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C2.7 · Connect simple compound structures with chemical formulas

Learn to connect simple compound structures with chemical formulas through clear examples and targeted practice.

Ontario Grade 9 Science

Chemistry: The Nature of Matter

Ontario Grade 9 Science study topic C2.7

Water, the gas in some fizzy drinks, and the fuel in a candle all involve substances made from more than one kind of atom. Chemists use models to show how atoms are joined, and formulas to record which atoms are present and how many of each are in a simple unit. The key question in this lesson is: How can we move from a model of a compound to its chemical formula? We will use simple particle models and familiar formulas. These models help us count atoms, but they are not pictures of atoms at their actual size.

What you will learn

Identify atoms and elements in a simple compound model.
Connect the kinds and numbers of atoms in a model to a chemical formula.
Read element symbols and subscripts in a formula.
Explain what a formula tells us and what it leaves out.

1. Start with atoms and compounds

Everything around us is made of tiny particles called atoms. An element is a pure substance made of only one kind of atom. Oxygen and hydrogen are examples of elements. Each element has a short symbol. The symbol for hydrogen is H, and the symbol for oxygen is O. A symbol begins with a capital letter. If it has a second letter, that letter is lowercase, as in Cl for chlorine.

A compound is a substance whose particles contain atoms of two or more different elements joined together. Water is a familiar compound. Each water particle has hydrogen atoms joined to an oxygen atom. A particle made of joined atoms is often called a molecule. For the simple compounds in this lesson, a molecule model shows which atoms are joined.

The number and kinds of atoms matter. A model with two hydrogen atoms and one oxygen atom represents a different structure from a model with one hydrogen atom and two oxygen atoms. The chemical formula records the kinds and numbers of atoms in a compound's particle.

An element contains one kind of atom.
A compound contains atoms of at least two different elements joined together.
An atom symbol identifies an element.

2. Read a formula as an atom count

A chemical formula uses element symbols to show which elements are in a substance. A small number written just below and to the right of a symbol is called a subscript. It tells how many atoms of that element are in one represented particle. In a formula, no subscript means there is one atom of that element.

For example, the formula for water has H followed by a 2, then O with no subscript. It means two hydrogen atoms and one oxygen atom. The formula for carbon dioxide has one carbon atom and two oxygen atoms. The capital letters identify the elements; the subscripts give the counts.

A useful way to connect a model to a formula is to count each type of atom. First, name the element for each atom by its symbol. Next, count how many atoms of each element appear in one particle. Finally, write each element symbol with its count as a subscript, leaving out a subscript of 1. Check that the formula accounts for every atom in the model.

The table compares three simple models and formulas. The counts refer to one particle in each model.

A subscript gives the number of atoms of the symbol immediately before it.
No subscript means one atom.
The order and capitalization of element symbols matter.

3. A formula is useful, but it is not a full picture

A formula is a compact record of the atom types and counts in a simple compound particle. A model can add information by showing which atoms are connected. For instance, a water model can show one oxygen atom joined to two hydrogen atoms. Its formula records the count, but does not draw those connections.

A drawn molecule model is simplified. Its circles are not the true sizes of atoms, and the spaces and bonds in the picture are not necessarily to scale. A formula also does not show the shape of the particle. Use a model when you need to discuss connections, and use a formula when you need a compact way to name the atom types and counts.

Evidence for a substance's composition comes from scientific investigation and reliable scientific records, not from the drawing alone. In this lesson, the examples are established formulas used to practise reading simple models; the diagrams are teaching models, not measurements. A social or ethical opinion about a substance is a different kind of judgment and does not determine its formula.

A model can show connections between atoms.
A formula shows element types and counts, not the full arrangement or shape.
A simplified diagram is a learning aid, not a photograph or a measurement.

4. A careful method for connecting structure and formula

When working from a particle model, count atoms by element rather than by colour or position. Check the key or labels to see which symbol each atom represents. Then write each symbol once with its count. Do not count a bond as an atom: a bond is the connection between atoms in the model.

When working in the other direction, read each symbol and its subscript to build a particle with the stated count. If there is no subscript, include one atom. Compare your particle with the formula, element by element. This reverse check helps catch missing or extra atoms.

These steps apply to the simple compound structures in this lesson. A formula is not a drawing, and a drawing can be arranged in different ways while keeping the same atom counts. For this expectation, focus on making the correct connection between the simple structure shown and the chemical formula that records its composition.

Count atoms of each element separately.
Do not mistake a bond for an atom.
Check the finished formula against the model in both directions.

Atom counts and formulas in simple particle models

Compound	Atoms in one particle	Formula
Water	2 hydrogen, 1 oxygen	$\mathrm{H_2O}$
Carbon dioxide	1 carbon, 2 oxygen	$\mathrm{CO_2}$
Ammonia	1 nitrogen, 3 hydrogen	$\mathrm{NH_3}$

Worked example

Two hydrogen atoms and one oxygen atom

A particle model shows two H atoms, each joined to one O atom. Connect the model to its chemical formula.

