Giant Molecular Structures (Or giant covalent structures)

Allotropes: different forms of the same element. Carbon has several allotropes – the best known being diamond and graphite

Structure of part of a diamond molecule

Contains only carbon atoms 3 dimensional tetrahedral structure Strong covalent bonds Strong substance High melting point and boiling point MP 3550℃ All the valence electrons are used in bonding, so diamond doesn’t conduct

Graphite

Graphite is another allotrope of carbon Contains a 2 dimensional giant covalent structure Each layer is strong, but there are only weak forces between layers. Layers can slide over each other, hence graphite is used as a lubricant and in pencils. Each carbon only makes 3 bond The remaining delocalised electrons are free to move, hence graphite is a conductor. Similar mp to diamond (? – you could research this. There is some contradictory evidence around)

Fullerene C60

The fullerenes are actually a whole chemical family. They are allotropes of carbon. They are spherical molecules consisting of 5 and 6 membered rings of carbon atoms. C60 is the one which has been studied most.

Each carbon is bonded to 3 other atoms, so there are delocalised electrons. However, because the surface isn’t flat the electrons cannot easily flow from one molecule to another. C60 has a conductivity greater than diamond, but MUCH less than graphite. Unusually, C60 is soluble in non polar solvents It has a lower MP than diamond or graphite (800K)

Nanotubes

Nanotubes are another allotrope of carbon. They are “state of the art” technology We will study them more in the option “Chemistry in Industry and Technology”

Silicon compounds Silicon is in the same group as carbon So the structure of silicon is very similar to that of diamond. The larger atoms means that the covalent bonding is not as strong So silicon is weaker than diamond and has a lower MP and BP. MP 1414 °C Silicon doesn’t have any allotropes like graphite or fullerenes.

Silicon dioxide also has a similar structure to diamond.

Silicon dioxide

It is giant molecular It has a tetrahedral structure Each silicon atom is bonded to 4 oxygen atoms Each oxygen atom is bonded to 2 silicon atoms