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CI 5.2

Molecules and Networks

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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 meltingpoint sand is largely SiO2

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Why the difference??

Carbon is a small atom so it can form

double bonds with oxygen

This means that each atom in CO2effectively has a full outer shell:

O C O

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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 littleenergy is

needed to

break these

forces

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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.

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This is achieved by each oxygen bonding to

another silicon atom so a giant network is built

up.

Silicon atom

Oxygen atom

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Covalent molecular structures

Small discrete molecules

Strong covalent bonds within molecules

Weak intermolecular forces betweenmolecules

Many of these structures dissolve in

organic solvents; some dissolve in water Typical examples CO2 , H2O

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Covalent network structures

Giant repeating lattices of very many

covalently bonded atoms

Insoluble solids with high melting and

boiling points

Typical example is SiO2

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Elements with molecular structures

Some non-metal elements exist as

molecules

H2, N2, O2, F2, Cl2 are all diatomicmolecules and gases at room temperature

Phosphorus and sulphur are both soft

solids with low melting points

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Elements with network structures

Silicon exists as a

giant network ofthousands of silicon

atoms joined by strong

covalent bonds

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Carbon has different forms

In graphite each

carbon atom is

covalently bonded

to 3 others

Graphite has a

layered structure

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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

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Fullerenes

These are moleculesof carbon

Each one has a

definite number ofcarbon atoms joinedby strong covalentbonds

There is muchresearch into possibleuses of the fullerenes

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Allotropes

Different structures of the same substance

are called allotropes

Graphite, diamond and

buckminsterfullerene are all allotropes of

carbon