1. 1
   An electrician selects copper wire for household wiring because it conducts electricity extremely well, and separately selects aluminium cable for overhead power lines because of its low density.

   (a)

   In the metallic structure of copper, the outer-shell electrons of each atom are described as 'delocalised'. What does the term 'delocalised electrons' mean in this context?

   [1 mark]

   • AElectrons that are free to move throughout the whole metal structure, not fixed to one particular atom
   • BElectrons that remain fixed to their original atom at all times
   • CElectrons that have been completely removed from the metal
   • DElectrons that are shared between only two neighbouring atoms

   (b)

   Copper's structure consists of positive copper ions arranged in a regular lattice, surrounded by a 'sea' of delocalised electrons. What holds the positive ions and the sea of delocalised electrons together in this structure?

   [1 mark]

   • ACovalent bonds between neighbouring copper atoms
   • BElectrostatic attraction between the positive ions and the negatively charged delocalised electrons
   • CWeak intermolecular forces between copper molecules
   • DHydrogen bonds between copper ions

   (c)

   Explain, in terms of its structure and bonding, why copper is an excellent conductor of electricity.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A blacksmith heats a bar of iron and hammers it into a curved horseshoe shape without it cracking or breaking, unlike a brittle ionic solid which would shatter under the same force.

   (a)

   Which property of metals, resulting from their structure, allows the iron to be shaped in this way without breaking?

   [1 mark]

   • APoor electrical conductivity, which prevents cracking
   • BBrittleness, meaning the metal shatters easily under force
   • CLow melting point, allowing it to be shaped only when fully molten
   • DMalleability, meaning the metal can be hammered or bent into shape

   (b)

   Unlike an ionic solid, which would shatter if struck with the same force, the iron bar simply bends into a new shape. Which explanation correctly accounts for this difference in behaviour?

   [1 mark]

   • AIn a metal, layers of ions repel each other strongly when struck, causing the metal to shatter
   • BIn a metal, the positive ions are held in a fixed pattern by strong covalent bonds that cannot be broken
   • CIn a metal, layers of positive ions can slide over one another while remaining surrounded by the sea of delocalised electrons, which continues to hold the structure together
   • DMetals contain no delocalised electrons, so their layers cannot slide past one another

   (c)

   Explain, in terms of structure and bonding, why metals such as iron are malleable, referring to the layers of ions and the delocalised electrons.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A construction engineer needs to choose a metal for a bridge cable and considers aluminium, which has a giant metallic lattice structure, and must weigh up its electrical conductivity and its resistance to bending forces.

   (a)

   Describe the arrangement of particles in a typical metallic lattice such as aluminium, referring to the positive ions and the delocalised electrons.

   [3 marks]

   (b)

   Explain, in terms of structure and bonding, why aluminium is both a good conductor of electricity and able to withstand being bent repeatedly without snapping, making it a suitable material for the bridge cable.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A metallurgist compares pure iron with a sample of stainless steel, an alloy of iron with chromium, nickel and carbon, and finds that the stainless steel is significantly harder and stronger than pure iron, even though both contain iron as the main component and both conduct electricity well.

   (a)

   Explain fully, in terms of structure and bonding, why pure iron conducts electricity and is malleable, and explain why introducing atoms of different sizes, as in stainless steel, makes the resulting alloy harder and stronger than the pure metal.

   [6 marks]

   (b)

   The metallurgist also examines brass, an alloy of copper and zinc used to make musical instruments, and finds that brass is harder than pure copper but still conducts electricity reasonably well and can be shaped into complex forms. Explain fully, using the concept of metallic bonding and the effect of different-sized atoms in a lattice, why brass is harder than pure copper while still retaining useful electrical conductivity and some ability to be shaped.

   [6 marks]

   Total for question 4: 12 marks

End of questions