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Science lesson plan · 9th grade (Year 10 in England) · 30 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 · 4 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. Potassium (K) is in group 1. What charge does its ion have? (3− / 3+ / 1+ / 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 lithium (atomic number 3) forms its usual ion (charge 1+). How many electrons does the ion have? → 2Prompt: A neutral atom has as many electrons as protons.

Hook · 3 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 · 7 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 · 6 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 potassium oxide? (Ions: K⁺ and O²⁻) (KO₂ / K₃O / K₂O / KO) → K₂OPrompt: The total positive charge must equal the total negative charge.
  2. In AlCl₃ (aluminium chloride), how many chloride ions are there for each formula unit? → 3Prompt: Look at the small number after the symbol or bracket.

You do: independent practice · 7 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 6 questions are on the student sheet below, with answers in the key.

Exit ticket · 3 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. What is the overall charge of any ionic compound? (zero / always negative / always positive / the charge of the metal ion) → zeroPrompt: The positive and negative charges cancel.
  2. What is the formula of sodium sulfate? (Ions: Na⁺ and SO₄²⁻) (Na₂SO₄ / NaSO₄ / Na(SO₄)₂ / Na₃SO₄) → Na₂SO₄Prompt: 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/ 6

  1. Question 1

    What is the formula of ammonium chloride? (Ions: NH₄⁺ and Cl⁻)

    • A. NH₄Cl₂
    • B. NH₄Cl
    • C. (NH₄)₂Cl

  2. Question 2

    In Al₂(SO₄)₃ (aluminium sulfate), how many aluminium ions are there for each formula unit?

    Answer

  3. Question 3

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

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

  4. Question 4

    What is the formula of magnesium nitrate? (Ions: Mg²⁺ and NO₃⁻)

    • A. Mg₂(NO₃)₂
    • B. MgNO₃
    • C. Mg(NO₃)₂
    • D. Mg(NO₃)₃

  5. Question 5

    In KNO₃ (potassium nitrate), how many nitrate ions are there for each formula unit?

    Answer

  6. Question 6

    What is the overall charge of any ionic compound?

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

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Answer key: Build ionic formulae from ion charges

  1. 1. B. NH₄Cl

  2. 2. 2

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

  4. 4. C. Mg(NO₃)₂

  5. 5. 1

  6. 6. A. zero