What Is Molecular Polarity?

Molecular polarity describes how electrical charge is distributed across a molecule.

In some molecules, electrons are distributed relatively evenly. These molecules are nonpolar.

In other molecules, electrons are distributed unevenly, producing slightly positive and slightly negative regions. These molecules are polar.

Whether a molecule is polar depends mainly on two things:

• The polarity of its bonds.
• The three-dimensional shape of the molecule.

This means that having polar bonds does not automatically make the entire molecule polar.

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Electronegativity and Bond Polarity

Atoms do not always attract bonding electrons equally.

Electronegativity describes an atom's ability to attract shared electrons in a covalent bond.

If two bonded atoms have different electronegativities, the bonding electrons are attracted more strongly toward one atom.

This produces a polar covalent bond.

The atom attracting the electrons more strongly develops a slight negative charge:

δ−

The other atom develops a slight positive charge:

δ+

These are called partial charges.

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Example: The H–Cl Bond

In hydrogen chloride:

H–Cl

chlorine attracts the shared electrons more strongly than hydrogen.

Therefore:

Hδ+ — Clδ−

The bond is polar.

Because HCl contains only two atoms, there is no second bond pointing in another direction to cancel this polarity.

Therefore:

HCl is a polar molecule.

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Nonpolar Covalent Bonds

If two identical atoms share electrons, they have the same electronegativity.

The electrons are therefore shared equally.

Examples include:

H₂

O₂

N₂

Cl₂

These molecules contain nonpolar covalent bonds and are themselves nonpolar.

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Bond Dipoles

A polar bond creates a bond dipole.

A bond dipole has a direction:

toward the more electronegative atom

We can think of each polar bond as producing a small pull of electron density in a particular direction.

Whether the whole molecule is polar depends on how all of these bond dipoles combine.

This is where molecular shape becomes extremely important.

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Shape and Molecular Polarity

Consider two molecules:

CO₂

and:

H₂O

Both contain polar bonds.

However:

CO₂ is nonpolar

while:

H₂O is polar

Why?

Their molecular shapes are different.

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Carbon Dioxide: Polar Bonds, Nonpolar Molecule

Carbon dioxide has the structure:

O=C=O

Each C=O bond is polar because oxygen attracts electrons more strongly than carbon.

However, CO₂ is:

linear

with a bond angle of:

180°

The two bond dipoles point in opposite directions.

Because they are equal and opposite, they cancel.

Therefore:

CO₂ is nonpolar overall.

This is an important example because it demonstrates that:

polar bonds do not necessarily produce a polar molecule.

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Water: A Polar Molecule

Water contains two polar O–H bonds.

Oxygen attracts the shared electrons more strongly than hydrogen.

Therefore:

• Oxygen has a partial negative charge.
• The hydrogen atoms have partial positive charges.

If water were linear, the bond dipoles could oppose each other.

But water is bent.

Its bond angle is approximately:

104.5°

Because the O–H bond dipoles point in different directions and do not cancel, water has an overall dipole.

Therefore:

H₂O is polar.

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Methane: A Nonpolar Molecule

Methane has the formula:

CH₄

Its molecular shape is:

tetrahedral

The four bonds are arranged symmetrically around the carbon atom.

Any small bond dipoles are distributed symmetrically and cancel overall.

Therefore:

CH₄ is nonpolar.

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Ammonia: A Polar Molecule

Ammonia has the formula:

NH₃

It contains:

• Three N–H bonds.
• One lone pair on nitrogen.

Its molecular shape is:

trigonal pyramidal

The molecule is not symmetrical enough for the bond dipoles to cancel.

Therefore:

NH₃ is polar.

The lone pair is important because it affects the molecular geometry.

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Comparing Common Molecules

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Boron Trifluoride

Boron trifluoride:

BF₃

contains three strongly polar B–F bonds.

However, BF₃ has a:

trigonal planar

shape.

The three identical bonds are arranged symmetrically at approximately:

120°

The bond dipoles cancel because of this symmetry.

Therefore:

BF₃ is nonpolar overall.

Again:

polar bonds + symmetrical geometry can produce a nonpolar molecule.

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Symmetry and Polarity

Symmetry is often useful when predicting molecular polarity.

