Small molecules (AQA GCSE Chemistry Combined Science): Revision Notes
Small molecules
What are small molecules?
Small molecules are formed when atoms join together using covalent bonds. These molecules contain just a few atoms bonded together. Examples include water (H₂O), ammonia (NH₃), and hydrogen gas (H₂).
Small molecules are the simplest type of covalently bonded structures, typically containing between 2-20 atoms. Unlike giant structures, they exist as discrete, separate units.
How to show small molecules
There are different ways to draw and represent small molecules. Each method shows different information about the molecule and has its own advantages for understanding molecular structure.
Dot-and-cross diagrams
With electron shells:
- Shows all the bonding electrons and lone pairs
- Displays the electron shells around each atom
- Does not show the actual shape of the molecule
- Useful for understanding how electrons are shared
Without electron shells:
- Still shows bonding electrons and lone pairs
- Simpler to draw than the full shell version
- Does not show the shells or the real shape
Worked Example: Drawing Water (H₂O)
Step 1: Count the electrons
- Oxygen has 6 outer electrons
- Each hydrogen has 1 electron
- Total = 8 electrons
Step 2: Show the sharing
- Oxygen shares 1 electron with each hydrogen
- Oxygen has 2 lone pairs remaining
Two-dimensional diagrams
- Shows the covalent bonds as lines between atoms
- Much quicker to draw than dot-and-cross diagrams
- Does not show lone pairs of electrons
- Easy way to represent the molecule structure
Ball and stick models
- Shows how the atoms are actually arranged in 3D space
- Different sized balls represent different atoms
- Sticks represent the covalent bonds
- Does not show the electrons themselves
- Good for understanding molecular shape
Properties of small molecules
Small molecules have distinctive properties that set them apart from other types of chemical structures. Understanding these properties helps explain their behaviour in different conditions.
Low melting and boiling points
Small molecules typically have low melting and boiling points. This is because the forces between different molecules (intermolecular forces) are weak. Even though the covalent bonds inside each molecule are strong, it doesn't take much energy to separate the molecules from each other.
Usually gases or liquids at room temperature
Because of their low melting and boiling points, most small molecules exist as gases or liquids at room temperature. They don't form solid structures easily.
Do not conduct electricity
Small molecules cannot conduct electricity because they have no overall electric charge. The molecules are neutral, so there are no free charges that can move and carry an electric current.
Common Mistake to Avoid: Students often confuse the strength of bonds within molecules (strong covalent bonds) with the forces between molecules (weak intermolecular forces). Remember: strong bonds inside, weak forces between!
Important facts about bonding
Understanding the difference between intramolecular and intermolecular forces is crucial for explaining the properties of small molecules.
Key Bonding Concepts:
- Covalent bonds within molecules are strong - it takes a lot of energy to break the bonds between atoms in a molecule
- Intermolecular forces between molecules are weak - it's easy to separate molecules from each other
- Larger molecules have stronger intermolecular forces - bigger molecules are attracted to each other more strongly
Common examples to learn
You should be familiar with the structure and properties of these common small molecules. Practice drawing their diagrams using different representation methods.
Essential Small Molecules to Master:
- H₂ (hydrogen gas)
- Cl₂ (chlorine gas)
- O₂ (oxygen gas)
- N₂ (nitrogen gas)
- HCl (hydrogen chloride)
- H₂O (water)
- CH₄ (methane)
- NH₃ (ammonia)
Key Points to Remember:
- Small molecules are made when atoms join with covalent bonds
- They have low melting and boiling points due to weak forces between molecules
- Most are gases or liquids at room temperature
- They don't conduct electricity because they have no overall charge
- You can represent them using dot-and-cross diagrams, 2D diagrams, or ball and stick models
- The bonds inside molecules are strong, but the forces between molecules are weak