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
Examiner Tip
Giant covalent structures can also be called macromolecules.
Diamond and graphite
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
Examiner 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
Examiner 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!
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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