Simple molecular substances have low melting points because only weak forces between molecules are overcome. Ionic compounds have high melting points because strong electrostatic attractions between oppositely charged ions in a giant lattice must be overcome. The key is to name which bonds or forces are involved in each case.
This lesson compares the two main cases in bonding and structure. It depends on ion formation and covalent bonding, and it runs alongside giant structures.
What changes when a substance melts?
Melting is a physical change. The particles keep their identity but gain enough energy to move past each other. So the question is always what holds the particles in place in the solid.
- In a simple molecular solid, the particles are molecules. They are held by weak intermolecular forces. The covalent bonds inside each molecule are strong, but they do not break on melting.
- In an ionic solid, the particles are ions in a lattice. Each ion is attracted to several neighbours of opposite charge by strong electrostatic forces.
A useful trend: among the halogens, chlorine is a gas, bromine is a liquid and iodine is a solid at room temperature. Larger molecules generally have stronger intermolecular forces, so more energy is needed to separate them.
How to write the comparison, step by step
- Identify the type of substance from the data given.
- Name the particles that are in the solid: molecules or ions.
- Name what holds them together: intermolecular forces or ionic bonds.
- Compare strength: weak against strong.
- Link to energy and melting point: little energy, low melting point; a lot of energy, high melting point.
Worked example
Two substances have the data below. The values are invented for practice.
| Substance | Melting point / °C | Conducts when solid | Conducts when molten |
|---|---|---|---|
| P | −85 | no | no |
| Q | 780 | no | yes |
Identify each substance type and explain the difference in melting point.
P has a low melting point and does not conduct in either state, so it is a simple molecular substance. Its molecules are held by weak intermolecular forces, and only these forces are overcome on melting. The covalent bonds inside the molecules stay intact, so little energy is needed.
Q has a high melting point and conducts only when molten, so it is an ionic compound. Its ions are held in a giant lattice by strong electrostatic attractions. A lot of energy is needed to overcome them, so the melting point is high. Once molten, the ions can move, so it conducts.
The mistake to watch for
The most common error is to say that covalent bonds break when a molecular substance melts.
Mistaken answer: “P has a low melting point because its covalent bonds are weak.”
The covalent bonds inside molecules are strong. They are simply not the bonds that are broken.
The correction is to write that the intermolecular forces are weak and that they are the ones overcome. A second slip is to say the ionic bond is between molecules. In a lattice there are ions, not molecules.
Check yourself
Try these without looking back, then open each answer.
1. When water boils, what is overcome: covalent bonds or intermolecular forces?
Show answer
Intermolecular forces between H₂O molecules. The molecules stay as H₂O in the gas, so the covalent bonds inside them are not broken.
2. Sodium chloride melts at a much higher temperature than iodine. Explain why.
Show answer
Sodium chloride is a giant ionic lattice, held by strong electrostatic attractions between Na⁺ and Cl⁻ ions, which need a lot of energy to overcome. Iodine is simple molecular, so only weak intermolecular forces are overcome, which needs much less energy.
3. A student says that a substance with a low melting point must contain weak covalent bonds. Correct the statement.
Show answer
A low melting point means the forces between molecules are weak. The covalent bonds inside the molecules can still be strong, because they are not broken when the substance melts.
Where this leads next
Next, explain why the same two types behave differently in electricity tests in explaining conductivity in solid and molten states. Then use the bonding and structure practice set to test all five lessons together.
Students who keep attaching the wrong bond to the wrong substance often need a single clear routine, which our teachers build in online one-to-one Chemistry tuition.