Science lesson plan · 8th grade (Year 9 in England) · 60 minutes
Count the atoms in a chemical formula
Atoms, elements and the periodic table
CCSS NGSS.MS-PS1-1England: KS3 Chemistry: Atoms, elements and compounds
Objective and success criteria
Students will: Count the atoms in a chemical formula.
- 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: Chemical formulae
Leads to: the next skill in this area.
Materials: Periodic tables, Atom model kits, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).
Warm-up · 6 min
- Quick retrieval from earlier learning (Chemical formulae). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- 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.
- How many hydrogen atoms (H) are in NH₃? → 3Prompt: Find every H in the formula.
- How many different elements are in iron(III) oxide, Fe₂O₃? → 2Prompt: Count the different symbols. Each starts with a capital letter.
Hook · 6 min
- Show the periodic table and ask what patterns students can see before anything is explained.
- Share the objective: Count the atoms in a chemical formula. Read the success criteria together.
I do: model it · 12 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: How many atoms are there altogether in one molecule or formula unit of CH₄?
- Step 1: C: 1, H: 4
- Step 2: 1 + 4 = 5
- Step 3: So the answer is: 5
- Answer: 5
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 12 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- How many atoms are there altogether in one molecule or formula unit of CO₂? → 3Prompt: A small number after a symbol counts only that atom.
- How many Al atoms are in one formula unit of Al₂(SO₄)₃? → 2Prompt: A number after a bracket multiplies everything inside the bracket.
You do: independent practice · 17 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 · 7 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- How many C atoms are shown by 5C₂H₅OH? → 10Prompt: The big number in front multiplies the whole formula.
- How many atoms are there altogether in one molecule or formula unit of CaCO₃? → 5Prompt: A small number after a symbol counts only that atom.
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: Chemical formulae.
- 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
- 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-8.
Free lesson plan from eurekami.com/lesson-plan-builder