Giant Covalent Structures (Edexcel IGCSE Chemistry (Modular))
Revision Note
Giant covalent structures
Giant covalent structures are solids with high melting points
They have a huge number of non-metal atoms bonded to other non-metal atoms via strong covalent bonds
These structures can also be called giant lattices and have a fixed ratio of atoms in the overall structure
Three examples include diamond, graphite and C60 fullerene
All giant covalent structures have high melting points because:
There are strong covalent bonds between atoms
These require lots of energy to overcome
Exam Tip
Giant covalent structures can also be called macromolecules.
Diamond, graphite and C60 fullerene
Diamond
Diamond and graphite are allotropes of carbon
Both substances contain only carbon atoms but due to the differences in bonding arrangements they are physically completely different
In diamond, each carbon atom bonds with four other carbons, forming a tetrahedron
Each covalent bond is very strong
What are the properties of diamond?
Diamond is very hard because:
Each carbon atom is covalently bonded to four other carbon atoms
The covalent bonds are very strong
Diamond being hard makes it useful for cutting tool
Diamond has a high melting point because:
It has a giant covalent structure
There are strong covalent bonds between atoms which need lots of energy to break
The bonding and structure in diamond
Each carbon atom is bonded to four other carbon atoms
Exam Tip
Diamond is the hardest naturally occurring mineral, but it is by no means the strongest.
Students often confuse hard with strong, thinking it is the opposites of weak.
Diamonds are hard, but brittle – that is, they can be smashed fairly easily with a hammer.
The opposite of saying a material is hard is to describe it as soft.
Graphite
Each carbon atom in graphite is bonded to three others forming layers of hexagons, leaving one free electron per carbon atom
What are the properties of graphite?
Graphite is soft and slippery
Each carbon atom is bonded to three other carbon atom forming layers
The layers are free to slide over each other because there are only weak forces between the layers, not covalent bonds
Graphite can conduct electricity and heat
Due to each carbon atom only forming three bonds, one electron from each carbon atom is delocalised
The delocalised electrons are free to move
Graphite is similar to metals in that it has delocalised electrons
Graphite has a high melting point because:
It has a giant covalent structure
There are strong covalent bonds between atoms which need lots of energy to break
Bonding and structure in graphite
In graphite, each carbon is bonded to three other carbon atoms forming layers
Exam Tip
Don’t confuse pencil lead with the metal lead – they have nothing in common.
Pencil lead is actually graphite, and historical research suggests that in the past, lead miners sometimes confused the mineral galena (lead sulfide) with graphite; since the two looked similar they termed both minerals ‘lead’.
The word graphite derives from the Latin word ‘grapho’ meaning ‘I write’, so it is a well named mineral!
C60 fullerene
Fullerenes are a group of carbon allotropes which consist of molecules that form hollow tubes or spheres
Fullerenes can be used to trap other molecules by forming around the target molecule and capturing it, making them useful for targeted drug delivery systems
They also have a huge surface area and are useful for trapping catalyst molecules onto their surfaces making them easily accessible to reactants so catalysis can take place
Some fullerenes are excellent lubricants and are starting to be used in many industrial processes
The first fullerene to be discovered was buckminsterfullerene which is affectionately referred to as a “buckyball”
In this fullerene, 60 carbon atoms are joined together forming 20 hexagons and 12 pentagons which produce a hollow sphere that is the exact shape of a soccer ball
C60 fullerene
The structure n bonding in C60 fullerene - the football shaped molecule
What are the properties of C60 fullerene?
C60 fullerene has a high melting point because:
It has a giant covalent structure
There are strong covalent bonds between atoms which need lots of energy to break
Exam Tip
Remember: Explaining the melting point for any giant covalent structure is always the same:
They have giant covalent structures
There are many strong covalent bonds
These need lots of energy to break
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