Properties of Simple Molecular Compounds
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge IGCSE Chemistry (0620)
Two Different Forces
Simple molecular substances have two very different types of force acting on them, and it is essential not to confuse the two:
- Covalent bonds WITHIN each molecule: strong - these hold the atoms of one molecule together and do not break on melting or boiling
- Intermolecular forces BETWEEN separate molecules: weak - these hold molecules to their neighbours, and are what breaks when a substance melts or boils
You are not required to know the names of specific types of intermolecular force - only that they exist and are weak compared to covalent bonds.
When water boils, only the weak dashed forces between molecules break - the strong O-H bonds within each molecule stay intact, so steam is still made of H₂O molecules
Low Melting and Boiling Points
- Only the weak intermolecular forces need to be overcome to melt or boil a simple molecular substance, so this requires relatively little energy
- As a result, simple molecular substances typically have low melting and boiling points, and are often gases or liquids at room temperature (e.g. H₂O: 0-100°C, CH₄: -182°C, CO₂ sublimes at -78°C)
- This contrasts sharply with ionic compounds like NaCl (801°C), where every ion is held by strong electrostatic forces throughout a giant lattice
Worked Example: Comparing Structures
- Question: methane (CH₄) is a gas at room temperature, while sodium chloride (NaCl) is a solid with a melting point of 801°C. Explain this difference in terms of structure and bonding. [3]
- Answer: methane is a simple molecular substance - only weak intermolecular forces exist between separate CH₄ molecules, which need little energy to overcome [1]; sodium chloride is a giant ionic lattice - strong electrostatic forces of attraction exist between Na⁺ and Cl⁻ ions throughout the whole structure [1]; overcoming the ionic forces to melt NaCl requires far more energy than overcoming the intermolecular forces in methane [1]
Poor Electrical Conductivity
Simple molecular compounds do not conduct electricity in any state (solid, liquid, or gas).
- The molecules are electrically neutral overall
- There are no ions and no free (delocalized) electrons available to carry charge
Common Mistakes
- ❌ Thinking that when a simple molecular substance melts or boils, the molecules themselves break apart into separate atoms
- ✓ only the weak forces between molecules break; the strong covalent bonds within each molecule remain intact, so the molecules stay whole
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