2.2.2Structure, Bonding and the Properties of Matter
Properties of Simple Molecules
5 min read 5 parts Lesson 8 of 14 1 quiz
Section 1 of 5Melting and Boiling Points
Simple molecules contain a fixed number of atoms joined by strong covalent bonds, but the molecules are held to each other only weakly.
Covalent bonds
- Atoms inside each molecule are joined by strong covalent bonds, and these bonds are not broken when the substance melts or boils.
Intermolecular forces
- Weak forces act between separate molecules, and only a small amount of energy is needed to overcome them.
Low melting point
- Simple molecular substances are usually gases or liquids at room temperature, or solids with low melting points.
Size effect
- Larger molecules have stronger intermolecular forces, so their melting and boiling points are generally higher.
| Substance | Structure | Melting point |
|---|---|---|
| Oxygen (O₂) | Simple molecular | About −219 °C |
| Iodine (I₂) | Simple molecular | About 114 °C |
| Water (H₂O) | Simple molecular | 0 °C |
Section 2 of 5Electrical Conductivity
Conduction needs charged particles that can move, and simple molecules contain neither ions nor free electrons.
1
Simple molecules have no ions and no free electrons, so there are no charged particles that can carry a current.
2
This remains true when the substance is molten, because the molecules are neutral whatever their state.
3
Most covalent substances therefore behave as insulators, and they do not conduct in the solid state either.
4
The plastic coating on electrical wiring is made from covalent molecules, which is why it stops current escaping.
Section 3 of 5Explaining Properties Correctly
Explaining Properties Correctly
Melting
- The weak intermolecular forces are overcome, not the covalent bonds inside the molecules.
Boiling
- Boiling separates molecules from each other, while the molecules themselves stay intact as a gas.
Conduction
- Without ions or delocalised electrons, no charge can flow through the substance.
Strength
- Covalent bonds are strong, so simple molecules do not break apart on melting or boiling.
Section 4 of 5Worked Example
Question Explain why iodine has a low melting point even though the atoms within each I₂ molecule are strongly bonded.
- Iodine is a simple molecular substance made of I₂ molecules.
- The two iodine atoms in each molecule are joined by a strong covalent bond.
- Melting only requires the weak intermolecular forces between the molecules to be overcome.
Answer Little energy is needed to separate the molecules from each other, so the melting point is low despite the strong bonds within each molecule.
Section 5 of 5Exam Checklist & Tips
2.2.2 – Key Points for the Exam
- Simple molecule A group of atoms joined by covalent bonds, with a fixed number of atoms in each molecule.
- Intermolecular force A weak attraction between separate molecules.
- Covalent bond A shared pair of electrons between two atoms.
- Insulator A substance that does not conduct electricity.
Common Mistakes
- Saying covalent bonds break when a simple molecular substance melts or boils.
- Describing simple molecular substances as conductors when they are molten.
- Forgetting that larger molecules have stronger intermolecular forces and therefore higher boiling points.
- Confusing intermolecular forces with the covalent bonds within a molecule.