1. CI 5.2 Molecules and Networks OCOOCO OCOOCO

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

3. Why the difference?? Carbon is a small atom so it can form double bonds with oxygen This means that each atom in CO 2 effectively has a full outer shell: O C O

4. CO 2 is made up of individual molecules The molecules are joined to each other by weak intermolecular forces O C O O C O Weak intermolecular force Very little energy is needed to break these forces

5. Silicon is bigger than carbon Silicon cannot form double bonds This means that each silicon bonds to 4 oxygen atoms oxygensilicon 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. Silicon atom 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 – CO 2, H 2 O

8. Covalent network structures Giant repeating lattices of very many covalently bonded atoms Insoluble solids with high melting and boiling points Typical example is SiO 2

9. Elements with molecular structures Some non-metal elements exist as molecules H 2, N 2, O 2, F 2, Cl 2 are all diatomic molecules and gases at room temperature Phosphorus and sulphur are both soft solids with low melting points

10. P4P4 S8S8

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. GraphiteDiamond Conducts electricityIs 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