Chemists show the same substance in different ways. A symbol names an element; a formula (H₂O) gives the types and numbers of atoms; a structural formula shows which atoms are joined; a 3D model shows the shape. Each shows something the others don't, so you pick the one that suits the job.
H₂O, a drawing of three balls, and the word 'water' all mean the same thing. Why do chemists need so many ways to say it?
You can tell elements, compounds and mixtures apart by their particles. This lesson asks how to show them.
Make your prediction first. Help opens once you've locked it in.
The table gives each particle's formula and number of atoms.
Colours stand for elements here (red oxygen, white hydrogen, blue nitrogen), as in a model kit.
Inspect each compound and compare its model with its formula.
Use the board. This ticks when you get there.
These models have no symbols. Inspect them: what can you work out from the colours and the number of balls?
Use the board. This ticks when you get there.
In your book: show carbon dioxide four ways (name, formula, structural formula, a ball drawing). Under each, write one thing it shows and one thing it doesn't.
Match each representation to what it shows best.
How many atoms are in one particle of methane, CH₄?
How many oxygen atoms are in 3CO₂ (three carbon dioxide particles)?
Every representation leaves something out. Match each to one of its limits.
Inspect all three substances.
Use the board. This ticks when you get there.
Inspect the compounds again, this time looking for what the model can't show.
Use the board. This ticks when you get there.
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Students compare symbols, formulas, structural formulas, models and particle diagrams, say what each shows, read formulas, and choose a representation for a purpose (AC9S8U06, node 5). About 24 minutes.
Open the teacher guide: answers, solutions, slides and a worksheet.
A concept tutorial for introducing or revising the idea; model kits in class do this best. Set the width with ?width=minimum, typical (default) or wider. It closes the Elements and compounds unit, after Mixed, not joined?
Not a hinge node, but formula-reading slips carry into Year 9 equations: the small number applied to both symbols, and big and small numbers confused. The prediction and the 3CO₂ check target these.
Board notes: the ball models are flat (2D), atoms not to scale, and molecules are rigid. Water is drawn bent and carbon dioxide straight, as in reality.
Big numbers versus small numbers. When a student miscounts, ask them to draw the particles for 3CO₂ before counting, rather than telling them the rule again.
Stretch: Glucose is C₆H₁₂O₆. How many atoms in one particle, and why would a 3D model of it be more useful than the formula to a biologist? (24 atoms; its shape decides how enzymes fit it.)
Support: Give a formula decoder card: element symbols, small number = atoms of the element before it, big number = particles.
Next: Same particles, new look? (physical change, AC9S8U07 node 1). In Year 9 formulas become balanced equations.
In class: model kits for H₂O, CO₂, CH₄, NH₃; draw the same substance four ways and swap with a partner to decode.