CI 5.2

Molecules and Networks

OC

O

OC

O

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

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

CO2 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

Weak intermolecular force

Very little energy is needed to break these forces

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.

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

Silicon atom

Oxygen atom

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

Covalent network structures

• Giant repeating lattices of very many covalently bonded atoms

• Insoluble solids with high melting and boiling points

• Typical example is SiO2

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

P4

S8

Elements with network structures

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

Carbon has different forms

•In graphite each carbon atom is covalently bonded to 3 others

•Graphite has a layered structure

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

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

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

Allotropes

• Different structures of the same substance are called allotropes

• Graphite, diamond and buckminsterfullerene are all allotropes of carbon