• Quick retrieval from earlier learning (Predict ion charges from the periodic table; Chemical formulae). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Aluminium (Al) is in group 13 (3). What charge does its ion have? (2+ / 1+ / 3− / 3+) → 3+Prompt: Metals lose their outer electrons; non-metals gain electrons to fill their outer shell.
2. How many atoms in total are in one molecule (formula unit) of sulfuric acid, H₂SO₄? → 7Prompt: A small number after a symbol tells you how many of that atom.
3. An atom of sodium (atomic number 11) forms its usual ion (charge 1+). How many electrons does the ion have? → 10Prompt: A neutral atom has as many electrons as protons.

• Show the periodic table and ask what patterns students can see before anything is explained.
• Share the objective: Build ionic formulae from ion charges. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: What is the formula of potassium sulfate? (Ions: K⁺ and SO₄²⁻)
• Step 1: 2 × (+1) and 1 × (−2) give zero overall charge.
• Step 2: So the answer is: K₂SO₄
• Answer: K₂SO₄
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. What is the formula of ammonium nitrate? (Ions: NH₄⁺ and NO₃⁻) (NH₄NO₃ / (NH₄)₂NO₃ / NH₄(NO₃)₂) → NH₄NO₃Prompt: The total positive charge must equal the total negative charge.
2. In MgCO₃ (magnesium carbonate), how many magnesium ions are there for each formula unit? → 1Prompt: Look at the small number after the symbol or bracket.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Why is the formula of calcium hydroxide written Ca(OH)₂ and not CaOH₂? (Calcium has two atoms / CaOH₂ means the same thing / The bracket shows two whole hydroxide ions, each with one O and one H / Brackets show the charge) → The bracket shows two whole hydroxide ions, each with one O and one HPrompt: What would CaOH₂ mean about the hydrogen atoms?
2. In Na₂S (sodium sulfide), how many sulfide ions are there for each formula unit? → 1Prompt: Look at the small number after the symbol or bracket.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.