Solids: MDCAT Notes and MCQs
Crystalline vs amorphous
- Crystalline solids: particles in a definite, repeating 3-D arrangement; sharp melting point, definite geometry, anisotropy (properties vary with direction), cleavage planes, symmetry. E.g. NaCl, diamond, ice.
- Amorphous solids: no long-range order, melt over a range, isotropic. E.g. glass, plastics, rubber.
Crystal lattice
- A crystal lattice is the 3-D array of points representing particle positions. The smallest repeating part is the unit cell, defined by edge lengths a, b, c and angles α, β, γ.
- 7 crystal systems (cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, rhombohedral) and 14 Bravais lattices.
- Isomorphism (same crystal form, different substances, e.g. NaF/MgO) and polymorphism (same substance, different forms, e.g. CaCO3 as calcite/aragonite); allotropy for elements.
Factors affecting shape of ionic crystals
- Size of ions (radius ratio r+/r−) — determines the coordination number (NaCl: 6; CsCl: 8).
- Charge on ions — crystal must be electrically neutral (ratio of cations to anions).
- Packing / polarizability — ions pack to give maximum attraction and minimum repulsion.
Ionic vs molecular crystals
| Ionic crystals | Molecular crystals |
|---|---|
| Positive & negative ions held by strong electrostatic forces | Molecules held by weak van der Waals / dipole / H-bond forces |
| Hard, brittle, high m.p. | Soft, low m.p., volatile |
| Conduct electricity when molten/aqueous | Non-conductors |
| NaCl, KBr, CsCl | Ice, iodine, dry ice (CO2), sugar, naphthalene |
Other types: covalent network (diamond, SiO2 — very hard, very high m.p.) and metallic (sea of electrons — conductors).
Lattice energy
Energy released when 1 mole of an ionic crystal is formed from its gaseous ions (or energy needed to separate it). NaCl ≈ −787 kJ/mol. Larger for smaller ions and higher charges (MgO ≫ NaCl). Higher lattice energy → higher m.p., lower solubility.
PMDC learning outcomes for this unit
- Crystalline Solids: 5.1 Describe crystalline solid
- Crystalline Solids: 5.2 Name three factors that affect the shape of the ionic crystals.
- Crystalline Solids: 5.3 Give brief description of ionic and molecular crystals.
- Crystalline Solids: 5.4 Explain the structure of a crystal lattice
- Crystalline Solids: 5.5 Define lattice energy