Knowledge Organiser: Simple Molecules
Key Terms
- Simple molecular: small molecules with covalent bonds within and weak forces between
- Intermolecular forces: weak attractions BETWEEN molecules
- Intramolecular bonds: strong covalent bonds WITHIN molecules
Must-Know Facts
- Low MP/BP — weak intermolecular forces break (not covalent bonds)
- Do NOT conduct — no ions or free electrons
- Often gases or liquids at room temperature
- Larger molecule = more intermolecular forces = higher MP
- Examples: H₂O, CO₂, O₂, N₂, CH₄, NH₃, Cl₂
Key Equations
- No calculation equations — descriptive topic
- Boiling point comparison: larger molecule → more intermolecular forces → higher BP
- No conduction: no free ions or free electrons in simple molecular compounds
Common Mistakes
- Saying covalent bonds break when simple molecules melt: It is the INTERMOLECULAR FORCES between molecules that break — the covalent bonds within molecules stay intact
- Saying simple molecular substances conduct electricity: They have no free ions or free electrons — no electrical conduction is possible
- Thinking all covalent substances have low melting points: Only simple molecular ones do — giant covalent structures (diamond, graphite) have very high melting points
- Ignoring molecule size when comparing boiling points: Larger molecules have more electrons → stronger intermolecular forces → higher boiling point
Memory Aid
- Larger molecule → more intermolecular forces → higher melting point
- O₂ (tiny) = gas at room temp; wax (large) = solid at room temp
- O₂, N₂, CO₂, CH₄, NH₃, HCl → all gases (tiny molecules, very weak forces)
- H₂O, Br₂, Hg → liquids (slightly stronger forces)
- I₂, wax → solids (larger molecules, stronger forces)