AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Bonding, structure, and the properties of matter › How bonding and structure are related to the properties of
Practise Giant covalent structures. 6 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.
Key facts
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
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
diagram
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.
Silicon dioxide (left) is one giant network. Carbon dioxide (right) is separate small molecules.
Not giant covalent: small, complete molecules; chains in brackets with n (polymers); + and − ions (ionic).
How to answer each type of question
Explain why a giant covalent substance has a very high melting point
3 marksGrade 5
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 answerHide 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).
Don’t lose marks
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.
More tips
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.
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.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name examples of giant covalent structuresDiamond and graphite (both forms of carbon) and silicon dioxide (silica).
Grade 4
State that they have very high melting pointsThey are solids at room temperature with very high melting and boiling points.
Grade 5
Explain their very high melting pointsMany strong covalent bonds must be broken, which needs a lot of energy.
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