If a molecule:

• Has identical surrounding atoms.
• Has a symmetrical shape.
• Has bond dipoles arranged equally around the central atom.

the bond dipoles may cancel.

The molecule may therefore be nonpolar.

Examples include:

CO₂

BF₃

CH₄

SF₆

However, symmetry should be used together with an understanding of the bonds and molecular geometry rather than as a rule by itself.

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Asymmetrical Molecules

If polar bonds are arranged asymmetrically, their dipoles usually do not cancel completely.

The molecule then has a net dipole.

Examples include:

H₂O

NH₃

HCl

These molecules are polar.

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Lone Pairs Can Affect Polarity

Lone pairs often influence molecular polarity because they affect molecular shape.

Consider:

CH₄

Four bonds and no lone pairs.

Shape:

tetrahedral

Overall:

nonpolar

Now consider:

NH₃

Three bonds and one lone pair.

Shape:

trigonal pyramidal

Overall:

polar

And:

H₂O

Two bonds and two lone pairs.

Shape:

bent

Overall:

polar

Lone pairs can therefore prevent bond dipoles from cancelling by changing molecular geometry.

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A Method for Predicting Molecular Polarity

When deciding whether a molecule is polar or nonpolar, use the following process.

Identify the bonds

Ask:

Are any of the bonds polar?

If there are no significant bond dipoles, the molecule is generally nonpolar.

Determine the molecular shape

Use the arrangement of bonding regions and lone pairs to determine the three-dimensional shape.

Consider the direction of the bond dipoles

Imagine each bond dipole as an arrow pointing toward the more electronegative atom.

Decide whether the dipoles cancel

If they cancel completely:

nonpolar molecule

If they do not cancel:

polar molecule

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Worked Example: CO₂

Bonds

C=O bonds are polar.

Shape

CO₂ is linear.

Dipole arrangement

The two dipoles are equal and point in opposite directions.

Result

They cancel.

CO₂ is nonpolar.

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Worked Example: H₂O

Bonds

O–H bonds are polar.

Shape

H₂O is bent.

Dipole arrangement

The two bond dipoles do not point directly opposite each other.

Result

They do not cancel.

H₂O is polar.

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Worked Example: NH₃

Bonds

N–H bonds are polar.

Shape

NH₃ is trigonal pyramidal.

Dipole arrangement

The three bond dipoles do not cancel because of the molecular geometry.

Result

NH₃ is polar.

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Worked Example: BF₃

Bonds

B–F bonds are polar.

Shape

BF₃ is trigonal planar.

Dipole arrangement

The three identical bond dipoles are arranged symmetrically.

Result

They cancel.

BF₃ is nonpolar.

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Molecular Dipole

When bond dipoles do not cancel, the molecule has an overall:

molecular dipole

One region of the molecule is relatively:

δ−

while another is relatively:

δ+

This uneven charge distribution influences how the molecule interacts with other molecules.

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Polar Molecules Attract One Another

Polar molecules can attract each other because opposite partial charges attract.

The δ+ region of one molecule can attract the δ− region of another.

These attractions are called:

dipole-dipole forces

These are intermolecular forces because they act between molecules.

They are not the same as the covalent bonds holding atoms together inside a molecule.

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Hydrogen Bonding

Some highly polar molecules can form particularly strong intermolecular attractions called hydrogen bonds.

Hydrogen bonding commonly occurs when hydrogen is bonded directly to:

• Nitrogen.
• Oxygen.
• Fluorine.

Water is an important example.

Hydrogen bonding between water molecules contributes to many of water's unusual properties.

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Polarity and Solubility

Polarity strongly influences whether substances dissolve in one another.

A useful general rule is:

"like dissolves like."

This means:

• Polar substances tend to dissolve more readily in polar solvents.
• Nonpolar substances tend to dissolve more readily in nonpolar solvents.

This is a useful guideline rather than an absolute rule.

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Water as a Polar Solvent

Water is strongly polar.

It can interact with:

• Ions.
• Polar molecules.

This helps explain why many salts and polar substances dissolve well in water.

When sodium chloride dissolves, for example, polar water molecules surround the Na⁺ and Cl⁻ ions.

The oxygen side of water is attracted toward positive ions.

