To relate a structure to a property, name the particles, name the bonds or forces between them, and say what must happen for the property to be shown. A giant structure has a very large number of strong bonds, so melting it, cutting it or scratching it needs a lot of energy.
Giant structures appear throughout bonding and structure. This lesson covers the giant covalent ones, diamond, graphite and silicon dioxide, plus a short look at giant ionic and metallic lattices. It builds on ion formation and covalent bonding.
What makes a structure “giant”?
In a simple molecule such as water, the atoms form a small unit of a few atoms. In a giant structure there is no small unit. Every atom or ion is bonded to its neighbours, which bond to their neighbours, forming one continuous lattice.
- Diamond: each carbon atom is covalently bonded to 4 others in a tetrahedral arrangement.
- Graphite: each carbon atom is bonded to 3 others in flat layers. Weak forces hold the layers together.
- Silicon dioxide, SiO₂: each silicon atom is bonded to 4 oxygen atoms, and each oxygen is bonded to 2 silicon atoms, so the ratio of Si to O is 1:2.
How to write a structure-to-property explanation
- State the property the question is about.
- Name the structure: giant covalent, giant ionic or giant metallic.
- Name the particles and the bonds that hold them, with “strong” or “weak”.
- Say what has to be overcome, such as breaking many strong covalent bonds.
- Link to the result: a lot of energy is needed, so the melting point is high. Or layers slide, so it is soft.
Worked example
Explain why diamond has a very high melting point and does not conduct electricity.
Melting point: diamond is a giant covalent structure. Every carbon atom is joined to 4 others by strong covalent bonds, in a continuous lattice. To melt it, a very large number of strong covalent bonds must be broken. This needs a lot of energy, so the melting point is very high.
Conduction: all 4 outer electrons of each carbon atom are used in covalent bonds. There are no free electrons and no ions, so there are no charged particles free to move. Diamond does not conduct electricity.
Notice that both explanations name the particle (carbon atoms), the bond (strong covalent) and the consequence.
The mistake to watch for
A common error is to write as if diamond were made of separate molecules, or to say that it melts by breaking “intermolecular forces”.
Mistaken answer: “Diamond has a high melting point because it has strong intermolecular forces.”
Diamond does not contain molecules. The forces that must be broken are covalent bonds.
The correction is to say that strong covalent bonds between atoms must be broken. Reserve “intermolecular forces” for simple molecular substances, which you meet in contrasting molecular and ionic melting behaviour.
Check yourself
Try these without looking back, then open each answer.
1. Explain why graphite is soft enough to be used in a pencil.
Show answer
Graphite has layers of carbon atoms. Within each layer the covalent bonds are strong, but the forces between layers are weak. The layers slide over each other easily, so the graphite rubs off onto paper.
2. Silicon dioxide is used in sand and quartz. Explain why its melting point is very high.
Show answer
Silicon dioxide is a giant covalent structure. Each silicon atom is joined to 4 oxygen atoms by strong covalent bonds, in a continuous lattice. Melting needs a lot of energy to break many strong covalent bonds, so the melting point is very high.
3. A substance is hard and brittle, has a high melting point, and does not conduct when solid. Name two giant structures that could fit and say which property helps tell them apart when molten. (Invented data for practice.)
Show answer
It could be giant covalent (such as silicon dioxide) or giant ionic (such as sodium chloride). When molten, a giant ionic substance conducts because its ions can move, while a giant covalent substance does not conduct because it has no free charged particles.
Where this leads next
Next, compare these with small molecules in contrasting molecular and ionic melting behaviour. Then check your understanding on the bonding and structure practice set.
If you often know the property but cannot build the particle explanation, our teachers can work through it with you in online one-to-one Chemistry tuition.