Lesson focus: Molecular models reveal atomThe tiny particle that everything is made of; each element has its own kind of atom. count and connection but use conventional colours and distorted scales.
Interpret ball-and-stick modelA model that shows atoms as balls and the links between them as sticks. and translate among model, name and formulaSymbols and numbers that show which atoms are in a substance, such as H₂O for water..
Build on earlier learning
Compounds and moleculesRepresent molecular elements and compounds identified previously.
You will be successful when you can…
- Count atoms by element.
- Write a simple formula from a model.
- Use a colour key rather than memory.
- Evaluate two model styles.
Words that carry the science
- ball-and-stick model
- Representation emphasising atoms and their connections.
- space-filling model
- Representation emphasising occupied shape and relative size.
- bond
- Attractive interaction holding atoms together.
- formula
- Symbolic statement of elements and their ratio or count.
- convention
- Agreed representation, such as model colour.
Read what a model shows—and what it hides
01Count using a key
A model key defines which colour represents each element. formulaSymbols and numbers that show which atoms are in a substance, such as H₂O for water. subscripts record atom count in a molecule: CO₂ has one carbon and two oxygen atoms.
02Models answer different questions
Ball-and-stick makes connections visible; space-filling better communicates overall shape and occupied space.
03Every model distorts
Sticks are not literal rods, colours are conventionAn agreed way of showing something so everyone understands it, such as drawing oxygen atoms red. and sizes/distances are often changed for clarity.
Translate a model
A model has one black central atom joined to two red atoms; key: black C, red O.
- Count carbon atoms: 1.
- Count oxygen atoms: 2.
- Write symbols with subscripts: CO₂.
- Two atom types bonded in one discrete group → molecular compound.
How to make the reasoning visible: Read the key, count, write formula, then classify.
Model translation
Use the key shown here: white H, red O and black C.
| Model count | Formula | Name | Classification |
|---|---|---|---|
| 2 H | H₂ | Hydrogen | Molecular element |
| 2 O | O₂ | Oxygen | Molecular element |
| 2 H + 1 O | H₂O | Water | Molecular compound |
| 1 C + 2 O | CO₂ | Carbon dioxide | Molecular compound |
Why can model colours vary between resources?
What does ball-and-stick show well?
A model shows one carbon atom joined to four hydrogen atoms. Write its formula.
Check your understanding
Answer from memory, then check against the suggested answer.
What is ball-and-stick model?
What is space-filling model?
What is bond?
What does the formula H₂O tell you?
What does a model key show?
A model has two identical bonded atoms. Can you identify the element without a key?
Compare space-filling and ball-and-stick models.
Give one way a ball-and-stick model is not like a real molecule.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A model contains two molecules. Each molecule has one carbon atom and two oxygen atoms.
Determine the formula of one molecule and the total number of each type of atom.
(3)Formula of one molecule: . Total carbon atoms: . Total oxygen atoms: .
Which best describes the sample?
(1)Total for this question: 4 marks
Command word: Subscripts describe one molecule; a coefficient or stated number multiplies the whole formula.