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B2.3 · Build models of simple organic molecules

Learn to build models of simple organic molecules through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Organic Chemistry

From familiar atoms to clear molecular models | Study-guide label: B2.3

A substance’s properties come from the particles that make it up, but individual molecules are too small to see directly. A model gives us a way to represent the atoms and bonds in a molecule. In this lesson, you will build and check models of simple organic molecules using familiar Grade 11 ideas about atoms, chemical symbols, and covalent bonds. A covalent bond is a bond formed when atoms share electrons. The goal is a model that shows a sensible arrangement of atoms and the connections between them—not a picture of a molecule at its actual size.

What you will learn

Use basic bonding rules to build a model of a simple organic molecule.
Check that a model shows the correct atoms and bonds for its formula.
Compare molecular, structural, and three-dimensional representations of the same molecule.

1. Start with the atoms and their bonding patterns

Before building a model, recall that a chemical formula uses element symbols and subscripts to tell how many atoms of each element are present. For example, the formula is not needed to count atoms; in the subscript indicates two hydrogen atoms. A formula does not, by itself, show how the atoms are connected.

Organic molecules contain carbon. In simple models, use the basic bonding patterns you learned earlier: a carbon atom forms four bonds, a hydrogen atom forms one, and an oxygen atom forms two. A bond can be shown as a line between atoms. In these models, one line represents one shared pair of electrons. These are useful counting rules for checking a model; they are not drawings of electrons moving around atoms.

Begin with a formula and count each type of atom. Then place the atoms and connect them with bonds. Make sure each carbon has four bonds, each hydrogen has one, and each oxygen has two. A bond between two atoms counts once for each atom connected by it. The completed model must include every atom in the formula, with no extra atoms.

A molecular formula gives the number of each kind of atom, not their arrangement.
Use four bonds for carbon, one for hydrogen, and two for oxygen in the simple models in this lesson.
A line between atom symbols represents a covalent bond.

2. Choose a representation that shows the structure

A model can be made with coloured balls and connectors, drawn with atom symbols and lines, or built in three dimensions. In a ball-and-stick model, balls stand for atoms and sticks stand for bonds. Colours help distinguish elements, but a colour has no chemical meaning unless a model’s key explains it. The model is simplified: atoms are not solid coloured balls, and sticks are not physical rods inside a molecule.

A molecular formula is a compact count of atoms. A structural formula shows which atoms are connected. For example, methane has the molecular formula and can be represented by a carbon connected to four hydrogen atoms. The structural representation gives information that the formula alone does not show: it shows the carbon as the central atom, attached to each hydrogen.

In a two-dimensional drawing, the page can make bonds appear to lie flat. A three-dimensional model can show that a molecule has a shape in space. When you build a simple model, arrange the connectors so they can attach to the correct number of bonds on each atom. Do not assume that the drawing’s position on the page tells you the molecule’s actual orientation.

Some molecules contain more than one element besides carbon. Ethanol, for example, contains carbon, hydrogen, and oxygen. Its molecular formula is , while a structural representation, , shows the order of the atoms along the molecule. In this representation, the oxygen has one bond to carbon and one to hydrogen. The formula and structural representation describe the same molecule in different ways.

Molecular formulas count atoms; structural formulas show connections.
Ball-and-stick models show atoms and bonds in a simple, visible form.
A model is a useful representation, not a photograph or full-size copy of a molecule.

3. Build, inspect, and revise a model

Use a repeatable process. First, write down the formula and count the atoms. Second, choose a likely arrangement of the atoms and connect them. Third, count the bonds at each atom. Fourth, compare the atoms in the model with the formula. If either check fails, revise the connections or arrangement.

For a small molecule, a carbon chain is often a useful starting arrangement. Add other atoms where the chosen structural representation places them, then add hydrogen atoms to complete the carbon’s four bonds. Do not add hydrogen automatically if the given formula or structure does not allow it. Always check the whole model against the stated formula.

A model may use different layouts and still show the same connections. Turning or rearranging the model does not change the molecule if every atom remains connected to the same neighbours. By contrast, moving a connector so that an atom attaches to a different neighbour changes the structural representation. This is why a model should be checked by tracing its bonds, not only by looking at its overall outline.

For example, a structural formula can be read bond by bond: each line joins two atoms, and each atom must satisfy its bonding pattern. In a model with a carbon–oxygen connection, check the oxygen’s remaining bond as well as the carbon’s remaining bonds. This local check helps catch errors before you compare the total atom count.

