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B2.6 · Build molecular and ionic models and write structural formulas

Learn to build molecular and ionic models and write structural formulas through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Matter, Chemical Trends, and Chemical Bonding

SCH3U study topic B2.6: Build molecular and ionic models and write structural formulas

A substance can look like a single piece of material, even though it is made of tiny particles. In a molecular substance, atoms are joined in separate groups called molecules. In an ionic substance, positive and negative ions are arranged together in a repeating structure. Models help show these differences. In this lesson, you will connect familiar formulas to particle models and learn what a structural formula can—and cannot—show.

What you will learn

1. Review: atoms, symbols, and valence electrons

A chemical formula uses element symbols and subscripts. A subscript tells how many atoms of an element are represented. For example, the formula for water is H2O\mathrm{H_2O}: it represents two hydrogen atoms and one oxygen atom in each water molecule. A subscript applies only to the symbol immediately before it.
The outer electrons of an atom are called valence electrons. They are important because they take part in the bonds represented in simple models. For the elements used here, hydrogen has one valence electron, oxygen has six, carbon has four, magnesium has two, and chlorine has seven. You can use the periodic table to find these patterns at this course level.
A covalent bond is a bond in which atoms share electrons. In a simple structural formula, each line between two atoms represents one shared pair of electrons. An ionic bond model instead represents attractions between positive and negative ions. Ions are atoms or groups of atoms with an electric charge.

2. From particles to models

Many substances are too small to inspect particle by particle. A model is a simplified representation that helps us reason about their particles. In a molecular model, each atom is shown as a separate piece, and bonds connect atoms that belong to the same molecule. A ball-and-stick model is one possible way to show this. The balls represent atoms; the sticks represent bonds.
For example, a water molecule contains two hydrogen atoms joined to one oxygen atom. Its structural formula can show the connections as H−O−H\mathrm{H-O-H}. This formula shows which atoms are connected, but it does not give an exact picture of their size or spacing.
Ionic compounds are modelled differently. Their particles are positive and negative ions arranged in a repeating structure, rather than separate molecules. The written formula gives the simplest whole-number ratio of ions. This ratio is called a formula unit. For magnesium chloride, the formula unit contains one magnesium ion for every two chloride ions: MgCl2\mathrm{MgCl_2}. A model should show that ratio and the opposite charges. It should not draw the formula as a single molecule with covalent sticks.

3. Writing and reading structural formulas

A structural formula is a drawing made with element symbols and bond lines. It shows how atoms are connected in a molecule. The formula is not a drawing of the molecule’s exact size or shape. For this lesson, focus on simple structures made from single covalent bonds.
To build a basic molecular model, first count the atoms from the molecular formula. Then use the usual simple bonding patterns for the atoms in the model: hydrogen usually forms one bond, oxygen usually forms two, and carbon usually forms four. Connect the atoms so each has its expected number of bonds. These patterns are a course-level model for the examples here; they are not a replacement for the formula.
For instance, carbon dioxide has one carbon atom and two oxygen atoms. A structural formula can show carbon connected to each oxygen with a double bond: O=C=O\mathrm{O=C=O}. The double lines show two shared pairs between the indicated atoms. The formula and structural formula communicate related information in different ways: the formula counts atoms, while the structural formula also shows connections.
For an ionic model, preserve the ion symbols and charges, then arrange the ions so the positive and negative charges balance in the ratio given by the formula unit. In a magnesium chloride model, one Mg2+\mathrm{Mg^{2+}} ion pairs in the ratio with two Cl−\mathrm{Cl^-} ions. The total charge is zero. Keep the ions distinct in the model; do not connect them with covalent bond lines.

4. Choosing a model that fits the substance

A model is useful only if it represents the kind of particles in the substance. A molecular formula such as H2O\mathrm{H_2O} refers to separate molecules. An ionic formula such as MgCl2\mathrm{MgCl_2} gives a ratio of ions in an ionic structure. The formulas may look similar on the page, but the particle models are not the same.
When you build or draw a model, check three things. First, are all atoms or ions present in the stated numbers? Second, does each molecular atom have the expected simple bonding pattern? Third, for an ionic model, do the ion ratio and charges agree with the formula unit? A model should make these features easy to check.
Structural formulas are most useful for showing connections in molecules. They do not show every feature of a particle. In particular, do not interpret a line between ions as a covalent bond just because lines are used between atoms in a molecular structural formula.

