AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The structures of the following crystals as examples of these four types…
Full specification wording
Bonding and physical properties: The structures of the following crystals as examples of these four types of crystal structure: diamond graphite ice iodine magnesium sodium chloride.
Six named crystals illustrate the four types.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Six named crystals illustrate the four types. Diamond is giant covalent, each carbon bonded to four others tetrahedrally at 109.5°, so it is extremely hard, melts very high and does not conduct. Graphite is also giant covalent but each carbon bonds to only three, leaving one delocalised electron per atom, so it conducts along its layers, which are held together by weak London forces and slide easily. Iodine is molecular, a lattice of I₂ molecules held by London forces. Ice is molecular, held by hydrogen bonds in an open tetrahedral cage. Magnesium is giant metallic with 2⁺ ions and two delocalised electrons per atom. Sodium chloride is giant ionic with 6:6 co-ordination.
What earns the marks
- The correct structure type assigned to the named crystal.
- The arrangement described, such as each carbon bonded to four others in diamond or three in graphite.
- Naming the force broken when that crystal melts.
- The property explained from that structure, such as conduction in graphite from delocalised electrons.
Common mistakes and how to put them right
Saying graphite conducts because it contains ions rather than delocalised electrons.
Giving the bond angle in diamond as 90° or 120° instead of 109.5°.
Describing iodine as a giant covalent structure because it is a solid that sublimes.
Explaining the low density of ice without naming hydrogen bonds holding an open lattice.
Exam tips
- Learn the six crystals as a table of structure, bonding, melting point and conductivity.
- For graphite, mention both the strong covalent bonds within layers and the weak forces between them.
- Quote co-ordination numbers where you know them; they show you can describe the lattice, not just name it.
Try the lesson
The start of the Bonding and physical properties: The structures of the following crystals as examples of these four types of crystal structure: diamond graphite ice iodine magnesium sodium chloride. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following crystals is an example of a macromolecular (giant covalent) structure?
Listen
Six named crystals illustrate the four types. Diamond is giant covalent, each carbon bonded to four others tetrahedrally at 109.5°, so it is extremely hard, melts very high and does not conduct. Graphite is also giant covalent but each carbon bonds to only three, leaving one delocalised electron per atom, so it conducts along its layers, which are held together by weak London forces and slide easily. Iodine is molecular, a lattice of I₂ molecules held by London forces. Ice is molecular, held by hydrogen bonds in an open tetrahedral cage. Magnesium is giant metallic with 2⁺ ions and two delocalised electrons per atom. Sodium chloride is giant ionic with 6:6 co-ordination. A strong answer earns marks for these points. the correct structure type assigned to the named crystal. the arrangement described, such as each carbon bonded to four others in diamond or three in graphite. naming the force broken when that crystal melts. the property explained from that structure, such as conduction in graphite from delocalised electrons. Exam tip: Learn the six crystals as a table of structure, bonding, melting point and conductivity.
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Key words
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diamond and graphite
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ice and iodine
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magnesium lattice
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sodium chloride lattice
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See every definition
- diamond and graphite
- Both are giant covalent, but each carbon in diamond bonds to four others tetrahedrally at 109.5°, while in graphite each carbon bonds to only three, leaving one delocalised electron per atom.
- ice and iodine
- Both are molecular: iodine is a lattice of I₂ molecules held by London forces, and ice is held by hydrogen bonds in an open tetrahedral cage.
- magnesium lattice
- A giant metallic structure with 2⁺ ions and two delocalised electrons per atom.
- sodium chloride lattice
- A giant ionic structure with 6:6 co-ordination.
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