Small molecules (AQA GCSE Chemistry): Revision Notes
Small molecules
What are small molecules?
When atoms join together using covalent bonds, they can create small molecules. These are groups of atoms that are held together strongly within the molecule, but have weak forces between different molecules.
Critical Understanding: The bonds inside the molecule are strong, but the forces between separate molecules are weak. This fundamental difference explains all the properties of small molecules.
Ways to show small molecules
There are different methods to draw and represent small molecules, each showing different levels of detail:
Dot-and-cross diagrams
- With electron shells: Shows all the electrons and how they're shared between atoms
- Without shells: Simpler version that still shows shared electrons and lone pairs
- Both methods show bonding electrons and lone pairs clearly
Two-dimensional diagrams
- Shows covalent bonds as simple lines between atoms
- Does not show lone pairs or electron detail
- Quick and easy way to represent molecules
Ball and stick models
- Shows how atoms are arranged in space
- Gives a 3D view of the molecule's shape
- Does not show electrons or bonding detail
Essential Molecules to Practice: You need to be able to draw diagrams for these key molecules:
- (hydrogen gas)
- (chlorine gas)
- (oxygen gas)
- (nitrogen gas)
- (hydrogen chloride)
- (water)
- (methane)
- (ammonia)
Properties of small molecules
Small molecules have three fundamental properties that directly result from their bonding and structure:
1. Low melting and boiling points
Small molecules have weak intermolecular forces between them. This means it doesn't take much energy to separate the molecules from each other. The bigger the molecule, the stronger these forces become.
The strength of intermolecular forces increases with molecular size, which is why larger molecules tend to have higher melting and boiling points.
2. Usually gases or liquids at room temperature
Because of their low melting and boiling points, most small molecules exist as gases or liquids when we encounter them in everyday conditions.
3. Do not conduct electricity
Small molecules have no overall electric charge. Without charged particles that can move freely, they cannot conduct electricity.
Understanding intermolecular forces
Here's the crucial distinction that explains all small molecule behaviour:
The Key Difference:
- Covalent bonds (inside the molecule) = STRONG
- Intermolecular forces (between molecules) = WEAK
When a substance like iodine melts or boils, the covalent bonds stay intact. Only the weak forces between molecules are broken. This is why the molecules remain as even in the gas state.
Real-world example
The halogen elements demonstrate this pattern perfectly. As you move down the group from fluorine to iodine:
Halogen Trend Demonstration:
- Fluorine (): Smallest molecules → weakest intermolecular forces → gas at room temperature
- Chlorine (): Larger molecules → stronger forces → gas at room temperature
- Bromine (): Even larger → stronger forces → liquid at room temperature
- Iodine (): Largest molecules → strongest intermolecular forces → solid at room temperature
Key Points to Remember:
- Small molecules are formed when atoms join with covalent bonds
- Strong bonds hold atoms together inside each molecule
- Weak intermolecular forces exist between different molecules
- This gives small molecules low melting and boiling points
- They are usually gases or liquids at room temperature
- They do not conduct electricity because they have no overall charge