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:
AX3E1 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:
AX3E0 S O

9. AX2E0 AX3E0 AX2E1 AX4E0 AX3E1 AX2E2 AX5E0 AX4E1 AX3E2 AX2E3 AX6E0
06/10/99
Electron pair geometry
AXE
Molecule geometry
linear
AX2E0
trigonal planar
AX3E0
AX2E1
bent
tetrahedral
AX4E0
AX3E1
Trigonal pyrimidal
AX2E2
trigonal bipyramidal
AX5E0
AX4E1
See-saw
AX3E2
T-shape
AX2E3
octahedral
AX6E0
AX5E1
Square pyramidal
AX4E2
AX3E3
AX2E4

10. Molecular Geometries S O AX3E0 Geometry: Trigonal Planar
Bond Angle: 120º
Example: SO3 S O

11. Molecular Geometries S O VSEPR Formula: AX2E1 Geometry: Bent (Angular)
Bond Angle: Less than 120º
Example: SO2 S O

12. 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

13. Molecular Geometries C Cl AX4E0 Geometry: Tetrahedral
Bond Angle: 109.5º
Example: CCl4 C Cl

14. 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

15. Molecular Geometries N H VSEPR Formula: AX3E1
Geometry: Trigonal Pyramidal
Bond Angle: Less than 109.5º
Example: NH3 N H

16. 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

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

18. Five Electron Groups AX5E0 In a molecule of PCl5
There are five electron groups around P.
Repulsion is minimized by placing five electron groups at angles of 120°, and 90o which is a trigonal bipyramidal arrangement.
The shape with five bonded atoms is trigonal bipyramidal
AX5E0

19. Five Electron Groups AX4E1 In a molecule of SF4
There are five electron groups around S - 4 bonding pairs and 1 non-bonding pair.
Repulsion is minimized by placing five electron groups at angles of 120°, and 90o which is a trigonal bipyramidal arrangement.
The non-bonding pair is on the trigonal plane
The shape with four bonded atoms is see-saw
AX4E1

20. Five Electron Groups AX3E2 In a molecule of ClF3
There are five electron groups around Cl - 3 bonding pairs and 2 non-bonding pair.
Repulsion is minimized by placing five electron groups at angles of 120°, and 90o which is a trigonal bipyramidal arrangement.
The two non-bonding pairs are on the trigonal plane
The shape with four bonded atoms is T-shape
AX3E2

21. Five Electron Groups AX2E3 In a molecule of XeF2
There are five electron groups around Xe - 2 bonding pairs and 3 non-bonding pair.
Repulsion is minimized by placing five electron groups at angles of 120°, and 90o which is a trigonal bipyramidal arrangement.
The three non-bonding pairs are on the trigonal plane
The shape with four bonded atoms is linear
AX2E3

22. Six Electron Groups AX6E0 In a molecule of SF6
There are six electron groups around S - 6 bonding pairs and 0 non-bonding pair.
Repulsion is minimized by placing four electron groups at angles of 90o on a plane and 2 perpendicular to the plane at 90o
The shape with six bonded atoms is octahedral
AX6E0

23. Six Electron Groups AX5E1 In a molecule of BrF5
There are six electron groups around Br - 5 bonding pairs and 1 non-bonding pair.
Repulsion is minimized by placing four electron groups at angles of 90o on a plane and 2 perpendicular to the plane at 90o
The lone pair does not go in the plane but above or below it
The shape with six bonded atoms is square pyramidal
AX5E1

24. Six Electron Groups AX4E2 In a molecule of XeF4
There are six electron groups around Xe - 4 bonding pairs and 2 non-bonding pairs.
Repulsion is minimized by placing four electron groups at angles of 90o on a plane and 2 perpendicular to the plane at 90o
The lone pairs go above or below the plane
The shape with six bonded atoms is square planar

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

26. 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°

27. 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

28. 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

29. 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

30. 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

31. 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

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

33. 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