atoms attached to the central atom and the number of lone pairs.

ShapesLinear

Trigonal Planar

Bent

Shapes

Seesaw

T-shape

d

Trigonal Bipyramidal

Tetrahedral

Trigonal Pyramidal

Shapes

Square Pyramida

l

Octahedral

Square Planar

Th

e V

SEP

R

Ch

art

Total Domains

Electron Geometry

Generic Formula

Bonded Atoms

Lone Pairs

Molecular Shape

Example

1 Linear AB 1 0 Linear H2

2Linear

AB22 0 Linear CO2

ABE 1 1 Linear CN-

3 Trigonal Planar

AB33 0 Trigonal

PlanarBF3

AB2E 2 1 Bent O3

ABE21 2 Linear O2

4Tetrahedr

al

AB44 0 Tetrahedr

alCH4

AB3E 3 1 Trigonal pyramid

NH3

AB2E22 2 Bent H2O

ABE31 3 Linear Cl2

A=

Centr

al A

tom

B=

Bonded

Ato

ms

E=

Lone p

air

s

Th

e V

SEP

R

Ch

art

Total Domains

Electron Geometry

Generic Formula

Bonded Atoms

Lone Pairs

Molecular Shape

Example

5Trigonal

Bipyramid

AB55 0 Trigonal

BipyramidPF5

AB4E 4 1 See Saw SF4

AB3E23 2 T shape ClF3

AB2E32 3 Linear I3

-

6Octahedra

l

AB66 0 Octahedra

lSF6

AB5E 5 1 Square pyramidal

IF5

AB4E24 2 Square

planarXeF4

A=

Centr

al A

tom

B=

Bonded

Ato

ms

E=

Lone p

air

s

http://www.sciencegeek.net/Chemistry/Presentations/VSEPR/

http://intro.chem.okstate.edu/1314f00/lecture/chapter10/vsepr.html

http://www.sciencegeek.net/APchemistry/Presentations/8_VSEPR/index.html

http://www.sciencegeek.net/Chemistry/Presentations/VSEPR/

http://intro.chem.okstate.edu/1314f00/lecture/chapter10/vsepr.html



Example 0: H2

STEP 1: Draw the Lewis structure› Remember how many valence electrons

the central atom has (in this example, H) One molecule can only have one central

atom.› Remember the valence electrons the

bound atoms have (the other hydrogen) just so you know how the chemical bonds are being formed. The lone pairs of these B atoms don’t

determine the molecule’s structure in this VSEPR model.

Example 0: H2

STEP 2: Count how many atoms have bonded to the central atom.› ANSWER: 1

STEP 3: Count how many lone pairs the central atom has› ANSWER: 0 One B

atom

No lone pairs in atom A (central atom)

ONLY in diatomic molecules, chose one or other atom to be the central atom, as both have the same characteristics. You determine which one is A and which one is B just to keep track of the electrons in bonds and lone pairs.

Example 0: H2

STEP 4: Check the VSEPR chart: › Total number of

domains (1+0=1)› Bonded atoms: 1› Lone pairs: 0

ANSWER: LINEAR

Example 1: CO2

STEP 1: Draw the Lewis structure› Remember how many valence

electrons the central atom has (in this example, C)

› Remember the valence electrons the bound atoms have (the two oxygens) just so you know how the chemical bonds are being formed. The lone pairs of these B atoms

don’t determine the molecule’s structure in this VSEPR model.

Example 1: CO2


STEP 3: Count how many lone pairs the central atom has› ANSWER: 0

Two B atoms


Example 1: CO2



ANSWER: LINEAR

Remember the repulsion between electrons: as each electron pair repel each other, they’ll try to be as far as possible from each other. They can be as far as 180ª from each other (the largest bond angle being 180ª). As there are no lone pairs impeding the formation of this angle, the structure is then linear.

Example 2: H2O

STEP 1: Draw the Lewis structure› Remember how many valence electrons the central atom

has (in this example, O)› Remember the valence electrons the bound atoms have

(the hydrogens) just so you know how the chemical bonds are being formed. The lone pairs of these B atoms don’t determine the molecule’s

structure in this VSEPR model.

Example 2: H2O



Two B atoms

Two lone pairs (E) in atom A (central atom)

Example 2: H2O



ANSWER: BENT

Remember the repulsion between electrons: as each electron pair repel each other, they’ll try to be as far as possible from each other. The two lone pair electrons of the central atom push the electrons from the bonds; the 180ª degree can no longer be achieved.

Example 3: O3

STEP 1: Draw the Lewis structure› Remember how many valence electrons the central atom has (in

this example, O)› Remember the valence electrons the bound atoms have (the

other two oxygens) just so you know how the chemical bonds are being formed. The lone pairs of these B atoms don’t determine the molecule’s

structure in this VSEPR model.

This is a resonant molecule

Example 3: O3



Two B atoms

One lone pair (E) in atom A (central atom)

Example 3: O3



ANSWER: BENT?

Actually, this is a trick questions:

It has been previously stated that this is a resonant molecule

One can’t successfully draw a VSEPR model for a resonant molecule.

REAL ANSWER: NO MODEL

Example 4: NH3


electrons the central atom has (in this example, N)

› Remember the valence electrons the bound atoms have (the hydrogens) just so you know how the chemical bonds are being formed. The lone pairs of these B atoms don’t


Example 4: NH3



Three B atoms

One lone pair (E) in atom A (central atom)

Example 4: NH3



ANSWER: TRIGONAL PYRAMID

This would be a trigonal planar with bond angles of 120ª between each hydrogen if it weren’t for the nitrogen’s lone pair that pushes hydrogen away. Remember that lone pairs repel bonds as well.

Example 5: CH4

STEP 1: Draw the Lewis structure› Remember how many valence electrons

the central atom has (in this example, C)

› Remember the valence electrons the bound atoms have (the hydrogens) just so you know how the chemical bonds are being formed. The lone pairs of these B atoms don’t


Example 5: CH4



Four B atoms


Example 5: CH4



ANSWER: TETRAHEDRAL

Example 6: PF5


electrons the central atom has (in this example, P)

› Remember the valence electrons the bound atoms have (the fluorines) just so you know how the chemical bonds are being formed. The lone pairs of these B atoms


Example 5: CH4



Five B atoms


Example 5: CH4

STEP 4: Check the VSEPR chart: › Total number of domains

(5+0=4)› Bonded atoms:5› Lone pairs: 0

ANSWER: TRIGONAL BIPYRAMIDAL

Example 7: ClF3


electrons the central atom has (in this example, Cl)

› Remember the valence electrons the bound atoms have (fluorines) just so you know how the chemical bonds are being formed. The lone pairs of these B atoms


Example 7: ClF3



Three B atoms

Two lone pairs (E) in atom A (central atom)

Example 7: ClF3



ANSWER: T-SHAPED

The two lone pairs push the fluorines away.NOTE: notice that both chlorine and fluorine are halogens. Bonds between halogens and noble gases tend to form planar structures.

Education

VSEPR