The physical properties of ionic substances
5 marks · HardCrystals of sodium chloride are often seen as large, regular cubes under the microscope.
State what is meant by the term 'crystalline' when describing a substance.
Describe the arrangement of ions in the giant ionic lattice of sodium chloride as it relates to its crystalline appearance.
Explain why the regular arrangement of ions results in sodium chloride crystals having a regular shape.
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(a) It means the substance has a regular, repeating arrangement of particles forming a definite pattern.
(b) In the lattice, each Na^+ ion is surrounded by six Cl^- ions and each Cl^- by six Na^+ ions—this arrangement repeats in all directions throughout the crystal.
(c) Because the ions are arranged in a regular three-dimensional lattice, the crystal as a whole forms a regular shape that reflects this internal structure.
Common mistakes: Equating crystalline with being hard rather than regular structure; Describing NaCl as molecules
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Marking points
B1 (a) · 1 mark · Defines crystalline as a regular, repeating arrangement of particles or ions.
B2 (b) · 1 mark · Describes that each sodium ion is surrounded by six chloride ions and vice versa.
B3 (b) · 1 mark · Correctly states this pattern is repeated throughout the lattice (not limited to one part).
B4 (c) · 1 mark · Links regular ionic arrangement to the overall regular (crystalline) shape of the crystal.
B5 (c) · 1 mark · Mentions the lattice arrangement extends throughout the crystal leading to the regular (often cubic) shape.
Worked solution
a) Crystalline means having a regular, repeating pattern of ions or particles in the structure.
b) Sodium chloride forms a giant ionic lattice where each sodium ion is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions. This pattern repeats in all directions so the arrangement is regular throughout.
c) The regularity of the lattice leads to a regular, often cube-shaped crystal. The geometric shape of the crystal reflects the repeating three-dimensional pattern of the ions inside the lattice.