Molecules and Networks

• 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

• 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

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
O C O Very little energy is needed to break these forces
Weak intermolecular force

• 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.

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

• 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

• Giant repeating lattices of very many covalently bonded atoms
• Insoluble solids with high melting and boiling points
• Typical example is SiO
2

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

P
4
S
8

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

•In graphite each carbon atom is covalently bonded to 3 others
•Graphite has a layered structure

•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

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

• 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

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