AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to explain why alloys are harder than pure metals…
Full specification wording
Properties of metals and alloys: Students should be able to explain why alloys are harder than pure metals in terms of distortion of the layers of atoms in the structure of a pure metal.
Pure metals have a regular giant lattice in which identical atoms are arranged in layers.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 5 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Pure metals have a regular giant lattice in which identical atoms are arranged in layers. Because the atoms are all the same size, the layers can slide over one another when a force is applied, so the metal is soft and can be bent or shaped. An alloy is a mixture of a metal with other elements, often other metals. The atoms of the added element are a different size from the atoms of the main metal. When these differently sized atoms sit in the lattice, they disrupt the regular arrangement and distort the layers. The distorted layers cannot slide over one another as easily, so the alloy is harder and stronger than the pure metal.
For example: Pure iron is relatively soft, but adding carbon and other elements produces steel, which is harder and more suitable for tools, bridges and car bodies.
What earns the marks
- Pure metals contain layers of identical atoms that can slide over one another, making the metal soft.
- An alloy contains atoms of at least one different element, and these atoms have a different size from the atoms of the main metal.
- The differently sized atoms distort the regular layers in the metal lattice.
- Distorted layers cannot slide over one another as easily, so the alloy is harder than the pure metal.
- A named example, such as steel being harder than pure iron, can support the explanation.
Common mistakes and how to put them right
Saying that alloys are harder because the added atoms are heavier.
Put it right: The key point is the difference in atomic size causing distortion, not mass.
Stating that alloys have stronger bonds between atoms.
Put it right: The explanation concerns the disruption of layer sliding, not a change in bond strength.
Describing the layers as melting or breaking.
Put it right: The layers are distorted and prevented from sliding, not destroyed.
Exam tips
- Use the phrase layers of atoms and explain that they slide in a pure metal before contrasting this with an alloy.
- Refer explicitly to atoms of a different size causing distortion, rather than saying the structure is just different.
- Link the distortion directly to the consequence: the layers cannot slide, so the alloy is harder.
Try the lesson
The start of the Properties of metals and alloys: Students should be able to explain why alloys are harder than pure metals in terms of distortion of the layers of atoms in the structure of a pure metal. lesson, made from checked teaching notes for this specification point.
Have a go first
Pure copper is made of layers of identical copper atoms. Brass is an alloy of copper that contains zinc atoms of a different size. Why is brass harder than pure copper?
Listen
Pure metals have a regular giant lattice in which identical atoms are arranged in layers. Because the atoms are all the same size, the layers can slide over one another when a force is applied, so the metal is soft and can be bent or shaped. An alloy is a mixture of a metal with other elements, often other metals. The atoms of the added element are a different size from the atoms of the main metal. When these differently sized atoms sit in the lattice, they disrupt the regular arrangement and distort the layers. The distorted layers cannot slide over one another as easily, so the alloy is harder and stronger than the pure metal. For example, pure iron is relatively soft, but adding carbon and other elements produces steel, which is harder and more suitable for tools, bridges and car bodies. A strong answer earns marks for these points. Pure metals contain layers of identical atoms that can slide over one another, making the metal soft. An alloy contains atoms of at least one different element, and these atoms have a different size from the atoms of the main metal. The differently sized atoms distort the regular layers in the metal lattice. Distorted layers cannot slide over one another as easily, so the alloy is harder than the pure metal. Exam tip: Use the phrase layers of atoms and explain that they slide in a pure metal before contrasting this with an alloy.
See it
Key words
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Giant metallic lattice structure
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Layers of atoms and sliding
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Alloy as a mixture containing different-sized atoms
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Relationship between structure and physical properties
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See every definition
- Giant metallic lattice structure
- A regular arrangement in pure metals in which identical atoms are arranged in layers.
- Layers of atoms and sliding
- Because the atoms are all the same size, the layers can slide over one another when a force is applied, so the metal is soft.
- Alloy as a mixture containing different-sized atoms
- A mixture of a metal with other elements, often other metals, whose atoms are a different size from the atoms of the main metal.
- Relationship between structure and physical properties
- Differently sized atoms distort the layers so they cannot slide as easily, making the alloy harder and stronger than the pure metal.
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