O C O O C O CI 5.2 Molecules and Networks
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
O C O Very little energy is needed to break these forces CO2 is made up of individual molecules • The molecules are joined to each other by weak intermolecular forcesO C O O C O Weak intermolecular force
oxygen silicon Silicon is bigger than carbon • Silicon cannot form double bonds • This means that each silicon bonds to 4 oxygen atoms • This gives silicon a full outer shell, but each oxygen needs one more electron.
Silicon atom Oxygen atom This is achieved by each oxygen bonding to another silicon atom – so a giant network is built up.
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
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
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