Worked example

Model water and magnesium chloride

For water, use the molecular formula H2O\mathrm{H_2O} to write a simple structural formula. For magnesium chloride, use MgCl2\mathrm{MgCl_2} to state what an ionic model must contain.
  1. Count the water atoms
    The subscript in H2O\mathrm{H_2O} means there are two hydrogen atoms. Oxygen has no subscript, so there is one oxygen atom.
    2 H, 1 O2\ \mathrm{H},\ 1\ \mathrm{O}
  2. Connect the water atoms
    Hydrogen usually forms one bond in this simple model, and oxygen usually forms two. Put oxygen between the two hydrogen atoms and draw one bond to each.
    H−O−H\mathrm{H-O-H}
  3. Read the ionic formula unit
    Magnesium forms a 2+2+ ion and chlorine forms a 1−1- ion. The subscript means that two chloride ions are needed for each magnesium ion, so the positive and negative charges balance.
    Mg2++2 Cl−\mathrm{Mg^{2+}}+2\,\mathrm{Cl^-}
Answer: Water has the structural formula H−O−H\mathrm{H-O-H}. An ionic model of magnesium chloride contains magnesium and chloride ions in a one-to-two ratio.
Check: Two chloride ions contribute a total charge of −2-2, which balances the +2+2 charge of one magnesium ion. The model contains the same ion ratio as MgCl2\mathrm{MgCl_2}.

Common mistakes and how to avoid them

Drawing an ionic compound as a separate molecule with covalent bond lines.
Correction: Show the positive and negative ions and their repeating ratio. Use structural bond lines for connections within molecules.
Forgetting that a missing subscript means one atom.
Correction: Read an element symbol without a subscript as one atom. For example, H2O\mathrm{H_2O} contains one oxygen atom.
Changing a subscript to make a drawn model look balanced.
Correction: Keep the formula’s atom or ion counts. Correct the model to match the formula rather than changing the formula.
Treating a structural formula as a picture of exact particle size or shape.
Correction: Use it to show which atoms are connected and how many bonds are represented. It is a simplified model.

Lesson summary

Check your understanding

Question 1

Which representation best shows the connections between atoms in one water molecule?
  1. H−O−H\mathrm{H-O-H}
  2. Mg2+\mathrm{Mg^{2+}} beside Cl−\mathrm{Cl^-}
  3. H2O\mathrm{H_2O} with no bonds shown
  4. Two chloride ions for each magnesium ion
Show answer and explanation
H−O−H\mathrm{H-O-H}
The structural formula H−O−H\mathrm{H-O-H} shows the two hydrogen atoms connected to the oxygen atom.

Question 2

What ion ratio should an ionic model of magnesium chloride show?
  1. One magnesium ion to one chloride ion
  2. One magnesium ion to two chloride ions
  3. Two magnesium ions to one chloride ion
  4. Two magnesium ions to two chloride ions
Show answer and explanation
One magnesium ion to two chloride ions
One Mg2+\mathrm{Mg^{2+}} ion balances the charges of two Cl−\mathrm{Cl^-} ions, matching the formula unit MgCl2\mathrm{MgCl_2}.

Question 3

What does one line between atoms in a simple structural formula represent?
  1. One shared pair of electrons
  2. One positive ion
  3. One formula unit
  4. One extra atom
Show answer and explanation
One shared pair of electrons
In this model, one covalent bond line represents one shared pair of electrons.

Key terms

Valence electrons
Electrons in an atom’s outer region that take part in the simple bonding models used here.
Covalent bond
A bond represented as atoms sharing electrons.
Ion
An atom or group of atoms with an electric charge.
Structural formula
A formula that uses symbols and bond lines to show how atoms in a molecule are connected.
Formula unit
The simplest whole-number ratio of ions represented by an ionic compound’s formula.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation B2.6. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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