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C2.2 · Model and diagram molecules in simple reactions

Learn to model and diagram molecules in simple reactions through clear examples and targeted practice.

Ontario Grade 10 Science

Chemistry: Chemical Reactions

Using particle pictures to show how atoms are rearranged

A candle burning, a bicycle rusting, and food changing as it cooks are examples of chemical changes. In each, substances change into other substances. We cannot usually see the individual particles, so we use models. A model is a simplified way to represent something. In this lesson, circles and letters stand for atoms, and joined circles stand for atoms connected in a molecule. The model helps us show how atoms are rearranged in a simple reaction.

What you will learn

  • Tell the difference between an atom, a molecule, and a compound.
  • Use a particle diagram to show molecules before and after a simple reaction.
  • Check that a reaction model shows the same number of each kind of atom before and after.
  • Connect a particle diagram to a simple chemical equation.

1. A Grade 9 bridge: atoms, molecules, and compounds

An atom is a tiny particle that represents one element. An element is a pure substance made of one kind of atom. In a particle diagram, each kind of atom can be shown with a different colour or letter. For example, a white circle might represent hydrogen and a red circle might represent oxygen. The colours are model choices; they are not the atoms’ actual appearance.
A molecule is a group of two or more atoms joined together. A molecule can contain atoms of one element, as in oxygen gas, or atoms of different elements, as in water. A compound is a substance whose particles contain atoms of different elements joined together. Water is a compound; a molecule made only of oxygen atoms is not.
A chemical reaction happens when atoms in the starting substances are rearranged to make different substances. Starting substances are called reactants. The substances made are called products. The atoms do not vanish or turn into different kinds of atoms in the simple reaction models used here. Instead, their connections change.
A useful rule for checking a model is to count each kind of atom on both sides. The number of each kind must match. This does not mean the molecules stay the same. It means the atoms are represented in a way that accounts for all of them.
  • A molecule contains joined atoms.
  • A compound contains more than one kind of atom.
  • In a reaction model, atoms are rearranged into new molecules.

2. From an observable change to a particle model

Imagine a teacher showing a video of a small piece of iron slowly forming rust in air. The visible colour and surface change are evidence that a new substance has formed. A particle model can help represent the change, but it is not a photograph of what happens. It is a simplified explanation built from symbols for atoms and molecules.
For a simple model, suppose iron atoms combine with oxygen molecules to make iron oxide. The oxygen molecules contain two oxygen atoms each. A model must show those oxygen atoms in the products too. The product particles contain iron and oxygen atoms joined in a new arrangement.
A diagram should make clear which circles represent which atoms, which atoms are joined, and which particles are present before and after. Put reactants on one side and products on the other. A plus sign separates different substances. An arrow means “changes into”; it does not mean that the molecules simply move in that direction.
A chemical equation is a compact symbol model. Its formulas identify the kinds and numbers of atoms in a particle. A number placed in front of a formula tells how many of those particles are represented. The small number within a formula tells how many atoms of that element are in each particle. These numbers have different jobs: changing a front number changes the number of particles, while changing a small number changes what particle is represented.
When drawing a model, make the particle count match the equation. Then check the atom count. A model that shows the right kinds of particles but loses or adds atoms is not a sound model of the reaction.
reactants→products\text{reactants} \rightarrow \text{products}
  • A particle diagram is a model, not a direct view of atoms.
  • The arrow separates reactants from products.
  • Check both the particles shown and the number of each kind of atom.

3. Build, compare, and check a reaction model

Start with a concrete particle picture. Suppose one molecule contains two hydrogen atoms joined together, and another molecule contains two oxygen atoms joined together. They can be represented with letters as hydrogen and oxygen molecules. If the products are water molecules, each water molecule contains two hydrogen atoms and one oxygen atom.
The first picture cannot make just one water molecule from those reactants: that would leave one oxygen atom unaccounted for. A better model uses two water molecules. Those products contain four hydrogen atoms and two oxygen atoms. The reactants must therefore also show four hydrogen atoms and two oxygen atoms, which means two hydrogen molecules and one oxygen molecule.
This is a model-building check, not a claim that a reaction always happens in exactly this small amount. The numbers show a smallest whole-particle arrangement that represents the atoms consistently. The same idea works when a diagram uses coloured circles: count circles of each colour on both sides.
A particle diagram and a chemical equation support each other. The drawing makes the particles and their connections visible. The equation is shorter and easier to compare. Neither representation should suggest that atoms disappear, appear from nowhere, or change into another element during the reaction.
2H2+O2→2H2O2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}
  • Draw complete molecules, not loose symbols, when the model calls for molecules.
  • Use particle counts that account for every atom.
  • A front number counts particles; a small number in a formula counts atoms in one particle.

4. Evidence, safe learning, and model limits

In a classroom investigation, observations such as a lasting colour change, gas formation, or a temperature change may be clues that a chemical change occurred. One clue alone may not tell the whole story. A particle model is used to represent an explanation for the change; it is not itself experimental evidence.
This lesson’s examples are models, not reports of measured results. Do not try chemical reactions at home. If a class investigation is used, follow the teacher’s instructions and the school’s safety rules. Do not taste substances or handle unknown materials.
Models leave details out. A circle diagram does not show an atom’s real size, and its colours are labels. Its purpose here is narrower: to show which atoms are present, how they are grouped into molecules, and how the groups change in a simple reaction.
  • Observations can provide evidence of a change; a model represents an explanation.
  • Use classroom materials only as directed by the teacher.
  • A useful model is clear about what it shows and what it leaves out.

