AQA GCSE Combined Science (8464), Higher tier · Chemistry › Bonding, structure, and the properties of matter › How bonding and structure are related to the properties of
Practise Giant covalent structures. 12 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Substances such as diamond, graphite and silicon dioxide, in which every atom is joined to others by strong covalent bonds in a huge network. You need to explain their very high melting points, recognise them from diagrams, and contrast them with substances made of small molecules.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name examples of giant covalent structuresDiamond and graphite (both forms of carbon) and silicon dioxide (silica).
4
State that they have very high melting pointsThey are solids at room temperature with very high melting and boiling points.
5
Explain their very high melting pointsMany strong covalent bonds must be broken, which needs a lot of energy.
6
Recognise giant covalent structures from diagramsA network of atoms all joined by covalent bonds, continuing in all directions, with no separate molecules.
7
Contrast giant covalent and small molecular substancesMelting a giant structure breaks covalent bonds; melting small molecules only overcomes intermolecular forces.
Notes
What a giant covalent structure is
In a giant covalent structure, all the atoms are linked to other atoms by strong covalent bonds, forming a huge network.
There are no separate molecules.
Examples: diamond and graphite (forms of carbon) and silicon dioxide, SiO2 (silica, the main compound in sand).
In silicon dioxide, each silicon atom is bonded to four oxygen atoms and each oxygen atom to two silicon atoms, which gives the 1 : 2 ratio in SiO2.
Properties
To melt or boil a giant covalent substance, covalent bonds must be broken.
There are a huge number of strong bonds, so a very large amount of energy is needed. So these substances are solids with very high melting and boiling points.
Most do not conduct electricity, because they have no charged particles that are free to move. Graphite is the exception: it has delocalised electrons.
Recognising them from diagrams
Look for many atoms joined by lines (bonds) in a repeating network, with bonds going off the edges of the diagram.
Diamond: each carbon atom joined to four others in 3D. Graphite: flat layers of hexagons. Silicon dioxide: silicon and oxygen atoms alternating in a network.
Not giant covalent: small, complete molecules; chains in brackets with n (polymers); + and − ions (ionic).
Giant versus small: silicon dioxide and carbon dioxide grade 7+
Carbon dioxide, CO2, is made of small molecules with weak intermolecular forces between them, so it is a gas at room temperature.
Silicon dioxide, SiO2, is a giant covalent structure with strong covalent bonds throughout, so it does not melt until over 1600 °C.
Cheatsheet
Giant covalent: all atoms joined by strong covalent bonds in a network
Melting breaks many strong covalent bonds → very high melting points
No separate molecules
Usually do not conduct (no charged particles free to move); graphite is the exception
SiO2: each Si bonded to 4 O, each O bonded to 2 Si
How to answer each type of question
Explain why a giant covalent substance has a very high melting point
3 marks5
Name the structure: giant covalent.
Say there are many strong covalent bonds (every atom is linked).
Say these must be broken, which needs a lot of energy.
Example. Silicon dioxide does not melt until over 1600 °C. Explain why, in terms of its structure and bonding.
Show the model answer
Silicon dioxide has a giant covalent structure (1). There are many strong covalent bonds between the atoms (1). A lot of energy is needed to break these bonds (1).
Identify a giant covalent substance from data
3 marks6
Look for a very high melting point.
Look for 'does not conduct', even when molten (so it is not ionic or metallic).
Give the structure and quote the evidence.
Example. Substance W melts at 3500 °C. It does not conduct electricity when solid or when molten. Give the type of structure of W. Give two reasons from the data.
Show the model answer
Giant covalent (1). It has a very high melting point (1). It does not conduct electricity even when molten, so it is not ionic or metallic (1).
Explain the difference between a giant covalent and a small molecular substance
4 marks7
Identify each structure: small molecules or giant covalent.
Small molecules: weak intermolecular forces, little energy needed to overcome them.
Giant covalent: many strong covalent bonds must be broken, a lot of energy needed.
Make clear that no covalent bonds break when the small molecules melt or boil.
