Giant Covalent Structures: Diamond, Graphite and Silicon Dioxide
Three substances with covalent bonds but no molecules. Every property follows from the structure, and the explanations are worded very specifically.
1. What “giant covalent” means
A giant covalent (macromolecular) structure is a huge lattice of atoms joined by STRONG COVALENT BONDS throughout.
There are NO separate molecules and NO intermolecular forces. Describing the forces in a giant structure as “intermolecular” was a recorded error twice — there is nothing between molecules because there are no molecules.
The general properties, and their reasons:
Very high melting and boiling points — because many STRONG COVALENT BONDS must be broken, which takes a great deal of energy Very hard (except graphite) Insoluble in water Do not conduct electricity (except graphite)
Say “strong covalent bonds throughout the structure” — not “strong forces”. And do not call them electrostatic forces; confusing electrostatic forces with covalent bonding in diamond was recorded.
2. Diamond
Each carbon atom is bonded to FOUR other carbons by strong covalent bonds, in a rigid tetrahedral 3D lattice.
Properties and explanations:
| Property | Because |
|---|---|
| Very hard — hardest natural substance | rigid 3D network of strong covalent bonds in all directions |
| Very high melting point | many strong covalent bonds to break |
| Does NOT conduct | no free electrons — all four outer electrons are used in bonding |
Four bonds per carbon. Misunderstanding the number of bonds in diamond was recorded — and it is exactly why there are no spare electrons to conduct.
Uses: cutting tools, drill tips, jewellery — all from its hardness.
3. Graphite
Each carbon atom is bonded to only THREE others, forming layers of hexagonal rings. The layers are held together by weak forces, and the fourth outer electron is DELOCALISED.
Properties and explanations:
| Property | Because |
|---|---|
| SOFT and slippery | the layers can SLIDE over each other, as the forces between layers are weak |
| CONDUCTS electricity | each carbon has one DELOCALISED (free) electron that can move through the structure |
| Very high melting point | the covalent bonds within the layers are strong |
Graphite is SOFT — not hard. Confusing it with a hard substance was recorded. This is the property that most distinguishes it from diamond, despite both being pure carbon.
Use the phrase “free moving, DELOCALISED electrons”. Not initially mentioning it was a recorded error, and it is the mark-earning phrase for graphite’s conductivity.
Graphite conducts even though it is a solid non-metal — a recorded misconception assumed solids can’t conduct. It is the only non-metal that conducts well.
The forces between the layers are weak — but do not call them intermolecular. They are weak forces between layers, not between molecules.
Graphite is an ELEMENT, not a compound — it is pure carbon. Recorded as an error.
Uses: pencil “lead” and lubricants (from its softness/slipperiness), electrodes (from its conductivity, plus a high melting point and inertness).
4. Diamond vs graphite — the comparison
Both are allotropes of carbon — different forms of the same element.
| Diamond | Graphite | |
|---|---|---|
| Bonds per carbon | 4 | 3 |
| Structure | 3D tetrahedral network | layers of hexagons |
| Hardness | very hard | soft, slippery |
| Conductivity | no | yes |
| Free electrons | none | one per atom |
The difference in properties comes entirely from the difference in structure — same element, different arrangement. That sentence is often worth a mark on its own.
5. Silicon dioxide (silica)
Silicon dioxide has a giant covalent structure similar to diamond: each silicon atom is bonded to four oxygens, and each oxygen to two silicons, in a 3D lattice.
Properties: very high melting point, hard, insoluble, does not conduct.
Silicon dioxide has a HIGH melting point, not a low one. A recorded error assumed low — it is a giant structure, so many strong covalent bonds must be broken.
Uses: sand, glass, and lining furnaces.
6. Comparing the four structure types
| Structure | Melting point | Conducts? | Example |
|---|---|---|---|
| Simple molecular | low | no | CO₂, H₂O |
| Giant covalent | very high | no (except graphite) | diamond, SiO₂ |
| Giant ionic | high | when molten/aqueous | NaCl |
| Giant metallic | high | yes, always | copper |
Simple molecular and giant covalent are completely different despite both having covalent bonds. Confusing them was recorded — the difference is whether there are separate molecules (low melting point) or one continuous lattice (very high).
Only graphite and metals conduct as solids. Ionic compounds need to be molten or aqueous; simple molecular substances never conduct.
7. Mistakes that cost marks
Calling the forces in a giant structure “intermolecular”.
Saying graphite is hard.
Omitting “delocalised electrons” for graphite’s conductivity.
Saying graphite can’t conduct because it’s a solid.
Calling graphite a compound.
Giving diamond three bonds per carbon, or graphite four.
Saying silicon dioxide has a low melting point.
Confusing simple molecular with giant covalent.
Saying covalent bonds break when a simple molecular substance melts.
Frequently asked questions
What is a giant covalent structure? A huge lattice of atoms joined by strong covalent bonds throughout, with no separate molecules.
How many bonds does each carbon form in diamond? Four, in a tetrahedral 3D network.
Why is diamond hard? A rigid 3D network of strong covalent bonds in all directions.
Why doesn’t diamond conduct? No free electrons — all four outer electrons are bonded.
Why is graphite soft? Its layers slide over each other because the forces between layers are weak.
Why does graphite conduct? Each carbon has one delocalised electron free to move.
How many bonds does each carbon form in graphite? Three.
Are diamond and graphite compounds? No — both are pure carbon, so they are elements (allotropes).
Why does silicon dioxide have a high melting point? Many strong covalent bonds in a giant lattice must be broken.
What’s the difference between simple molecular and giant covalent? Simple molecular has separate molecules with weak forces between them (low melting point); giant covalent is one continuous lattice (very high).
Quick revision checklist
- I can define a giant covalent structure
- I never call its forces intermolecular
- I know diamond has four bonds per carbon and why it’s hard
- I know why diamond doesn’t conduct
- I know graphite has three bonds and layers
- I can explain graphite’s softness via sliding layers
- I use “delocalised electrons” for its conductivity
- I know graphite is an element
- I can compare diamond and graphite in a table
- I know silicon dioxide’s structure and high melting point
- I can distinguish all four structure types
- I know which structures conduct, and when
These notes cover giant covalent structures in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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