Worked example

Modeling water formation

A model starts with hydrogen molecules and oxygen molecules and makes water molecules. Choose particle counts that show the same number of hydrogen and oxygen atoms before and after. Then write the equation.
  1. Count atoms in one product molecule
    One water molecule contains two hydrogen atoms and one oxygen atom. Two water molecules therefore contain four hydrogen atoms and two oxygen atoms.
    2H2O=4H+2O2\mathrm{H_2O} = 4\mathrm{H} + 2\mathrm{O}
  2. Choose matching reactant particles
    Each hydrogen molecule contains two hydrogen atoms, so two hydrogen molecules provide four. One oxygen molecule provides two oxygen atoms. These counts match the products.
    2H2+O22\mathrm{H_2} + \mathrm{O_2}
  3. Write the complete model
    Place the matching reactants before the arrow and the two water molecules after it. The count of each atom is the same on both sides.
    2H2+O2→2H2O2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}
Answer: Two hydrogen molecules and one oxygen molecule form a model with two water molecules.
Check: Both sides contain four hydrogen atoms and two oxygen atoms.

Worked example

Checking a carbon dioxide model

A simple model shows carbon reacting with oxygen molecules to form carbon dioxide molecules. How many carbon atoms and oxygen molecules are needed to make two carbon dioxide molecules?
  1. Read the product particle
    Each carbon dioxide molecule contains one carbon atom and two oxygen atoms. Two product molecules therefore need two carbon atoms and four oxygen atoms.
    2CO2=2C+4O2\mathrm{CO_2} = 2\mathrm{C} + 4\mathrm{O}
  2. Match the reactant particles
    Oxygen is shown as molecules containing two oxygen atoms. Two oxygen molecules supply four oxygen atoms, and two separate carbon atoms supply the two carbon atoms.
    2C+2O22\mathrm{C} + 2\mathrm{O_2}
  3. Compare both sides
    The products have two carbon atoms and four oxygen atoms, just like the reactants. The model therefore accounts for every atom.
    2C+2O2→2CO22\mathrm{C} + 2\mathrm{O_2} \rightarrow 2\mathrm{CO_2}
Answer: Use two carbon atoms and two oxygen molecules to model two carbon dioxide molecules.
Check: Each side contains two carbon atoms and four oxygen atoms.

Worked example

Finding what is missing from a diagram

A student draws one nitrogen molecule and three hydrogen molecules before the arrow, then one ammonia molecule after it. What should be changed so the model accounts for every atom?
  1. Count the starting atoms
    One nitrogen molecule contains two nitrogen atoms. Three hydrogen molecules contain six hydrogen atoms.
    N2+3H2=2N+6H\mathrm{N_2} + 3\mathrm{H_2} = 2\mathrm{N} + 6\mathrm{H}
  2. Count the atoms in the shown product
    One ammonia molecule contains one nitrogen atom and three hydrogen atoms. That product uses fewer atoms than the reactants provide.
    NH3=1N+3H\mathrm{NH_3} = 1\mathrm{N} + 3\mathrm{H}
  3. Add a second product molecule
    Two ammonia molecules contain two nitrogen atoms and six hydrogen atoms. This matches the starting counts, so the diagram should show two product molecules.
    N2+3H2→2NH3\mathrm{N_2} + 3\mathrm{H_2} \rightarrow 2\mathrm{NH_3}
Answer: Change the product side to show two ammonia molecules.
Check: Both sides then contain two nitrogen atoms and six hydrogen atoms.

Common mistakes and how to avoid them

Changing a small number inside a formula to make the atom counts match.
Correction: Keep each substance’s formula intact. Change the number of whole particles shown in the model instead.
Thinking that molecules stay unchanged during a reaction.
Correction: The model shows atoms rearranged into different molecules or particles.
Counting molecules but not checking each kind of atom.
Correction: Count hydrogen, oxygen, and any other represented atoms separately on both sides.

Lesson summary

  • Atoms are represented by symbols or circles; joined atoms form molecules.
  • Reactants are the starting substances, and products are the substances made.
  • A reaction model shows atoms rearranged into new particles.
  • Check that each kind of atom has the same count before and after the reaction.

Check your understanding

Question 1

In a particle diagram, what does a group of joined circles represent?
  1. A molecule
  2. A reaction arrow
  3. A single element in every case
  4. correctIndex":0,"explanation":"Joined circles represent atoms connected in a molecule. The atoms may be the same kind or different kinds."}
Show answer and explanation
A molecule
Joined circles represent atoms connected in a molecule. The atoms may be the same kind or different kinds.

Question 2

A product side has two molecules, each containing one carbon atom and two oxygen atoms. How many oxygen atoms are shown on that side?
  1. Two
  2. Three
  3. Four
  4. correctIndex":2,"explanation":"Each molecule contains two oxygen atoms. Two molecules contain four oxygen atoms."}
Show answer and explanation
Four
Each molecule contains two oxygen atoms. Two molecules contain four oxygen atoms.

Question 3

Why is a particle model with fewer hydrogen atoms in the products than in the reactants not a good model of the reaction?
  1. It does not account for all the hydrogen atoms.
  2. Molecules cannot contain hydrogen atoms.
  3. The arrow must point from products to reactants.
  4. correctIndex":0,"explanation":"A reaction model must account for the same number of each kind of atom on both sides."}
Show answer and explanation
It does not account for all the hydrogen atoms.
A reaction model must account for the same number of each kind of atom on both sides.

Key terms

Atom
A tiny particle representing one element.
Molecule
Two or more atoms joined together.
Compound
A substance whose particles contain atoms of different elements joined together.
Reactant
A starting substance in a chemical reaction.
Product
A substance made in a chemical reaction.
Particle diagram
A drawing that uses symbols or shapes to represent particles and their arrangement.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation C2.2. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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