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C2.4 · Predict molecular polarity from shape and electronegativity

Learn to predict molecular polarity from shape and electronegativity through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Structure and Properties of Matter

Using electronegativity and molecular shape

A molecule’s shape can help explain why some molecules have an uneven distribution of electrical charge while others do not. To predict molecular polarity, consider two things: whether the bonds are polar and how those bonds are arranged in space. This lesson reviews the needed ideas and applies them to familiar molecular formulas.

What you will learn

Explain how electronegativity differences make bonds polar.
Use a molecule’s shape to decide whether its bond polarities cancel.
Predict whether a molecule is polar or non-polar and explain the prediction.

1. Review: bonds and electronegativity

Atoms in a molecule share electrons in covalent bonds. Electronegativity describes how strongly an atom attracts the shared electrons in a bond. When bonded atoms have different electronegativities, the electrons are drawn more strongly toward one atom.

This unequal sharing gives the atoms partial charges. The atom with the stronger pull is slightly negative, and the other atom is slightly positive. The word “partial” matters: these are not full ionic charges. We can mark them with

\delta-

and

\delta+

A polar bond is a covalent bond with unequal sharing of electrons. A non-polar bond has equal or nearly equal sharing. A bond-polarity arrow is a way to show the direction of the unequal sharing: it points toward the more electronegative atom. You can compare electronegativities using a course reference chart. No calculation is needed for this prediction.

\delta+\;\longrightarrow\;\delta-

Different electronegativities can produce a polar bond.
The more electronegative atom has the partial negative charge, $\delta-$ .
A bond’s polarity and a molecule’s polarity are not the same question.

2. Shape determines whether bond polarities cancel

A molecule can contain polar bonds and still be non-polar overall. To see why, picture each bond’s polarity as a pull in the direction of its bond-polarity arrow. If the molecule’s shape places equal pulls in opposite directions, they cancel. If they do not cancel, the molecule has an overall uneven charge distribution and is polar.

Molecular shape means the arrangement of atoms around a central atom in three dimensions. A shape model is useful because a flat drawing can hide the direction of bonds that point out of the page. For example, a tetrahedral arrangement has bonds directed into three-dimensional space, not four bonds all lying in one flat plane.

Use the shape learned through electron-pair repulsion models: regions of shared or unshared electrons around a central atom affect how its atoms are arranged. An unshared pair is a pair of valence electrons on the central atom that is not part of a bond. It can make the arrangement of bonded atoms uneven. Use the molecule’s actual shape, not just its formula, when judging cancellation.

A practical rule is to check the bonds first, then the shape. If all the bonds are non-polar, the molecule is non-polar. If some bonds are polar, ask whether the arrangement makes their pulls cancel. A symmetrical arrangement with matching outer atoms can allow cancellation. Different outer atoms, or an uneven arrangement, can prevent it. Symmetry is a guide; the bond directions and polarities are what determine the result.

Molecular polarity depends on both bond polarity and molecular shape.
Opposite, matching bond-polarity pulls can cancel.
Polar bonds do not automatically make a molecule polar.

3. A reliable prediction method

For each molecule, follow the same sequence. First, identify the bonds and compare the electronegativities of the atoms in each bond. Mark which bonds are polar and which atom in each polar bond is partially negative. Second, identify the molecule’s shape around the central atom. Use the three-dimensional shape, including the effect of unshared pairs where relevant.

Third, consider the directions of the bond-polarity arrows together. Ask whether matching pulls cancel because of the shape. If they cancel, predict a non-polar molecule. If they do not cancel, predict a polar molecule. In your explanation, name the polar bonds and connect the final prediction to the shape.

This method predicts polarity from the structure and electronegativity information. It does not require a numerical value for the molecule’s overall polarity. The result is a category: polar or non-polar.

\text{bond polarity} + \text{molecular shape} \longrightarrow \text{molecular polarity}

Identify polar bonds before deciding molecular polarity.
Use the actual three-dimensional molecular shape to judge cancellation.
State whether the molecule is polar or non-polar and give the reason.

4. What the prediction tells you

In a polar molecule, the partial positive and partial negative regions do not balance out across the molecule. In a non-polar molecule, the bond polarities may be absent, or their effects may cancel because of the arrangement of atoms.

Keep the two levels of description separate. A bond describes the sharing between two bonded atoms. Molecular polarity describes the overall distribution across the whole molecule. In a written answer, it is not enough to report that a bond is polar; you must also use shape to decide what happens to the molecule as a whole.

