• 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. Nitrogen (N) is in group 15 (5). 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 nitrogen (atomic number 7) forms its usual ion (charge 3−). 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 calcium sulfate? (Ions: Ca²⁺ and SO₄²⁻) (Ca₂SO₄ / CaSO₄ / Ca(SO₄)₂) → CaSO₄Prompt: The total positive charge must equal the total negative charge.
2. In KBr (potassium bromide), how many potassium 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. What is the formula of lithium oxide? (Ions: Li⁺ and O²⁻) (LiO₂ / Li₂O / Li₂O₂ / LiO) → Li₂OPrompt: The total positive charge must equal the total negative charge.
2. In NH₄Br (ammonium bromide), how many ammonium 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.