Grade 7 · Science · 30 minutes
Unit 2 · Changes in Matter and Motion · Lesson 1 of 2
Rearrange Atoms Without Losing Them
Use paper tokens to model conservation in a chemical change.
Mr. S Teaches · Original starter mini-lesson · MS7-SCI-03Learning objective
Construct before-and-after molecule models with identical atom counts and explain the conservation claim.
Before you begin
- Count symbols and distinguish an atom from a group of atoms.
Gather your materials
- Paper or notebook
- Pencil and the passage, model directions or data printed in this lesson
Language to know
- atom
- A basic unit represented by one labeled token.
- molecule
- A group of bonded atoms represented together.
- reactant
- A starting substance in a chemical reaction.
Understand the concept
Particle-model passage: A reaction rearranges atoms into new groupings. In a closed system, the number of atoms of each type is unchanged. Model two H–H pairs and one O–O pair becoming two H–O–H groups. The groups differ, but four H tokens and two O tokens remain. A gas can leave an open container and make a scale reading decrease even though the atoms still exist outside that container. These paper symbols describe the reaction; do not attempt it physically.
Teach it step by step
- Read the passage and count H and O separately before moving any tokens.
- Arrange four H and two O paper tokens into the described reactants.
- Regroup the same tokens into two H–O–H molecules and record a before-and-after count table.
- Explain what an open-container scale can and cannot establish about conservation.
Make the thinking visible
Worked example or model response
Before: two H2 groups contain four H atoms; one O2 group contains two O atoms. After: two H2O groups contain four H and two O. The molecules are new groupings, but the model has not created or destroyed either atom type.
Guided practice
Together critique a drawing showing two H–H pairs plus O–O becoming only one H–O–H group. Identify the missing tokens.
Independent practice
Draw a corrected before-and-after model. Write a paragraph using the passage to explain why gas escaping an open container does not show that matter disappeared.
Quick assessment
Can conserving the total number of dots alone prove each atom type is conserved?
Answer key & assessment criteria
Answers and teaching guidance
- Guided: One H–O–H group accounts for only two H and one O; the model needs a second such group or must show where all other atoms remain.
- Independent: The corrected model preserves four H and two O. Escaped gas crosses the chosen boundary and still contains matter.
- Quick check: No. Count each labeled atom type separately; exchanging one type for another could preserve dot totals while violating the model.
Look for: Look for a claim that answers this lesson’s question, accurate use of the supplied evidence and an explanation of a limit or alternative. Ask the learner to point to the detail supporting each inference.
Make it work for your learner
More support
Use large H and O paper labels and a counting mat with before and after columns.
Extension challenge
Choose a larger whole-number batch, such as three times the starting groups, and show that the atom-type totals still match.
Selected contributions · Official sources
Standards connections
Selected NGSS band contribution; not complete performance expectation coverage.
Contributes to NGSS performance expectation
MS-PS1-5NGSS · Grades 6–8 band
Model conserved atoms during a chemical rearrangement.
Read the official performance expectation ↗A selected paper model shows atom conservation and regrouping. It is not a laboratory reaction or a full account of all reactions, mass measurement and molecular structures.
Source and mapping checked 2026-11-04. This mini-lesson contributes selected skills; it does not claim full attainment.
Supporting Common Core literacy connection
RST.6-8.7Common Core · Grades 6–8
Connect scientific prose with a diagram, model or table.
Read the official standard ↗The provided scientific passage/model is explicitly analyzed in the activity. This selected literacy practice does not establish subject-content alignment or complete coverage of the standard.
Source and mapping checked 2026-11-04. This mini-lesson contributes selected skills; it does not claim full attainment.