A compound is two or more types of atom chemically joined in a fixed ratio. It is a new substance with new properties, and a formula like H₂O tells you which atoms and how many.
Sodium is a metal that fizzes and can burst into flame in water. Chlorine is a poisonous gas. Put them together and you can sprinkle the result on your chips.
Last lesson: atoms come in types, one for each element. This lesson: what happens when different types join.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: sodium and chlorine
ExploreTry it on the board and compare what happens with your prediction.
Explore: mixed or joined?
Both boards contain only hydrogen and oxygen atoms. Look at how they are arranged.
Same two atom types. What is different?
Try it yourselfNothing is saved: start again as often as you like.
Inspect the particles in the mixture of hydrogen and oxygen.
Use the board. This ticks when you get there.
Inspect the particles of water.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: compounds and formulas
The small number belongs to the atom before it: H₂O has 2 H and 1 O.
In your book: why can you eat sodium chloride when sodium and chlorine are both dangerous? Use "compound", "joined" and "new substance".
Element, compound or mixture?
O₂One type of atom.
H₂OHydrogen and oxygen atoms joined.
CO₂
He
Hydrogen gas and oxygen gas in one jarTwo elements together, not joined.
NaCl
AirSeveral substances together, each with its own particles.
N₂
Match each formula to the atoms in one particle.
H₂O
CO₂
NH₃
CH₄
Elaborate TypicalUse the same idea in a new context.
Elaborate: find the compounds
Try it yourselfNothing is saved: start again as often as you like.
Inspect all five gases.
Use the board. This ticks when you get there.
How many atoms are in one particle of methane, CH₄?
Glucose is C₆H₁₂O₆. How many atoms are in one glucose particle?
Elaborate WiderUse the same idea in a new context.
Elaborate: making a compound
The panel shows the atoms at the start and now.
Try it yourselfNothing is saved: start again as often as you like.
Watch hydrogen and oxygen react.
Use the board. This ticks when you get there.
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
A compound is two or more types of atom chemically joined in a fixed ratio.
A compound is a new substance with new properties (sodium chloride is not like sodium or chlorine).
In a mixture, substances are together but not joined, and each keeps its properties.
A formula gives the atoms in one particle: H₂O is 2 hydrogen and 1 oxygen.
For teachers
Students define a compound (different atom types chemically joined in a fixed ratio, a new substance with new properties), tell it apart from a mixture, and read formulas (AC9S8U06, node 3, a hinge node). About 25 minutes.
I can define a compound as two or more types of atom chemically joined in a fixed ratio.
I can read a formula to find the atom types and how many of each.
I can explain that a compound has different properties from the elements it is made from.
Before class
A concept tutorial for introducing or revising the idea; it does not replace models or demonstrations. Set the width with ?width=minimum, typical (default) or wider.
The hinge risk is students treating a compound as a mixture of its elements ("water is hydrogen and oxygen mixed"). The two Explore boards hold the atom types constant so that only joining differs.
Accuracy note: sodium chloride is not made of NaCl molecules; it is a lattice of ions (Year 10). The lesson uses its formula only as a ratio and never draws it as a particle.
Running the lesson
Engage: if you can, show a short video of sodium in water, then a salt shaker.
Explore: ask 'what is the same on both boards?' (atom types) and 'what is different?' (joined or not).
Explain: the first question is the checkpoint. The sort mixes formulas and descriptions on purpose.
In your own words: the salt paragraph is the best evidence for the node.
The hard part (on purpose)
Seeing a compound as a new substance, not its elements mixed. When a student calls water a mixture, ask whether water burns the way hydrogen does.
When students get stuck
A student says water is a mixture of hydrogen and oxygen. Ask: Would water let things burn, like hydrogen and oxygen do? Fix: No: it puts fires out. The atoms are joined into a new substance with new properties.
A student reads H₂O as two waters. Ask: Which atom does the small 2 sit next to? Fix: The small number belongs to the atom before it: two hydrogens, one oxygen.
A student says O₂ is a compound. Ask: How many types of atom are joined? Fix: One type: oxygen. A compound needs different types.
Stretch and support
Stretch: Hydrogen peroxide, H₂O₂, is made of the same atom types as water but is a bleach. What does that say about the ratio of atoms?
Support: Give a formula card: element symbols with colours, and an example (H₂O → H H O) to copy while reading formulas.
Where it leads
Next: chemical change (AC9S8U07), where compounds are made and broken: a new substance is the test.
In class: build H₂O, CO₂, NH₃ and CH₄ with a model kit, then pull apart a water model to show that the elements' properties do not survive inside the compound.
What each step is for
Engage: sodium and chlorine: Students who think a compound is its elements mixed will predict it keeps their properties; salt shows a compound is a new substance.
Explore: mixed or joined?: The same two atom types, first mixed and then joined, separates 'compound' from 'mixture' at the particle level.
Explain: compounds and formulas: Defining a compound by joining, fixed ratio and new properties, then reading formulas as atom counts, is the hinge for chemical change and equations.
Elaborate: find the compounds: Reading formulas from particle pictures, and back, builds both directions of the representation.
Elaborate: making a compound: Seeing elements join into a compound links this node to chemical change: the compound is a new substance made by rearranging atoms.
Now you can
Define a compound as two or more types of atom chemically joined in a fixed ratio.
Read a formula to find the atom types and how many of each.
Explain that a compound has different properties from the elements it is made from.