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Covalent Molecules and Covalent Networks

O C O. O C O. Covalent Molecules and Covalent Networks. Carbon and Silicon oxides. Carbon and silicon – both in Group 14 Group members are predicted to have similar properties However carbon and silicon oxides are very different. CO 2 sublimes from solid to gas at -78 o C

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Covalent Molecules and Covalent Networks

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  1. O C O O C O Covalent Molecules and Covalent Networks

  2. Carbon and Silicon oxides Carbon and silicon – both in Group 14 Group members are predicted to have similar properties However carbon and silicon oxides are very different. CO2 sublimes from solid to gas at -78oC SiO2 is a hard solid with a high melting point 1610°C – 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 O C O Weak intermolecular force The molecules are attracted to each other by weak intermolecular forcesO C O

  5. oxygen silicon Silicon is bigger than carbon • Silicon (larger atom) cannot form double bonds • This means that each silicon bonds to 4 oxygen atomswith strong covalent bonds • This gives silicon a full outer shell, but each oxygen needs one more electron to achieve a full outer shell.

  6. Silicon atom Oxygen atom This is achieved by each oxygen bonding to another silicon atom – so a covalent 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, Br2, S8

  8. Covalent network structures Giant repeating lattices of large numbers of 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(P4) and sulphur(S8) are both soft solids with low melting points Br2 is liquid

  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 other carbons. The 4th electron is delocalised between the layers • 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, as it has very strong, short bonds

  14. Fullerenes • These are molecules of carbon as they dissolve in benzene • 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 element are called allotropes Graphite, diamond and buckminsterfullerene are all allotropes of carbon

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