Chemistry · Ch 11 — The Solid State
Classification of Crystalline Solids
11.3
Classification of Crystalline Solids
Most solid substances are crystalline rather than amorphous. Metals such as iron, copper and silver; non-metals such as sulphur, phosphorus and iodine; and compounds such as sodium chloride, zinc sulphide and naphthalene all form crystalline solids.
Crystalline solids can be classified in several ways depending on the purpose. Here we classify them by the nature of the intermolecular forces or bonds that hold their particles together. These forces are of four kinds:
- Van der Waals forces
- Ionic bonds
- Covalent bonds
- Metallic bonds
On this basis crystalline solids fall into four categories — molecular, ionic, metallic and covalent (network) solids — which we examine one by one in the following sub-sections.
Different types of solids (Table 1.2)
| Type of solid | Constituent particles | Bonding / forces | Examples | Physical nature | Electrical conductivity | Melting point |
|---|---|---|---|---|---|---|
| Molecular — (i) non-polar | Molecules | Dispersion / London forces | Ar, CCl₄, H₂, I₂, CO₂ | Soft | Insulator | Very low |
| Molecular — (ii) polar | Molecules | Dipole–dipole | HCl, SO₂ | Soft | Insulator | Low |
| Molecular — (iii) H-bonded | Molecules | Hydrogen bonding | H₂O (ice) | Hard | Insulator | Low |
| Ionic | Ions | Coulombic / electrostatic | NaCl, MgO, ZnS, CaF₂ | Hard but brittle | Insulator in solid state; conducts when molten or in solution | High |
Table 1.2Different Types of Solids
| Type of Solid | Constituent Particles | Bonding/Attractive Forces | Examples | Physical Nature | Electrical Conductivity | Melting Point |
|---|---|---|---|---|---|---|
| Molecular solids (i) Non polar | Molecules | Dispersion or London forces | Ar, CCl4, H2, I2, CO2 | Soft | Insulator | Very low |
| Molecular solids (ii) Polar | Molecules | Dipole-dipole interactions | HCl, SO2 | Soft | Insulator | Low |
| Molecular solids (iii) Hydrogen bonded | Molecules | Hydrogen bonding | H2O (ice) | Hard | Insulator | Low |
| Ionic solids | Ions | Coulombic or electrostatic | NaCl, MgO, ZnS, CaF2 | Hard but brittle | Insulators in solid state but conductors in molten state and in aqueous solutions | High |
| Metallic solids | Positive ions in a sea of delocalised electrons | Metallic bonding | Fe, Cu, Ag, Mg | Hard but malleable and ductile | Conductors in solid state as well as in molten state | Fairly high |