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Science lesson plan · 9th grade (Year 10 in England) · 45 minutes

Build ionic formulae from ion charges

Atoms, elements and the periodic table

CCSS NGSS.HS-PS1-2England: GCSE Combined Science: Bonding, structure and the properties of matter

Objective and success criteria

Students will: Build ionic formulae from ion charges.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: Predict ion charges from the periodic table; Chemical formulae

Leads to: Predict the products of electrolysis

Materials: Periodic tables, Atom model kits, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 5 min

  • 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. Chlorine (Cl) is in group 17 (7). What charge does its ion have? (3− / 3+ / 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 methane, CH₄? → 5Prompt: A small number after a symbol tells you how many of that atom.
  3. An atom of aluminium (atomic number 13) forms its usual ion (charge 3+). How many electrons does the ion have? → 10Prompt: A neutral atom has as many electrons as protons.

Hook · 5 min

  • 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.

I do: model it · 10 min

  • 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.

We do: guided practice · 10 min

  • 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⁻) (CaSO₄ / Ca₂SO₄ / Ca(SO₄)₂) → CaSO₄Prompt: The total positive charge must equal the total negative charge.
  2. In NH₄NO₃ (ammonium nitrate), how many nitrate ions are there for each formula unit? → 1Prompt: Look at the small number after the symbol or bracket.

You do: independent practice · 10 min

  • 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.

Exit ticket · 5 min

  • 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? (CaOH₂ means the same thing / Brackets show the charge / The bracket shows two whole hydroxide ions, each with one O and one H / Calcium has two atoms) → The bracket shows two whole hydroxide ions, each with one O and one HPrompt: What would CaOH₂ mean about the hydrogen atoms?
  2. What is the formula of calcium nitrate? (Ions: Ca²⁺ and NO₃⁻) (CaNO₃₂ / Ca₂(NO₃)₂ / Ca(NO₃)₂ / Ca₂NO₃) → Ca(NO₃)₂Prompt: The total positive charge must equal the total negative charge.

Watch out for

  • Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
  • Remembering facts without the reason: ask "why?" and "what is the evidence?"

Support and stretch

Support

  • Review first: Predict ion charges from the periodic table.
  • Use concrete materials: periodic tables.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Move on to: Predict the products of electrolysis.
  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

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

Free lesson plan from eurekami.com/lesson-plan-builder

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Build ionic formulae from ion charges

Name

Date

Score/ 8

  1. Question 1

    In KCl (potassium chloride), how many potassium ions are there for each formula unit?

    Answer

  2. Question 2

    What is the formula of sodium carbonate? (Ions: Na⁺ and CO₃²⁻)

    • A. Na₂CO₃
    • B. NaCO₃
    • C. Na₃CO₃
    • D. Na₂(CO₃)₂

  3. Question 3

    Why is the formula of calcium hydroxide written Ca(OH)₂ and not CaOH₂?

    • A. Brackets show the charge
    • B. Calcium has two atoms
    • C. CaOH₂ means the same thing
    • D. The bracket shows two whole hydroxide ions, each with one O and one H

  4. Question 4

    In NaCl (sodium chloride), how many chloride ions are there for each formula unit?

    Answer

  5. Question 5

    What is the formula of ammonium bromide? (Ions: NH₄⁺ and Br⁻)

    • A. NH₄Br
    • B. NH₄Br₂
    • C. (NH₄)₂Br

  6. Question 6

    In CaBr₂ (calcium bromide), how many calcium ions are there for each formula unit?

    Answer

  7. Question 7

    What is the formula of aluminium chloride? (Ions: Al³⁺ and Cl⁻)

    • A. AlCl
    • B. AlCl₄
    • C. Al₂Cl₃
    • D. AlCl₃

  8. Question 8

    What is the overall charge of any ionic compound?

    • A. always positive
    • B. the charge of the metal ion
    • C. zero
    • D. always negative

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Answer key: Build ionic formulae from ion charges

  1. 1. 1

  2. 2. A. Na₂CO₃

  3. 3. D. The bracket shows two whole hydroxide ions, each with one O and one H

  4. 4. 1

  5. 5. A. NH₄Br

  6. 6. 1

  7. 7. D. AlCl₃

  8. 8. C. zero