Edexcel IGCSE Chemistry · Spec 1.50
Diamond, Graphite and C60 Compared
Covers compare bonding in three carbon allotropes, Explain hardness and conductivity and Explain melting-point differences.
Chemistry revision video
Diamond, Graphite and C60 Compared
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Explained
Three forms of carbon, side by side
Diamond, graphite and C sixty are allotropes: different structural forms of the same element. Every difference between them comes from how many other carbon atoms each atom is bonded to, and whether the result is one continuous structure or separate molecules.
Diamond
Each carbon atom forms four covalent bonds, giving a giant three dimensional lattice with no weak direction anywhere in it.
It is extremely hard, because any force has to break covalent bonds. It has a very high melting point, for the same reason. And it does not conduct electricity, because all four outer electrons are used in bonding, leaving none free to move.
Graphite
Each carbon atom forms three covalent bonds, giving flat layers of hexagons.
The layers are held to each other by weak forces, so they slide over one another easily. That makes graphite soft and slippery, useful as a lubricant and as pencil lead.
The fourth outer electron on each atom is not used in bonding. It is delocalised and free to move between the layers, so graphite conducts electricity, which no other non metal in this course does.
Its melting point is still very high, because melting means breaking the covalent bonds within the layers, not merely separating the layers from each other.
C sixty
A fullerene: a hollow ball of sixty carbon atoms, each bonded to three others, arranged as a pattern of hexagons and pentagons.
The critical difference is that this is a molecule, not a giant structure. The balls are held to each other by weak intermolecular forces, so C sixty is soft and melts at a low temperature, even though the covalent bonds inside each ball are just as strong as those in graphite.
It is the one carbon structure in this topic where the word intermolecular is correct, and knowing why it is correct here and wrong for the other two is the point of the comparison.
The comparison in one table
- Bonds per atom: four, three, three.
- Structure: giant three dimensional, giant layered, simple molecular.
- Hardness: very hard, soft and slippery, soft.
- Conducts electricity: no, yes, no.
- Melting point: very high, very high, low.
Notice that graphite and C sixty have the same number of bonds per atom and behave completely differently. Bond count alone does not decide the properties; whether the structure is continuous does.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme explaining diamond's high melting point wants three linked points: the covalent bonds are strong, there are many of them to break, and a large amount of energy is needed to break them.
Two short notes sit beside it. It ignores more energy, because that is a comparison with nothing to compare against and the question asks why the value is high. And it says not just heat, so thermal energy or heat energy is wanted rather than heat on its own.
The same note then withholds all three marks if intermolecular forces, forces between molecules or ions, ionic bonding or metallic bonding are mentioned anywhere in the answer.
Diamond and graphite contain no molecules, so intermolecular forces cannot exist in either. Reaching for the phrase describes a structure that is not there, and it costs the whole question rather than one mark.
Why the uses follow
Diamond cuts and drills because it is harder than what it is cutting. Graphite lubricates because its layers slide, and is used for electrodes because it conducts and has a high melting point, so it survives molten electrolytes. Both properties are needed for that job, which is why no other non metal will do.
Spec 1.50
What you need to know
- Compare bonding in three carbon allotropes
- Explain hardness and conductivity
- Explain melting-point differences
Active recall
Quick check
Answer each question before opening the answer.
Why is diamond hard?
It has strong covalent bonds in a three-dimensional lattice
Why does graphite conduct?
It has delocalised electrons within its layers
Why does C60 melt below diamond?
Only weak intermolecular forces act between separate C60 molecules
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