The complete lecture — solids come alive in the live panel as you read. Scroll down; the animation keeps pace, and you can explore the unit-cell viewer yourself.
1 — The solid state
Solid particles are packed closely in fixed positions and can only vibrate — so a solid has a definite shape and volume, is incompressible and rigid.
2 — Crystalline vs amorphous
- Crystalline — long-range order; sharp melting point; anisotropic (NaCl, quartz).
- Amorphous — no long-range order; softens over a range; isotropic (glass, rubber).
3 — Types of crystalline solids
| Type | Force |
|---|
| Ionic (NaCl) | electrostatic |
| Covalent (diamond) | covalent network |
| Molecular (ice) | van der Waals / H-bond |
| Metallic (Cu) | electron sea |
4 — Crystal lattice & unit cell
- Unit cell — smallest repeating unit of the lattice; defined by edges a, b, c and angles α, β, γ.
5 — The seven crystal systems
Classified by the edges & angles: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, rhombohedral.
6 — Cubic unit cells
Atoms per cellSC = 8×⅛ = 1 · BCC = 8×⅛+1 = 2 · FCC = 8×⅛+6×½ = 4
Move the slider in the live panel to switch the cubic type and watch the atom count change.
7 — Isomorphism & polymorphism
- Isomorphism — different substances, same crystal form.
- Polymorphism — same substance, different forms (calcite/aragonite).
8 — Allotropy
- Allotropy — an element existing in more than one form (diamond, graphite, fullerene).
9 — Lattice energy
Higher charge & smaller ions → greater lattice energy → higher melting point (MgO > NaCl).
10 — Properties of crystalline solids
- Sharp melting point.
- Cleave along definite planes.
- Anisotropic; definite geometric shape.
11 — Worked questions
atoms per cell
FCC: (8×⅛) + (6×½) = 4 atoms
reasoning
Graphite conducts (delocalised electrons); diamond does not (all 4 e⁻ bonded).
12 — Exam recap
- Solid properties; crystalline vs amorphous.
- Four crystal types & their properties.
- Lattice & unit cell; the 7 systems.
- Cubic cells SC/BCC/FCC; atoms per cell.
- Isomorphism, polymorphism, allotropy.
- Lattice energy & crystal properties.