From particles to material properties
A chemical bond is an attraction that keeps atoms or ions together. A crystal lattice is the ordered three-dimensional arrangement of particles in a solid. To explain a material, identify both the bonding and the particles occupying the lattice.
Learning goals: distinguish ionic, covalent and metallic bonding; distinguish ionic, molecular, atomic-network and metallic lattices; predict melting, conductivity, hardness and brittleness from structure.
1. Three bonding models
Ionic
Electrons are transferred in the model, forming oppositely charged ions held by electrostatic attraction. Example: Na+ and Cl− in sodium chloride.
Covalent
Atoms share electron pairs. Equal or nearly equal sharing gives a nonpolar bond, as in H2; unequal sharing gives a polar bond, as in HCl.
Metallic
Positive metal ions are attracted to mobile delocalized electrons. The mobility of these electrons explains electrical and thermal conductivity.
2. Four solid structures
| Lattice | Particles and forces | Typical properties | Example |
|---|---|---|---|
| Ionic | cations and anions; strong attraction in all directions | high melting point; brittle; conducts when molten or dissolved, not as a solid | NaCl |
| Molecular | molecules; weaker intermolecular forces between them | often low melting point; usually poor conductor | iodine, ice |
| Atomic network | atoms joined by covalent bonds throughout the crystal | very hard; high melting point; usually poor conductor | diamond, SiO2 |
| Metallic | metal ions and delocalized electrons | conductive; malleable; ductile; often high melting point | Cu, Fe |