Ionic Bonding
8 marks · HardSodium chloride and magnesium oxide are both ionic compounds with giant ionic lattice structures.
Describe, in terms of ions, the arrangement and structure of sodium chloride.
Magnesium oxide, like sodium chloride, forms a giant ionic lattice, but in magnesium oxide every magnesium ion is Mg^{2+} and every oxide ion is O^{2-}. Magnesium oxide has a higher melting point than sodium chloride. Using your answer to part (a), explain why magnesium oxide has a higher melting point than sodium chloride.
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(a) Sodium chloride has a giant ionic lattice structure. Sodium ions (Na^+) and chloride ions (Cl^-) are arranged in a regular, repeating 3D pattern. Each Na^+ ion is surrounded by six Cl^- ions, and each Cl^- ion is surrounded by six Na^+ ions. There are no individual molecules; the arrangement extends in all directions. · must show giant ionic lattice, alternating Na^+ and Cl^- ions, each ion surrounded by six oppositely charged ions, extends in all directions / regular repeating
(b) Magnesium oxide has a higher melting point than sodium chloride because the ions in magnesium oxide (Mg^{2+} and O^{2-}) have a greater charge than those in sodium chloride (Na^+ and Cl^-). The greater charge results in stronger electrostatic attractions between ions in the lattice, so more energy is required to break these bonds and melt the solid. · must show reference to higher charge on Mg^{2+} and O^{2-}, stronger electrostatic attraction, relating to melting point / more energy needed
Common mistakes: Describing sodium chloride as a molecule; Failing to mention the regular repeating arrangement of ions; Not linking strength of attraction to charge size
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Marking points
M1 (a) · 1 mark · Identifies sodium chloride as a giant ionic lattice and NOT a molecule.
M2 (a) · 1 mark · Describes the regular, repeating (3D) arrangement/structure.
A1 (a) · 1 mark · States that each sodium ion is surrounded by 6 chloride ions (and vice versa).
A2 (a) · 1 mark · States that the structure extends in all directions / all ions surrounded.
M3 (b) · 1 mark · Uses the idea of lattice/structure from (a) to compare to MgO. (follow-through)
M4 (b) · 1 mark · States that Mg^{2+} and O^{2-} have higher charges than Na^+ and Cl^-.
A3 (b) · 1 mark · Links higher charge to stronger electrostatic forces of attraction.
A4 (b) · 1 mark · States that more energy is needed to overcome these forces / melting point is higher. (follow-through)
Worked solution
a) Sodium chloride consists of a giant ionic lattice. In this structure, sodium ions (Na^+) and chloride ions (Cl^-) are arranged in a regular and repeating 3D pattern. Each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions. The lattice extends in all directions, with no individual molecules.
b) Magnesium oxide has a similar lattice structure to sodium chloride, but with Mg^{2+} and O^{2-} ions instead. The higher charge on the magnesium and oxide ions (2+ and 2- instead of 1+ and 1-) leads to much stronger electrostatic forces of attraction holding the lattice together. As a result, more energy is needed to overcome these forces when melting magnesium oxide, so it has a higher melting point than sodium chloride.