Giant ionic structures
2 marks · HardLead(II) bromide (PbBr_2) is an example of a giant ionic compound. Explain why solid lead(II) bromide does not conduct electricity, but molten lead(II) bromide does.
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Also accept: Solid PbBr_2: Ions are fixed in the lattice and cannot move, so no conduction. Molten PbBr_2: Ions are free to move, allowing electrical conduction.; In solids, ions are held in place and cannot carry charge. In molten, the ions move freely and carry current.
Common mistakes: Saying electrons carry current in ionic compounds.; Mixing up solid and molten states.
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Marking points
M1 · 1 mark · Describes that in solid lead(II) bromide, ions cannot move because they are held in the lattice, so cannot conduct electricity.
M2 · 1 mark · Describes that when molten, the ions are free to move and can carry charge, so PbBr_2 conducts electricity.
Worked solution
In solid PbBr_2, the ions are held rigidly in the lattice and cannot move, so they cannot conduct electricity.
When PbBr_2 is molten, the lattice breaks down, and the ions are free to move, allowing them to carry charge and conduct electricity.