Giant Covalent Structures (Edexcel IGCSE Chemistry: Double Science)

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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
  • Two examples include diamond and graphite
  • 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.

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Diamond & graphite

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

Diamond structure, IGCSE & GCSE Chemistry revision notes

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 is used as pencil lead 
  • 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 
    • It is often used as electrodes during electrolysis 
  • 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

Graphite structure, IGCSE & GCSE Chemistry revision notes

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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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.