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C2.3 · Predict and diagram simple molecular and ionic shapes with VSEPR

Learn to predict and diagram simple molecular and ionic shapes with vsepr through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Structure and Properties of Matter

A Grade 12 guide to simple molecular and ionic shapes

Molecules are too small to see directly, but their shapes affect how they fit and interact. A familiar example is water: its three atoms do not lie in a straight line. VSEPR is a course-level model that helps predict such shapes from a particle’s electron groups. You will use it for simple molecules and polyatomic ions, which are charged groups of bonded atoms.

What you will learn

Explain how repulsion between electron groups helps predict a particle’s shape.
Count bonding groups and lone pairs around a central atom.
Predict and describe common molecular and polyatomic-ion shapes.
Draw a clear shape diagram that distinguishes atoms, bonds, and lone pairs.

1. From atoms and bonds to electron groups

Before predicting shape, recall two ideas from earlier chemistry. A covalent bond is a shared pair of electrons between atoms. A lone pair is a pair of valence electrons that belongs to one atom and is not shared in a bond. A Lewis structure shows bonds and lone pairs as lines and pairs of dots.

VSEPR stands for Valence Shell Electron Pair Repulsion. It is a model for predicting the arrangement of groups of electrons around a central atom. Electron groups repel one another, so they arrange themselves as far apart as the model allows. This arrangement is called the electron-group arrangement.

For counting, each bond to another atom is one electron group. A single, double, or triple bond each counts as one group because each connects the central atom to one neighbouring atom. Each lone pair on the central atom also counts as one group. Do not count lone pairs on the surrounding atoms when predicting the central atom’s arrangement.

The shape name describes the positions of the atoms, not the positions of lone pairs. The shape of a molecule or ion is therefore not always the same as its electron-group arrangement. For example, lone pairs take up space in the arrangement, but they are not named as atoms in the molecular shape.

\text{electron groups} = \text{bonding groups} + \text{central-atom lone pairs}

Make or inspect a Lewis structure before counting.
Count each bond and each central-atom lone pair as one group.
Use the atom positions to name molecular shape.

2. Use the group count to predict a shape

Start by identifying the central atom, usually the atom bonded to the greatest number of other atoms. Hydrogen is never the central atom in these simple structures. Then count the electron groups around that atom and compare the count with the common arrangements below.

Two groups arrange in a line, giving a linear arrangement and a linear shape when there are two bonded atoms and no lone pairs. Three groups spread into a flat triangular arrangement. With three bonds and no lone pairs, the molecular shape is trigonal planar. With two bonds and one lone pair, the molecular shape is bent.

Four groups point toward the corners of a three-dimensional tetrahedron. With four bonds and no lone pairs, the shape is tetrahedral. With three bonds and one lone pair, it is trigonal pyramidal. With two bonds and two lone pairs, it is bent.

Five groups have a trigonal bipyramidal arrangement; six groups have an octahedral arrangement. These names describe the arrangement of groups. To name the molecular shape in such cases, use the positions of bonded atoms and account for any lone pairs. In simple applications, focus on the common shapes in the table and use the same counting process.

Lone pairs usually push bonding groups closer together more strongly than bonding groups push one another. That is why adding a lone pair can change a shape name, even when the number of surrounding atoms is unchanged. For the prediction task, identify the standard shape; exact bond angles are not needed. shape depends on bonded atoms and lone pairs around the central atom

Two groups: linear.
Three groups: trigonal planar arrangement; molecular shapes include trigonal planar and bent.
Four groups: tetrahedral arrangement; molecular shapes include tetrahedral, trigonal pyramidal, and bent.
Five and six groups give trigonal bipyramidal and octahedral arrangements.

3. Apply the model to molecules and ions

The same VSEPR steps apply to a neutral molecule and to a polyatomic ion. First use the formula and overall charge to make a correct Lewis structure. Then count electron groups at the central atom. The charge matters when drawing the Lewis structure, but it does not add a separate group by itself.

For a polyatomic ion, put the complete structure in square brackets and show its charge outside the brackets. A single-atom ion, such as a sodium ion, has no molecular shape to predict using this method. The method applies to ions made of bonded atoms, such as ammonium or nitrate.

To make a useful diagram, draw the central atom and surrounding atoms in the predicted arrangement. Show bonds as lines and show any central-atom lone pairs as pairs of dots. For three-dimensional shapes, use a wedge for a bond coming out of the page and a dashed bond for one going behind the page, if those drawing conventions are available. State the shape name beside the drawing.

A flat formula or a row of atom symbols is not automatically a shape diagram. It may hide the three-dimensional arrangement. A correct diagram should communicate which atoms are connected and where they sit relative to the central atom.

\left[\text{polyatomic ion structure}\right]^{\text{charge}}

Draw the Lewis structure before predicting shape.
Count electron groups around the central atom, not the whole particle.
Show the overall charge of a polyatomic ion outside square brackets.