The hydrogen side is attracted toward negative ions.

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Why Oil and Water Separate

Water is polar.

Most oils consist largely of nonpolar molecules.

The interactions between water molecules are much stronger and more favorable than interactions between water and many oil molecules.

As a result, oil and water tend to separate into different layers.

This familiar observation is an everyday example of molecular polarity.

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Soap and Polarity

Soap and detergent molecules have an interesting structure.

They usually contain:

• A polar or ionic hydrophilic head.
• A nonpolar hydrophobic tail.

The head interacts with water.

The tail interacts with oils and grease.

This allows soap molecules to form structures called micelles, trapping grease so that it can be carried away by water.

Polarity therefore explains an important part of how soaps and detergents work.

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Polarity and Boiling Point

Molecular polarity can affect boiling point.

If molecules attract each other strongly, more energy is required to separate them into the gas phase.

Polar molecules may experience:

• Dipole-dipole attractions.
• Hydrogen bonding in appropriate molecules.

Nonpolar molecules mainly experience London dispersion forces.

However, boiling point also depends on factors such as:

• Molecular size.
• Shape.
• Number of electrons.
• Strength of all intermolecular forces.

Therefore, polarity alone does not determine boiling point.

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Polarity and Biological Molecules

Polarity is extremely important in biology.

Cell membranes contain molecules called phospholipids.

A phospholipid contains:

• A polar head.
• Nonpolar tails.

In water, phospholipids naturally arrange themselves into a bilayer.

The polar heads face the watery environments.

The nonpolar tails point inward, away from the water.

This arrangement is a fundamental feature of cell membranes.

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Polarity and Molecular Interactions

Polarity can influence:

• Solubility.
• Boiling point.
• Melting point.
• Surface tension.
• Intermolecular attractions.
• Biological membrane behavior.
• Protein interactions.
• Drug interactions.

This demonstrates an important chemistry relationship:

molecular structure → polarity → intermolecular forces → properties

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Comparing Polar and Nonpolar Molecules

These are general patterns. Actual physical properties also depend on other aspects of molecular structure.

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Predicting Polarity from a Molecular Model

Suppose you are shown a molecular model.

Ask:

What atoms are bonded together?

↓

Which bonds are polar?

↓

What is the three-dimensional molecular shape?

↓

Is the arrangement symmetrical?

↓

Do the bond dipoles cancel?

↓

If yes:

NONPOLAR

If no:

POLAR

This method is much more reliable than simply looking for polar bonds.

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Challenge Example: CCl₄

Carbon tetrachloride has the formula:

CCl₄

Each C–Cl bond is polar.

Its shape is:

tetrahedral

The four identical chlorine atoms are arranged symmetrically around carbon.

The bond dipoles cancel.

Therefore:

CCl₄ is nonpolar overall.

This is another example of a molecule containing polar bonds but having no overall molecular dipole.

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Challenge Example: CH₃Cl

Now replace one chlorine atom in CCl₄ with hydrogen:

CH₃Cl

The molecule is still approximately tetrahedral around carbon.

However, the surrounding atoms are no longer identical.

The bond dipoles do not cancel completely.

Therefore:

CH₃Cl is polar.

This comparison shows how changing just one atom can change the polarity of a molecule.

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Real-World Connection: Removing Grease

Water alone often does not remove grease effectively.

Water is:

polar

Grease is largely:

nonpolar

They interact poorly.

Soap provides a connection between them because it contains both polar and nonpolar regions.

The nonpolar tails interact with grease while the polar heads interact with water.

This allows greasy material to be dispersed and washed away.

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Real-World Connection: Food

Polarity affects many processes in food chemistry.

For example:

• Sugar dissolves readily in water.
• Many oils do not.
• Emulsifiers help mixtures containing water and oil remain mixed.

Food products such as mayonnaise rely on molecules that help stabilize mixtures containing both polar and nonpolar substances.

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Common Mistakes

Assuming Every Molecule with Polar Bonds Is Polar

CO₂, BF₃, CCl₄ and other symmetrical molecules can contain polar bonds while remaining nonpolar overall.

Ignoring Molecular Shape

Polarity cannot always be predicted from a molecular formula alone.

Three-dimensional geometry matters.

