1. Shapes and Polarity of Molecules
VSPER
Shapes and Polarity of Molecules
Adapted from: Pearson Education, Inc.
Publishing as Benjamin Cummings

2. Shapes of Molecules
Molecules (covalent chemicals) form certain shapes depending on how many lone and bonding pairs of electrons it has.
Because the electron pairs repel each other we get certain shapes being formed. These are due to a certain rule called VSEPR (Valence Shell Electron Pair Repulsion)
Bonding pair – these can also be drawn as straight lines
Lone pair

3. VSEPR Theory
06/10/99
Based on Lewis structures we can know the shape or “geometry” of molecules
VSEPR, as the name suggests, predicts geometry based on the repulsion of electron pairs (bonding pairs and lone pairs)
Electrons around the central nucleus repel each other. Thus, resulting structures have atoms maximally spread out.

4. The 6 Basic Shapes for electron pair repulsion:2
Electron Repulsion
Zones 3 4 5 6
Shape:
Linear
Trigonal Planar (Flat)
Tetrahedral
Trigonal Bipyramid
Octahedral

5. Planar triangular
Tetrahedral
Octahedral
Trigonal bipyramidal

6. AXE
Lewis structures do not show geometry, only electron pair placement.
However, the 3-D shape (geometry) of a molecule can be determined from a properly-drawn Lewis structure.
All monocentric molecules can be represented by an AXE formula:
A = central atom
X = outer atoms (doesn’t matter what they actually are or how many bonds they are held by)
E = lone pairs of electrons on the central atom only.

7. Cl O What AXE formula corresponds to the chlorate ion, ClO3-1?
First draw a proper Lewis structure:
One central atom, three outer atoms, one lone pair:
AX3E Cl O -1

8. S O What AXE formula corresponds to sulfur trioxide, SO3?
Draw a Lewis structure.
1 central atom, 3 outer atoms, no lone pairs:
AX3 S O

9. AX2 AX3 AX2E AX4 AX3E AX2E2 AX5 AX4E AX3E2 AX2E3 AX6 AX5E AX4E2 AX3E3
06/10/99
Electron pair geometry
AXE
Molecule geometry
linear
AX2
trigonal planar
AX3
AX2E
bent
tetrahedral
AX4
AX3E
Trigonal pyrimidal
AX2E2
trigonal bipyramidal
AX5
AX4E
See-saw
AX3E2
T-shape
AX2E3
octahedral
AX6
AX5E
Square pyramidal
AX4E2
AX3E3
AX2E4

10. Molecular Geometries S O AX3 Geometry: Trigonal Planar
Bond Angle: 120º
Example: SO3 S O

11. Molecular Geometries S O VSEPR Formula: AX2E Geometry: Bent (Angular)
Bond Angle: Less than 120º
Example: SO2 S O

12. Molecular Geometries C Cl AX4 Geometry: Tetrahedral Bond Angle: 109.5º
Example: CCl4 C Cl

13. Four Electron Groups In a molecule of CH4
There are four electron groups around C.
Repulsion is minimized by placing four electron groups at angles of 109°, which is a tetrahedral arrangement.
The shape with four bonded atoms is tetrahedral.
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

14. Molecular Geometries N H VSEPR Formula: AX3E
Geometry: Trigonal Pyramidal
Bond Angle: Less than 109.5º
Example: NH3 N H

15. Three Bonding Atoms and One Lone Pair
In a molecule of NH3
Three electron groups bond to H atoms and the fourth one is a lone (nonbonding) pair.
Repulsion is minimized with 4 electron groups in a tetrahedral arrangement.
With three bonded atoms, the shape is pyramidal.
Copyright © by Pearson Education, Inc. Publishing as Benjamin Cummings

16. Molecular Geometries O H VSEPR Formula: AX2E2 Geometry: Bent (Angular)
Bond Angle: Less than 109.5º
Example: H2O O H

17. Two Bonding Atoms and Two Lone Pairs
In a molecule of H2O
Two electron groups are bonded to H atoms and two are lone pairs (4 electron groups).
Four electron groups minimize repulsion in a tetrahedral arrangement.
The shape with two bonded atoms is bent(~109).
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

18. Learning Check The shape of a molecule of N2O (N N O) is 1) linear
2) trigonal planar
3) bent (120°)

19. Solution The shape of a molecule of N2O (N N O) is 1) linear
In the electron-dot structure with 16 e-, octets
are acquired using two double bonds to the central N atom. The shape of a molecule with two electron groups and two bonded atoms (no lone pairs on N) is linear.
two electron groups
• • • •
: N :: N :: O :
• • • •
: N = N=O : linear, 180°

20. Learning Check
State the number of electron groups, lone pairs, and use VSEPR theory to determine the shape of the following molecules or ions.
1) tetrahedral ) pyramidal 3) bent
A. PF3
B. H2S
C. CCl4

21. Solution A. PF3 4 electron groups, 1 lone pair, (2) pyramidal B. H2S
4 electron groups, 2 lone pairs, (3) bent
C. CCl4
4 electron groups, 0 lone pairs, (1) tetrahedral

22. Polar Molecules A polar molecule Contains polar bonds.
Has a separation of positive and negative charge called a dipole indicated with + and -.
Has dipoles that do not cancel.
+  • •
H–Cl H—N—H
dipole H
dipoles do not cancel

23. Nonpolar Molecules A nonpolar molecule Contains nonpolar bonds.
Cl–Cl H–H
Or has a symmetrical arrangement of polar bonds.
O=C=O Cl
Cl–C–Cl Cl
dipoles cancel

24. Determining Molecular Polarity
STEP Write the electron-dot formula.
STEP Determine the polarity of the bonds.
STEP Determine if dipoles cancel.
Example: H2O
. .
H─O: H2O is polar │ H
dipoles do not cancel

25. Learning Check Identify each of the following molecules as
1) polar or 2) nonpolar. Explain.
A. PBr3
B. HBr
C. Br2
D. SiBr4

26. Solution Identify each of the following molecules as
1) polar or 2) nonpolar. Explain.
A. PBr3 1) pyramidal; dipoles don’t cancel; polar
B. HBr 1) linear; one polar bond (dipole); polar
C. Br2 2) linear; nonpolar bond; nonpolar
D. SiBr4 2) tetrahedral; dipoles cancel; nonpolar