• 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. Potassium (K) is in group 1. What charge does its ion have? (1+ / 2− / 2+ / 1−) → 1+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 calcium carbonate, CaCO₃? → 5Prompt: A small number after a symbol tells you how many of that atom.
3. An atom of chlorine (atomic number 17) forms its usual ion (charge 1−). How many electrons does the ion have? → 18Prompt: 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 aluminium hydroxide? (Ions: Al³⁺ and OH⁻) (Al₂(OH)₃ / Al₃OH / Al(OH)₄ / Al(OH)₃) → Al(OH)₃Prompt: The total positive charge must equal the total negative charge.
2. In K₂O (potassium oxide), how many oxide 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? (The bracket shows two whole hydroxide ions, each with one O and one H / Calcium has two atoms / CaOH₂ means the same thing / 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 Al₂S₃ (aluminium sulfide), how many aluminium ions are there for each formula unit? → 2Prompt: 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.