Water particle model

One oxygen atom is joined to two hydrogen atoms. The circles and connections are schematic and not to scale.

Identify the elements
The labels show that the particle contains hydrogen, H, and oxygen, O. There are two kinds of atom, so this model represents a compound.
Count each kind
Count two hydrogen atoms and one oxygen atom. The count of one is written without a subscript.
$2\ \mathrm{H},\ 1\ \mathrm{O}$
Write the formula
Write H with a subscript 2 for the two hydrogen atoms, followed by O for the one oxygen atom. This is the formula for water.
$\mathrm{H_2O}$

Answer: The formula is

\mathrm{H_2O}

Check: The formula represents two hydrogen atoms and one oxygen atom, matching the model.

Worked example

One carbon atom and two oxygen atoms

A model of one particle has one C atom connected to two O atoms. Find the formula and explain what its subscript means.

Carbon dioxide particle model

One carbon atom is connected to two oxygen atoms. The drawing shows connections, not true atom sizes or distances.

Read the atom labels
C is the symbol for carbon, and O is the symbol for oxygen. The model contains both elements.
Count the atoms
There is one carbon atom and there are two oxygen atoms. The carbon count of one needs no written subscript. The oxygen count is written as a subscript after O.
$1\ \mathrm{C},\ 2\ \mathrm{O}$
Record the counts
Write the carbon symbol followed by the oxygen symbol with a subscript 2. The oxygen subscript applies to O, not to C.
$\mathrm{CO_2}$

Answer: The formula is

\mathrm{CO_2}

. The subscript 2 means that each represented particle contains two oxygen atoms.

Check: The formula has one C and two O atoms, exactly as in the model.

Worked example

Build a particle from its formula

A formula is given as NH₃. Describe the atom count for one particle, then compare it with a model containing one N atom connected to three H atoms.

Ammonia particle model

One nitrogen atom is connected to three hydrogen atoms. The model is schematic; atoms are not drawn to scale.

Read each symbol
N is nitrogen and H is hydrogen. The formula includes two different elements.
Use the subscript rule
There is no subscript after N, so the formula represents one nitrogen atom. The subscript 3 after H means three hydrogen atoms.
$1\ \mathrm{N},\ 3\ \mathrm{H}$
Compare with the model
The model has one N atom and three H atoms. Its atom counts match the formula, so the model and formula describe the same simple particle composition.
$\mathrm{NH_3}$

Answer: One particle represented by

\mathrm{NH_3}

contains one nitrogen atom and three hydrogen atoms.

Check: The model's one N and three H atoms match the symbols and subscript in the formula.

Common mistakes and how to avoid them

Reading a missing subscript as zero atoms.

Correction: A missing subscript means one atom of that element.

Applying a subscript to every symbol in the formula.

Correction: A subscript applies only to the element symbol immediately before it.

Treating a bond in a model as an atom.

Correction: A bond shows a connection. Count only the labelled atom models.

Using a lowercase first letter or incorrect capitalization in an element symbol.

Correction: Use the correct element symbol, with a capital first letter and a lowercase second letter when there is one.

Lesson summary

A compound contains atoms of at least two different elements joined together.
Element symbols name the atom types in a formula.
A subscript gives the atom count for the symbol directly before it; no subscript means one.
A simple particle model can show connections, while a chemical formula records element types and counts.

Check your understanding

Question 1

A model contains one nitrogen atom and two oxygen atoms. Which formula matches its atom counts?

$\mathrm{NO_2}$
$\mathrm{N_2O}$
$\mathrm{NO}$
$\mathrm{N_2O_2}$

Show answer and explanation

\mathrm{NO_2}

One nitrogen is written N with no subscript. Two oxygen atoms are written O with a subscript 2.

Question 2

\mathrm{H_2O}

, how many oxygen atoms are represented in one particle?

Show answer and explanation

O has no subscript, so it represents one oxygen atom.

Question 3

What information does a simple chemical formula give?

The exact size and colour of each atom
The element types and their atom counts in a represented particle
The true shape and scale of the particle
The number of bonds in every possible model

Show answer and explanation

The element types and their atom counts in a represented particle

A formula records which elements and how many atoms of each are represented. It does not give true sizes, scale, or a complete picture of shape.

Key terms

Atom: A tiny particle that makes up matter.
Element: A pure substance made of only one kind of atom.
Compound: A substance whose particles contain atoms of two or more different elements joined together.
Molecule: A particle made of atoms joined together.
Chemical formula: A compact way to show the element types and atom counts in a represented particle.
Subscript: A small number after an element symbol that tells how many atoms of that element are represented.
Bond: A connection between atoms in a particle model.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 9 Science (SNC1W), expectation C2.7. 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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