Count atoms and check bonds as separate steps.
The total atoms must agree with the molecular formula.
A changed connection can represent a different structure, even if the same kinds and numbers of atoms are present.

4. Use the model as a communication tool

A useful model makes its meaning clear to another person. Include a title or formula when needed, and use a key if colours represent particular elements. If you draw a structure, label the atoms with their correct element symbols. Use a line only when it represents a bond. Keep the drawing uncluttered so the connections can be followed.

Models have limits. A molecular formula does not identify the connections by itself, and a flat structural drawing does not fully show three-dimensional shape. A ball-and-stick model makes connections and general shape easier to inspect, but its atom sizes and bond lengths are not to scale. Choose the representation that answers the question you are asking.

A strong model is not simply neat. It is chemically consistent: it includes the correct atoms, shows the intended connections, and follows the bonding patterns used here. When you explain a model, state what the representation shows and what it leaves out.

Label a model so another person can read it.
Use the representation that makes the needed information easiest to see.
Check both chemical consistency and clarity.

Worked example

Build a model of ethanol

Build a simple structural model for ethanol from its molecular formula, . Show the connections and check the bond count at each type of atom.

Count the atoms
The formula requires two carbon atoms, six hydrogen atoms, and one oxygen atom. Start by placing the two carbon atoms next to each other, then attach the oxygen to one of the carbons. This gives a simple chain of three heavy atoms.
$\mathrm{C_2H_6O}$
Add the hydrogen atoms
Each carbon needs four bonds. The carbon at the end of the chain has one bond to the other carbon, so it needs three bonds to hydrogen. The second carbon has one bond to carbon and one to oxygen, so it needs two bonds to hydrogen. The oxygen already has one bond to carbon, so give it one bond to hydrogen.
$\mathrm{CH_3-CH_2-OH}$
Check the completed model
The structural representation contains two carbon atoms, six hydrogen atoms, and one oxygen atom, matching the formula. Each carbon has four bonds, each hydrogen has one, and the oxygen has two. The model passes both the atom-count and bond-count checks.
$\mathrm{C_2H_6O}$

Answer: A suitable structural representation is . It contains the correct atoms and satisfies the bonding patterns used in this lesson.

Check: There are two carbon atoms, six hydrogen atoms, and one oxygen atom. The bond counts are four for each carbon, one for each hydrogen, and two for oxygen.

Common mistakes and how to avoid them

Treating a molecular formula as if it showed the arrangement of atoms.

Correction: Use the molecular formula to count atoms. Use a structural formula or a physical model to show their connections.

Giving carbon three bonds because only three neighbouring atoms are visible in a drawing.

Correction: Count bond lines, not just neighbouring atoms. A carbon may need another bond to hydrogen to reach four.

Giving oxygen one bond because it is connected to only one heavy atom.

Correction: Check all bonds to oxygen, including a bond to hydrogen when the structure includes one.

Assuming a model must look exactly like a drawing to represent the same molecule.

Correction: Compare which atoms are connected. A rotated or differently arranged model can show the same connections.

Lesson summary

A molecular formula gives the number of atoms of each element.
A structural formula or three-dimensional model shows how atoms are connected.
For the simple models here, check carbon for four bonds, hydrogen for one, and oxygen for two.
Check atom counts and bond counts independently.
Models help us reason about molecules but simplify their real size, appearance, and shape.

Check your understanding

Question 1

A model has one carbon connected by single bonds to four hydrogen atoms. Which formula matches it?

$\mathrm{CH_4}$
$\mathrm{CH_3}$
$\mathrm{C_2H_4}$
$\mathrm{H_4}$

Show answer and explanation

\mathrm{CH_4}

The model has one carbon atom and four hydrogen atoms, so its formula is

\mathrm{CH_4}

. The carbon has four bonds and each hydrogen has one.

Question 2

What information does a structural formula show that a molecular formula does not show by itself?

The connections between atoms
The total number of each kind of atom
The element symbols present
The overall atom count

Show answer and explanation

The connections between atoms

Both representations can communicate atom counts, but a structural formula also shows which atoms are connected.

Key terms

Atom: A basic unit of an element that keeps that element’s identity.
Covalent bond: A bond formed when atoms share electrons; a line represents one bond in the models here.
Molecular formula: A notation that gives the number of atoms of each element in a molecule.
Structural formula: A representation that shows how atoms in a molecule are connected.
Model: A simplified representation used to show or explore features of something too small or complex to inspect directly.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation B2.3. 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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