Assuming CO₂ Is Polar Because Oxygen Is Electronegative

The C=O bonds are polar, but their dipoles cancel because CO₂ is linear.

Assuming Water Is Linear

Water is bent because oxygen has two lone pairs.

Its bond dipoles therefore do not cancel.

Confusing Bond Polarity With Molecular Polarity

A bond can be polar while the entire molecule is nonpolar.

Assuming Nonpolar Means No Electrons Are Moving

Electrons are still present and moving. Nonpolar means there is no permanent overall separation of charge across the molecule.

Assuming Polarity Is the Only Factor Affecting Properties

Molecular size, shape, hydrogen bonding and other intermolecular forces also matter.

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Check Your Understanding

1. Define a polar molecule.

2. Define a nonpolar molecule.

3. What is electronegativity?

4. Explain how electronegativity differences produce polar bonds.

5. What do δ+ and δ− represent?

6. Explain what is meant by a bond dipole.

7. Why is HCl polar?

8. Why is O₂ nonpolar?

9. Explain why CO₂ is nonpolar even though it contains polar bonds.

10. Explain why H₂O is polar.

11. Why does molecular shape affect polarity?

12. Is CH₄ polar or nonpolar? Explain.

13. Is NH₃ polar or nonpolar? Explain.

14. Explain why BF₃ is nonpolar.

15. Predict whether CCl₄ is polar or nonpolar and explain your reasoning.

16. Why is CH₃Cl polar even though it has an approximately tetrahedral shape?

17. Explain the phrase "like dissolves like."

18. Why do oil and water tend to separate?

19. Explain how soap uses both polar and nonpolar regions to remove grease.

20. Describe how polarity contributes to the structure of cell membranes.

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Key Terms

• Polar molecule – molecule with an uneven distribution of electrical charge and an overall molecular dipole.
• Nonpolar molecule – molecule with no permanent overall separation of charge.
• Electronegativity – ability of an atom to attract shared bonding electrons.
• Polar covalent bond – covalent bond in which electrons are shared unequally.
• Partial charge – slight positive or negative charge produced by unequal electron distribution.
• δ+ – partial positive charge.
• δ− – partial negative charge.
• Bond dipole – separation of charge across a polar bond.
• Molecular dipole – overall separation of charge across a molecule.
• Dipole-dipole force – attraction between oppositely charged regions of polar molecules.
• Hydrogen bond – particularly strong intermolecular attraction involving hydrogen bonded to certain highly electronegative atoms.
• Hydrophilic – attracted to or interacting favorably with water.
• Hydrophobic – tending to avoid interaction with water.
• Symmetry – balanced arrangement of parts around a molecule.
• Intermolecular force – attraction acting between separate molecules.

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Key Takeaways

• Molecular polarity describes how electrical charge is distributed across a molecule.
• Polar molecules have an uneven distribution of charge.
• Nonpolar molecules have no permanent overall separation of charge.
• Differences in electronegativity can produce polar covalent bonds.
• Polar bonds contain partial positive and partial negative regions.
• Molecular polarity depends on both bond polarity and molecular shape.
• Polar bonds do not automatically make a molecule polar.
• Bond dipoles can cancel when arranged symmetrically.
• CO₂ contains polar bonds but is nonpolar because its linear bond dipoles cancel.
• BF₃ contains polar bonds but is nonpolar because of its symmetrical trigonal planar geometry.
• CH₄ is nonpolar because of its symmetrical tetrahedral geometry.
• H₂O is polar because its bent geometry prevents its bond dipoles from cancelling.
• NH₃ is polar because its trigonal pyramidal geometry produces a net molecular dipole.
• Lone pairs can influence polarity by changing molecular geometry.
• Polar molecules can experience dipole-dipole attractions.
• Some polar molecules can form hydrogen bonds.
• Polarity strongly influences solubility.
• Polar substances often interact well with polar solvents such as water.
• Nonpolar substances often interact more readily with other nonpolar substances.
• Soap works because its molecules contain both water-attracting and oil-attracting regions.
• Polarity is important in chemistry, biology, environmental science, food science and everyday life.
• To predict molecular polarity, examine the bonds, determine the molecular shape, and decide whether the bond dipoles cancel.