• 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. Phosphorus (P) is in group 15 (5). What charge does its ion have? (3− / 2− / 3+ / 1−) → 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 carbon dioxide, CO₂? → 3Prompt: 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 bromide? (Ions: Ca²⁺ and Br⁻) (Ca₂Br / Ca₂Br₂ / CaBr / CaBr₂) → CaBr₂Prompt: The total positive charge must equal the total negative charge.
2. In Li₂S (lithium sulfide), how many lithium ions are there for each formula unit? → 2Prompt: 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. In Li₂O (lithium oxide), how many lithium ions are there for each formula unit? → 2Prompt: Look at the small number after the symbol or bracket.
2. What is the overall charge of any ionic compound? (always negative / always positive / the charge of the metal ion / zero) → zeroPrompt: The positive and negative charges cancel.

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