4. A reliable prediction routine

Use the same routine each time. First, identify whether the species is a molecule or a polyatomic ion and note its charge. Second, draw a Lewis structure that accounts for the atoms, bonds, lone pairs, and total charge. Third, identify the central atom and count its electron groups. Fourth, name the electron-group arrangement. Fifth, name the molecular or ionic shape from the positions of the bonded atoms. Finally, sketch and label the shape.

Check the result in two ways. The group count must match the arrangement you chose. Also, the shape name must describe atom positions, not include lone pairs as if they were atoms. If the central atom has no lone pairs, the arrangement and shape names often match. If it has lone pairs, they may differ.

VSEPR gives a useful prediction, not a photograph of a particle. A diagram is a model. Keep it simple: show the atoms, bonds, relevant lone pairs, charge when needed, and the shape name. Avoid claiming that a two-dimensional sketch shows exact distances.

Follow a consistent structure-to-count-to-shape sequence.
Check whether lone pairs make the molecular shape differ from the electron-group arrangement.
Treat the drawing as a model of the predicted arrangement.

Common electron-group counts and shapes

Groups around central atom	Bonding groups	Lone pairs	Molecular shape
2	2	0	Linear
3	3	0	Trigonal planar
3	2	1	Bent
4	4	0	Tetrahedral
4	3	1	Trigonal pyramidal
4	2	2	Bent

Worked example

Predicting the shape of ammonia

Use VSEPR to predict and describe the shape of ammonia,

\mathrm{NH_3}

Build the electron picture
Nitrogen is the central atom, and each hydrogen forms one single bond to it. Nitrogen has one lone pair in the Lewis structure. The structure therefore has three bonding groups and one lone pair around nitrogen.
$3+1=4$
Find the arrangement
Four electron groups arrange in a tetrahedral pattern. This describes the positions of all four groups, including the lone pair.
$\text{4 groups}\rightarrow\text{tetrahedral arrangement}$
Name the molecular shape
Only three of the four groups are bonded atoms. The remaining group is a lone pair, so the atom positions form a trigonal pyramidal shape. In a sketch, put nitrogen near the centre, the three hydrogens around it, and the lone pair on nitrogen.
$\mathrm{NH_3}:\ \text{trigonal pyramidal}$

Answer: Ammonia has four electron groups around nitrogen: three bonding groups and one lone pair. Its electron-group arrangement is tetrahedral, and its molecular shape is trigonal pyramidal.

Check: The prediction uses the central atom’s three bonds and one lone pair. The shape name describes the three hydrogen positions, not the lone pair.

Common mistakes and how to avoid them

Counting a double bond as two electron groups.

Correction: A bond to one neighbouring atom counts as one group, whether it is single, double, or triple.

Using the electron-group arrangement as the shape name even when the central atom has lone pairs.

Correction: Name the electron-group arrangement first, then name the shape using only the bonded atoms.

Counting lone pairs on surrounding atoms when finding the central atom’s arrangement.

Correction: For the central arrangement, count bonds to the central atom and lone pairs on that central atom.

Treating the charge of an ion as an extra electron group.

Correction: Use the charge when drawing the Lewis structure. Then count the actual bonds and central-atom lone pairs.

Lesson summary

VSEPR predicts shape by arranging electron groups around a central atom to keep them apart.
Each bond and each lone pair on the central atom counts as one group.
The molecular shape names the arrangement of atoms; lone pairs affect the prediction but are not atoms.
Use a Lewis structure, count groups, identify the arrangement, name the shape, and draw the result.

Check your understanding

Question 1

A central atom has two bonds to surrounding atoms and two lone pairs. What is its molecular shape?

Linear
Bent
Trigonal planar
Tetrahedral

Show answer and explanation

Bent

There are four electron groups, so the arrangement is tetrahedral. With two bonded atoms and two lone pairs, the molecular shape is bent.

Question 2

When predicting a central atom’s electron-group arrangement, how should a double bond to one atom be counted?

As one electron group
As two electron groups
As three electron groups
It is not counted

Show answer and explanation

As one electron group

A double bond connects the central atom to one neighbouring atom, so it counts as one electron group.

Key terms

Central atom: The atom at the centre of a simple molecular or polyatomic-ion structure, bonded to surrounding atoms.
Covalent bond: A bond formed when atoms share electrons.
Lone pair: A pair of valence electrons on an atom that is not shared in a bond.
Electron group: A bond or a lone pair counted as one region of electron density around the central atom for VSEPR.
Electron-group arrangement: The arrangement of all electron groups around a central atom.
Molecular shape: The arrangement of bonded atoms around a central atom.
Polyatomic ion: A charged group of two or more atoms held together by bonds.

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