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CI 5.2

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 CO 2 sublimes from solid to gas at -78 o C

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CI 5.2

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

  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. 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

  5. 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.

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

  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. 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

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

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