Intermolecular Forces & Crystal Types
Objectives
- Distinguish between van der Waals forces and hydrogen bonds
- Compare the four crystal types (ionic, covalent network, molecular, metallic)
- Identify crystal type from physical properties
Intermolecular Forces
Weak forces acting between molecules are called intermolecular forces.
Strength of intermolecular forces: Hydrogen bonds > van der Waals forces. Van der Waals forces increase with molecular mass (more electrons).
Van der Waals forces: weak attractions between all molecules. Stronger for larger molecules.
Hydrogen bonds: H bonded to F, O, or N is attracted to lone pairs on F, O, or N of neighboring molecules. Much stronger than van der Waals forces.
Water () has a higher boiling point than (which has greater molecular mass) because of hydrogen bonding.
Four Types of Crystals
| Type | Particles | Bonding | Melting pt | Hardness | Conductivity | Examples |
|---|---|---|---|---|---|---|
| Ionic | Cations + anions | Ionic | High | Hard but brittle | Solid: no; Liquid/aq: yes | NaCl, CaO |
| Covalent network | Atoms | Covalent | Very high | Very hard | No (exception: graphite) | Diamond, SiO₂ |
| Molecular | Molecules | Intermolecular | Low | Soft | No | Ice, dry ice, I₂ |
| Metallic | Metal atoms + free e⁻ | Metallic | Varies | Varies | Yes | Fe, Cu, Au |
Comparison of crystal types
Crystal properties depend on the strength of bonding between particles. General melting point trend: covalent network > ionic > metallic > molecular.
“Molecular crystals have no covalent bonds” → Covalent bonds exist within the molecules. What’s weak is the intermolecular force between molecules, not the bonds within them.
How to identify crystal type:
- Dissolves in water and conducts electricity → ionic
- Extremely hard, very high melting point → covalent network
- Low melting point, some sublime → molecular
- Conducts electricity/heat, malleable/ductile → metallic
Check Your Understanding
Q1 Why does water have a higher boiling point than H₂S?
Q2 Dry ice (solid CO₂) is which crystal type?
Q3 When does an ionic crystal conduct electricity?
Exercises
Q1. Classify: diamond, NaCl, I₂, Cu, SiO₂, naphthalene
Solution
| Substance | Type | Reason |
|---|---|---|
| Diamond | Covalent network | C atoms linked by covalent bonds in 3D |
| NaCl | Ionic | Na⁺ and Cl⁻ ions |
| I₂ | Molecular | I₂ molecules held by van der Waals forces |
| Cu | Metallic | Cu atoms with metallic bonding |
| SiO₂ | Covalent network | Si-O covalent bonds in 3D network |
| Naphthalene | Molecular | C₁₀H₈ molecules, can sublime |