1. Molecules and Networks
O C O
CI 5.2
Molecules and Networks
O C O

2. Carbon and Silicon oxides
Carbon and silicon – both in Group 4
So we would expect similar properties
But compare carbon and silicon oxides
CO2 sublimes from solid to gas at -78oC
SiO2 is a hard solid with a high melting point – sand is largely SiO2

3. Why the difference??
Carbon is a small atom so it can form double bonds with oxygen
This means that each atom in CO2 effectively has a full outer shell: O C O

4. CO2 is made up of individual molecules
The molecules are joined to each other by weak intermolecular forces O C O
O C O
Very little energy is needed to break these forces
O C O
Weak intermolecular force

5. Silicon is bigger than carbon
Silicon cannot form double bonds
This means that each silicon bonds to 4 oxygen atoms
oxygen
silicon
This gives silicon a full outer shell, but each oxygen needs one more electron.

6. This is achieved by each oxygen bonding to another silicon atom – so a giant network is built up.
Oxygen atom

7. Covalent molecular structures
Small discrete molecules
Strong covalent bonds within molecules
Weak intermolecular forces between molecules
Many of these structures dissolve in organic solvents; some dissolve in water
Typical examples – CO2 , H2O

8. Covalent network structures
Giant repeating lattices of very many covalently bonded atoms
Insoluble solids with high melting and boiling points
Typical example is SiO2

9. Elements with molecular structures
Some non-metal elements exist as molecules
H2, N2, O2, F2, Cl2 are all diatomic molecules and gases at room temperature
Phosphorus and sulphur are both soft solids with low melting points

10. P4 S8

11. Elements with network structures
Silicon exists as a giant network of thousands of silicon atoms joined by strong covalent bonds

12. Carbon has different forms
In graphite each carbon atom is covalently bonded to 3 others
Graphite has a layered structure

13. Diamond In diamond each carbon atom is covalently bonded to 4 others
The structure is like that of silicon
Diamond is the hardest naturally occurring substance

14. Graphite
Diamond
Conducts electricity
Is a non-conductor
Acts as a lubricant – it is soft and brittle
Is the hardest natural substance – used in drill bits

15. Fullerenes These are molecules of carbon
Each one has a definite number of carbon atoms joined by strong covalent bonds
There is much research into possible uses of the fullerenes

16. Allotropes
Different structures of the same substance are called allotropes
Graphite, diamond and buckminsterfullerene are all allotropes of carbon