No units or significant figures are needed for this prediction because it is a qualitative classification, not a measured calculation. The evidence for the classification is the electronegativity comparison and the shape-based cancellation check.

A molecular polarity prediction is qualitative.
Support the prediction with both bond information and shape.

Worked example

Comparing carbon dioxide and water

Predict whether carbon dioxide,

\mathrm{CO_2}

, and water,

\mathrm{H_2O}

, are polar or non-polar. Explain how their shapes affect the polar bonds.

Check the bonds
Oxygen is more electronegative than carbon and hydrogen. Therefore, the carbon–oxygen bonds in carbon dioxide and the oxygen–hydrogen bonds in water are polar. In each bond, oxygen is the partially negative end.
Use the shape of carbon dioxide
Carbon dioxide has a linear shape, with the carbon atom between the two oxygen atoms. The two matching carbon–oxygen bond-polarity arrows point in opposite directions. Their effects cancel, so carbon dioxide is non-polar overall.
$\mathrm{O=C=O}$
Use the shape of water
Water has a bent shape. Its two oxygen–hydrogen bonds do not point in opposite directions along one straight line. Their polarities therefore do not cancel, so water is polar overall.
$\mathrm{H_2O}$
State the predictions
Both molecules contain polar bonds, but their shapes lead to different results. The linear arrangement cancels the bond polarities in carbon dioxide; the bent arrangement does not cancel them in water.
$\mathrm{CO_2}:\ \text{non-polar};\quad \mathrm{H_2O}:\ \text{polar}$

Answer: Carbon dioxide is non-polar because its two matching polar bonds are arranged linearly and cancel. Water is polar because its bent shape prevents its two polar bond effects from cancelling.

Check: The prediction distinguishes bond polarity from molecular polarity and uses shape to explain why the results differ.

Common mistakes and how to avoid them

Assuming that any molecule with polar bonds must be polar.

Correction: Check the molecular shape. Polar bond effects can cancel in a suitable arrangement, as in linear carbon dioxide.

Using a flat drawing to decide whether bond polarities cancel.

Correction: Use the molecule’s three-dimensional shape. A bent molecule and a linear molecule arrange their bonds differently.

Calling a molecule non-polar because it contains identical outer atoms.

Correction: Identical outer atoms can support cancellation in a symmetrical shape, but you must still check the shape and bond directions.

Giving only the electronegativity comparison as the explanation.

Correction: Electronegativity identifies polar bonds. Shape is also needed to predict the polarity of the whole molecule.

Lesson summary

Electronegativity differences can make covalent bonds polar.
Molecular shape determines how the polarities of bonds are arranged.
When bond-polarity effects cancel, the molecule is non-polar; when they do not, it is polar.
A complete prediction considers both the bonds and the three-dimensional shape.

Check your understanding

Question 1

A molecule has polar bonds arranged so that their effects cancel. What is the prediction for the molecule?

It is non-polar overall.
It is polar because at least one bond is polar.
Its shape has no effect on its polarity.
It has full ionic charges on every atom.

Show answer and explanation

It is non-polar overall.

When the polar bond effects cancel because of the arrangement, the molecule is non-polar overall.

Question 2

Why is water polar in the worked example?

Its bent shape prevents the polar oxygen–hydrogen bonds from cancelling.
Its two oxygen–hydrogen bonds are non-polar.
Its linear shape makes the bond effects point in the same direction.
Its atoms have full positive and negative ionic charges.

Show answer and explanation

Its bent shape prevents the polar oxygen–hydrogen bonds from cancelling.

The oxygen–hydrogen bonds are polar, and water’s bent shape prevents their effects from cancelling.

Question 3

Which information is needed to predict molecular polarity when a molecule has polar bonds?

The molecule’s shape and the directions of its polar bonds.
Only the number of atoms in its formula.
Only the identity of the central atom.
The mass of the molecule in grams.

Show answer and explanation

The molecule’s shape and the directions of its polar bonds.

Shape shows how the polar bond effects are arranged and whether they cancel.

Key terms

Electronegativity: How strongly an atom attracts shared electrons in a covalent bond.
Polar bond: A covalent bond in which electrons are shared unequally.
Partial charge: A small positive or negative charge caused by unequal sharing of electrons; written with the symbol $\delta$ .
Molecular shape: The three-dimensional arrangement of atoms in a molecule.
Molecular polarity: An uneven distribution of charge across a whole molecule when the polarities of its bonds do not cancel.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation C2.4. 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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