Example. Carbon dioxide is a gas at room temperature. Silicon dioxide is a solid with a very high melting point. Explain this difference in terms of structure and bonding.
Show the model answer
Carbon dioxide is made of small molecules (1) with weak intermolecular forces between them, which need little energy to overcome (1). Silicon dioxide has a giant covalent structure (1) in which many strong covalent bonds must be broken, which needs a lot of energy (1).
Shortcuts and memory tricks
Giant covalent: no molecules, so melting means breaking covalent bonds.
Sand (silica) and diamond both have very high melting points: both are giant covalent.
Very high melting point + doesn't conduct even when molten → giant covalent. Graphite is the one that does conduct.
Where marks are lost
Saying diamond or silicon dioxide has intermolecular forces. They have no separate molecules.
Saying silicon dioxide has a high melting point because of strong intermolecular forces.
Writing 'strong bonds' without 'covalent', or saying silicon dioxide has ionic bonds.
Confusing silicon dioxide (giant covalent) with carbon dioxide (small molecules).
Saying that all giant covalent substances are hard and don't conduct: graphite is soft and conducts.
Exam technique
The key phrase is 'many strong covalent bonds', which must be broken and need a lot of energy.
In compare questions, write about both substances: explaining only one earns at most half the marks.
Identify the structure first, from the formula, the diagram or the data, then explain the property.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Substance X has a giant covalent structure. Predict the state of X at room temperature.
solid
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Some substances have giant covalent structures.
(a) Which substance has a giant covalent structure? Tick (✓) one box.[1]
Silicon dioxide
Water
Sodium chloride
Oxygen
(b) Explain why substances with giant covalent structures have very high melting points.[2]
(c) Substance X has a giant covalent structure. Predict the state of X at room temperature.[1]
Show the answer and mark scheme
(a)Answer: Silicon dioxide
(b)Answer: All the atoms are linked by many strong covalent bonds, and a lot of energy is needed to break them.
(many) strong covalent bonds
a lot of energy is needed to break / overcome the bonds
(c)Answer: solid
solid
Question 2Medium6 marks
In silicon dioxide, each silicon atom is covalently bonded to four oxygen atoms. Each oxygen atom is covalently bonded to two silicon atoms.
(a) Explain why the formula of silicon dioxide is SiO2.[2]
(b) Explain why silicon dioxide has a high melting point.[3]
(c) Silicon dioxide does not conduct electricity. Suggest why.[1]
Show the answer and mark scheme
(a)Answer: Each oxygen atom is shared between two silicon atoms, so each silicon atom has 4 × ½ = 2 oxygen atoms.
each oxygen atom is shared between two silicon atoms
so there are 4 × ½ = 2 oxygen atoms for every silicon atom
(b)Answer: It is a giant covalent structure with many strong covalent bonds, which need a lot of energy to break.
giant covalent structure
many strong covalent bonds
a lot of energy is needed to break the bonds
(c)Answer: It has no delocalised electrons or ions that are free to move.
no delocalised electrons / no ions / no charged particles free to move
Question 3Hard6 marks
Silicon carbide, SiC, is used on cutting discs and grinding wheels. In silicon carbide, each silicon atom is bonded to four carbon atoms and each carbon atom is bonded to four silicon atoms, in a structure similar to diamond.
(a) Suggest two physical properties of silicon carbide.[2]
(b) Explain, in terms of structure and bonding, why silicon carbide is suitable for cutting discs.[3]
(c) Explain why the formula of silicon carbide is SiC and not SiC4.[1]
Show the answer and mark scheme
(a)Answer: Very hard; very high melting point.
very hard
very high melting point
does not conduct electricity
insoluble in water
(b)Answer: It is a giant covalent structure in which each atom is joined to four others by strong covalent bonds in a rigid 3D network, so it is very hard.
giant covalent structure
each atom is joined to four others by strong covalent bonds (in a rigid 3D network)
so it is very hard / a lot of energy is needed to break the bonds
(c)Answer: Each carbon atom is also bonded to four silicon atoms, so overall there is one carbon atom for every silicon atom.
each carbon atom is also bonded to four silicon atoms, so the ratio of atoms is 1 : 1