Giant ionic structures
5 marks · HardSodium sulfide (Na_2S) and aluminium oxide (Al_2O_3) are both giant ionic compounds.
Describe the arrangement of ions in the lattice of a giant ionic compound.
Explain, using your knowledge of structure, why the melting point of aluminium oxide is much higher than that of sodium sulfide.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Positive and negative ions are arranged in a regular, repeating 3D lattice. Each positive ion is surrounded by negative ions and vice versa.
(b) Al_2O_3 has a higher melting point because its ions have higher charges (Al^{3+} and O^{2-}), resulting in much stronger electrostatic attractions between ions than the Na^+ and S^{2-} ions in Na_2S. More energy is needed to break these attractions.
Common mistakes: Confusing covalent and ionic bonding.; Not linking charge to melting point.
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Marking points
M1 (a) · 1 mark · Describes lattice as a regular/repeating arrangement/structure.
M2 (a) · 1 mark · States ions alternate (positive surrounded by negative and vice versa) or that it is 3D.
M3 (b) · 1 mark · Compares charges: Al_2O_3 contains Al^{3+} and O^{2-} ions; Na_2S has Na^+ and S^{2-} ions.
M4 (b) · 1 mark · States greater charge leads to stronger electrostatic attractions in Al_2O_3.
M5 (b) · 1 mark · Explains that more energy (higher temperature) is needed to overcome these attractions, so Al_2O_3 has a much higher melting point.
Worked solution
a) In a giant ionic lattice, positive and negative ions are arranged in a regular three-dimensional structure. Each positive ion is surrounded by negative ions and each negative ion by positive ions.
b) Aluminium oxide has Al^{3+} and O^{2-} ions, which have higher charges than the Na^+ and S^{2-} ions in sodium sulfide. The stronger electrostatic attractions between Al^{3+} and O^{2-} ions mean much more energy is needed to overcome the forces and melt the lattice, so Al_2O_3 has a much higher melting point.