1. Shapes of Molecules & Intermolecular Forces

2. Shapes of covalent molecules
VSEPR – Valence Shell Electron Pair Repulsion
Use this to predict shapes of covalent molecules
Count number of bond pairs and lone pairs
Predict shape

3. 2 180 3 120 4 109.5 1 107 104.5 Linear BeCl2 BeH2 Triangular planar
No. electron pairs:
No. bond pairs:
No. lone pairs:
Shape molecule:
Bond angle:
Examples: 2
Linear
180
BeCl2
BeH2 3
Triangular planar
120
BCl3
AlH3 4
Tetrahedral
109.5
CH4
SiCl4 1
Pyramidal
107
NH3
PH3
V-shaped
104.5
H2O
H2S

4. BeCl2 Be in Group II – 2 e-’s in outer shell
Cl in Group VII – 7 e-’s in outer shell
2 bond pairs & 0 lone pairs
Linear
Cl Be Cl
1800

5. BCl3 B in Group III – 3 e-’s in outer shell
Cl in Group VII – 7 e-’s in outer shell
3 bond pairs & 0 lone pairs
Triangular planar Cl
1200 B
Cl Cl

6. CH4 C in Group IV – 4 e-’s in outer shell
H in Group I – 1 e- in outer shell
4 bond pairs & 0 lone pairs
Tetrahedral H
109.50
H C H

7. NH3 N in Group V – 5 e-’s in outer shell
H in Group I – 1 e- in outer shell
3 bond pairs & 1 lone pair
Pyramidal N
1070
H H H

8. H2O H in Group I – 1 e- in outer shell
O in Group VI – 6 e-’s in outer shell
2 bond pairs & 2 lone pairs
V-shaped O
H H

9. Ionic & Covalent Characteristics
Network of ions in a crystal
Individual molecules
Usually hard & brittle (metals)
Usually soft (non-metals)
High melting & boiling points
(mainly solids)
Low melting & boiling points (mainly liquids & gases)
Solid at room T
Liquid or gas at room T
Conduct electricity when molten or dissolved in water
Do not conduct electricity

10. Intramolecular bonding
Occurs between ions in ionic compounds
Hold ions together within molecules or crystal lattices
Example – sodium chloride (NaCl)

11. Intermolecular forces
Forces of attraction between covalent molecules
3 types – van der Waals, dipole-dipole & hydrogen bonds
When these forces are strong molecules are attracted to each other and form solids or liquids

12. Van der Waals forces Weakest intermolecular force
Exist between neutral atoms
As electrons move within atoms temporary dipoles can occur

13. Opposite ends of these temporary dipoles attract each other
Temporary so not very strong
+H – H H – H-
+H – H-

14. Dipole-dipole forces Stronger than van der Waals
In polar molecules the positive pole of one molecule is attracted to the negative pole of another
Permanent dipoles so quite strong

15. +H – Cl H – Cl-
+H – Cl-

16. Hydrogen bonding Strongest intermolecular force
When a H atom is bonded to small strongly electronegative atoms like N, O or F it gains a partial positive charge
This positive H atom is then attracted to the negative poles in other molecules forming a hydrogen bond

17. Strength of intermolecular forces:
H – C – C H+
-O – H O
H H+
Strength of intermolecular forces:
Van der Waals < dipole-dipole < hydrogen bonds

18. Boiling Points
Boiling points are increased when there are intermolecular forces present
Examples –
(a) H2 and O2:
Both molecules are non-polar so can only have
van der Waals forces
O2 has higher b.p. because its heavier
+H – H+ -O == O-
Mr = 2 Mr = 16

19. (b) C2H4 and HCHO:
C2H4 is non-polar so can only have van der
Waals forces
HCHO has a C == O bond so is polar so can have dipole-dipole forces

20. +H H O-
C C +H – C
+H H H+
non-polar polar
Dipole-dipole forces are stronger than van der
Waals so HCHO will have a higher boiling point

21. (c) H2O and H2S:
H2O has a polar O --- H bond and O is strongly electronegative so can form hydrogen bonds
H2S has a polar S --- H bond but S is not very electronegative so can only form dipole-dipole forces and not hydrogen bonds

22. O S
H H H H
O = S = 2.5
Hydrogen bonds are stronger than dipole-dipole forces so H2O will have a higher boiling point