Uses of aluminium
3 marks · NormalExplain, using ideas about atomic structure, why aluminium alloys are harder than pure aluminium. Include reference to the effect of atomic size differences.
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In pure aluminium, all atoms are the same size and arranged in regular layers, so the layers can slide easily, making the metal soft. In alloys, added elements have different sized atoms, which distort the lattice and prevent the layers from sliding, making the material harder.
Also accept: Alloys contain atoms of different sizes which prevent movement of layers, increasing strength
Common mistakes: Saying the presence of other elements weakens the metal
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Marking points
M1 · 1 mark · Mentions the regular arrangement/layers in pure aluminium allows sliding.
M2 · 1 mark · Correctly states that different sized atoms in the alloy disrupt the lattice.
A1 · 1 mark · Links disruption to prevention of layer movement, resulting in a harder alloy.
Worked solution
Pure aluminium consists of identical atoms arranged in regular layers, which can slide over each other easily, making it soft.
In alloys, the added atoms are a different size, disrupting the regular layers.
This distortion stops the layers from sliding, so the alloy is harder and stronger than pure aluminium.