A chemical bond acts between atoms inside a particle. An intermolecular force acts between separate molecules. Melting and boiling overcome the second, not the first.
This lesson is part of SPM Chemistry chemical bonding. It relies on the molecules drawn in drawing covalent structures.
How do I tell them apart?
Ask where the force is acting. If it joins atoms within one molecule or one lattice, it is a bond. If it holds one whole molecule to another, it is an intermolecular force.
| Chemical bond | Intermolecular force | |
|---|---|---|
| Acts between | atoms or ions | separate molecules |
| Relative strength | strong | weak |
| Example | O–H inside water | attraction between two water molecules |
| Broken when | a reaction occurs | a substance melts or boils |
Worked example: iodine and sodium chloride
Iodine is a simple molecular solid made of I₂ molecules. Sodium chloride is a lattice of Na⁺ and Cl⁻ ions.
When iodine melts, only the weak forces between I₂ molecules are overcome. The I–I bonds stay, so liquid iodine is still I₂. Little energy is needed, so the melting point is low.
When sodium chloride melts, the strong attractions between oppositely charged ions in the lattice are overcome. That takes a lot of energy, so the melting point is high.
The mistake that loses marks
A student writes “iodine has a low melting point because its covalent bonds are weak”. The I–I bond is strong, and it is not broken when iodine melts.
The fix is to name the particles and the force. The correct line is “weak intermolecular forces between iodine molecules are overcome”.
Check yourself
Naphthalene is a simple molecular solid that melts at a low temperature. Explain this, and say why it does not conduct electricity when molten.
Answer
Naphthalene is made of separate molecules. Only the weak forces between the molecules are overcome on melting, which needs little energy. The covalent bonds inside each molecule are not broken.
It does not conduct because the molten liquid contains neutral molecules with no freely moving ions or electrons.
What to study next
Apply the distinction in explaining properties from bonding and structure. Then use the practice set to test the wording.
Write repeated wording slips in the mistake log and paper-error review. For a teacher to go through your explanations with you, see online one-to-one Chemistry tuition.