--- title: "Symbols, formulas or models?" subtitle: "Year 8 Chemistry · Elements and compounds · Lesson 5" --- # Symbols, formulas or models? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** 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? :::teacher-note **In one sentence:** 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). **Students should be able to say:** I can read a chemical formula and count the atoms. I can say what symbols, formulas and models each show. I can choose a representation that suits the job. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **What does the formula H₂O tell you about a water particle?** ☐ It is a mixture of hydrogen and oxygen ☐ It has 2 hydrogen atoms and 2 oxygen atoms ☐ It is a bent shape with the oxygen in the middle ☐ It has 2 hydrogen atoms and 1 oxygen atom My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 2 hydrogen atoms and 1 oxygen atom. The small number belongs to the symbol just before it; no number means 1. A formula tells you which atoms and how many, but not the shape: for that you need a model. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: what each one shows :::keep-together ### Which shows what **Match each representation to what it shows best.** A. Which atoms are joined to which B. Which atoms, and how many of each C. Whether it is an element, compound or mixture, and how much of each D. The shape of the particle E. Which element {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Symbol (O)` | | | `Formula (CO₂)` | | | `Structural formula (O=C=O)` | | | `3D model` | | | `Particle diagram of a box` | | ::: :::teacher-note **Answer:** Symbol (O) → E. Which element; Formula (CO₂) → B. Which atoms, and how many of each; Structural formula (O=C=O) → A. Which atoms are joined to which; 3D model → D. The shape of the particle; Particle diagram of a box → C. Whether it is an element, compound or mixture, and how much of each Each shows something; none shows everything. ::: :::keep-together ### Atoms in methane **How many atoms are in one particle of methane, CH₄?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 5 1 carbon and 4 hydrogen: 5 atoms. ::: :::keep-together ### Big and small numbers **How many oxygen atoms are in 3CO₂ (three carbon dioxide particles)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 6 Each CO₂ has 2 oxygen atoms; 3 × 2 = 6. The big number in front counts particles; the small one counts atoms in each. ::: :::keep-together ### Choosing one **You want to show a friend that air is a mixture of nitrogen and oxygen. Which is best?** ☐ The formulas N₂ and O₂ ☐ A particle diagram of a box with separate N₂ and O₂ particles ☐ A 3D model of one oxygen particle ::: :::teacher-note **Answer:** A particle diagram of a box with separate N₂ and O₂ particles Pick the representation that shows what matters for the job: here, separate particles side by side. ::: :::keep-together ### What each leaves out **Every representation leaves something out. Match each to one of its limits.** A. Slow to build or draw, and no use in a written equation B. It doesn't show how many atoms are joined in each particle (oxygen gas is O₂) C. It doesn't show which atoms are joined to which, or the shape D. Drawn flat, it can hide the real 3D shape (water is bent) {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Symbol (O)` | | | `Formula (H₂O)` | | | `Structural formula (H–O–H)` | | | `3D model` | | ::: :::teacher-note **Answer:** Symbol (O) → B. It doesn't show how many atoms are joined in each particle (oxygen gas is O₂); Formula (H₂O) → C. It doesn't show which atoms are joined to which, or the shape; Structural formula (H–O–H) → D. Drawn flat, it can hide the real 3D shape (water is bent); 3D model → A. Slow to build or draw, and no use in a written equation Symbols and formulas are quick to write; a structural formula shows the joins; a 3D model shows the shape. Each has a limit, which is why chemists use several. ::: :::typical ## Elaborate: three more :::keep-together ### One more oxygen **Hydrogen peroxide is H₂O₂ and water is H₂O. What is the difference?** ☐ H₂O₂ is a mixture of water and oxygen ☐ H₂O₂ has one more oxygen atom in each particle, so it is a different compound ☐ None: both are water ::: :::teacher-note **Answer:** H₂O₂ has one more oxygen atom in each particle, so it is a different compound Change one small number and you have a new substance with new properties. ::: ::: :::wider ## Elaborate: where models mislead :::keep-together ### What it leaves out **Which of these does this flat ball model NOT show truthfully?** ☐ How many atoms are in each particle ☐ Which atoms are in each particle ☐ The real 3D shape, the real sizes and the huge empty space between particles ::: :::teacher-note **Answer:** The real 3D shape, the real sizes and the huge empty space between particles It is flat, the atoms aren't to scale, and real gas particles are much further apart. Every model simplifies; a good scientist knows how. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - A **symbol** (O, Na) names an element. - A **formula** (H₂O, CO₂) gives the types of atom and how many of each. The small number counts the atom before it; a big number in front counts particles (3CO₂). - A **structural formula** shows which atoms are joined; a **3D model** shows the shape; a **particle diagram** shows elements, compounds and mixtures. - No single representation shows everything: choose the one that suits the job. ## Exit ticket :::keep-together **How many atoms of each element are in 2NH₃?** {{lines-3}} ::: :::teacher-note **Answer:** 2 nitrogen and 6 hydrogen (each NH₃ has 1 N and 3 H, and there are 2 particles). :::