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General Chemistry I · Chemical Bonding & Molecular Geometry

Molecular Polarity

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Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

A molecule is polar if it has a net dipole moment — an uneven distribution of electron density that gives one side a partial negative charge and the other a partial positive charge. Each polar bond contributes a dipole (a vector pointing toward the more electronegative atom), but the molecule's overall polarity is the vector sum of all bond dipoles (plus lone-pair effects). If the individual dipoles cancel because of a symmetric geometry, the molecule is nonpolar even though its bonds are polar.

Why this matters

Polarity is the reason "like dissolves like." Polar water dissolves polar and ionic substances (salt, sugar) but not nonpolar oils; nonpolar solvents dissolve nonpolar solutes. Polarity also drives boiling points, solubility, and how molecules interact (hydrogen bonding, dipole–dipole forces). In biology, the polarity of water is the foundation of life's chemistry — membranes, protein folding, and DNA structure all depend on polar vs. nonpolar regions.

The college version

Key Ideas

  • Bond dipole: a polar bond has δ⁺ and δ⁻ ends; the dipole arrow points from δ⁺ toward δ⁻ (toward the more electronegative atom).
  • Molecular dipole: the vector sum of all bond dipoles and lone-pair contributions.
  • Symmetric geometries cancel: linear (CO₂), trigonal planar (BF₃), tetrahedral (CH₄, CCl₄), octahedral (SF₆), and trigonal bipyramidal (PCl₅) with identical outer atoms are nonpolar.
  • Asymmetric geometries add: bent (H₂O), trigonal pyramidal (NH₃), and any molecule with unlike outer atoms (CHCl₃) are polar.
  • Lone pairs on a central atom usually break symmetry and create a net dipole (H₂O, NH₃).

Equations and Variables

  • ΔEN = |EN(A) − EN(B)| sets each bond's polarity.
  • Net dipole μ = vector sum of bond dipoles: μ = μ₁ + μ₂ + … (μᵢ = each bond dipole, a vector quantity with magnitude and direction).
    • If the vector sum = 0, the molecule is nonpolar; otherwise it is polar.

How It Works

  1. Draw the Lewis structure and determine the geometry (VSEPR).
  2. Assign a dipole vector to each polar bond, pointing toward the more electronegative atom.
  3. Add the vectors: if they are equal in magnitude and symmetrically opposed, they cancel (net dipole = 0 → nonpolar).
  4. If they do not cancel (bent, pyramidal, or unlike outer atoms), a net dipole remains → polar.
  5. Lone pairs contribute electron density on one side, reinforcing the net dipole in bent and pyramidal molecules.

Worked Example

Classify CO₂, H₂O, BF₃, NH₃, and CH₄ as polar or nonpolar.

  1. CO₂ — nonpolar. Linear geometry; the two C=O dipoles point in opposite directions and cancel exactly.
  2. H₂O — polar. Bent geometry (~104.5°); the two O–H dipoles point toward O and do not cancel, plus oxygen's lone pairs add density on the O side → a net dipole toward oxygen.
  3. BF₃ — nonpolar. Trigonal planar; the three B–F dipoles point outward at 120° and cancel.
  4. NH₃ — polar. Trigonal pyramidal; the three N–H dipoles plus the lone pair on N produce a net dipole pointing up along the lone pair.
  5. CH₄ — nonpolar. Tetrahedral; the four C–H dipoles (which are individually small) cancel by symmetry.

Common Confusions

  • "Polar bonds always mean a polar molecule." — Wrong: symmetric molecules like CO₂, BF₃, and CH₄ have polar bonds whose dipoles cancel, making them nonpolar.
  • "A molecule with no polar bonds can still be polar." — Wrong: with no polar bonds there are no bond dipoles, so the molecule is nonpolar (only lone pairs could add asymmetry, but you still need a dipole source).
  • "Water is linear." — Wrong: water is bent, which is precisely why its bond dipoles don't cancel and it is polar.
  • "Symmetry is about the formula, not the 3-D shape." — Wrong: you must consider the actual geometry; CH₄ and H₂O both have the formula with one central atom, but only CH₄ is symmetric and nonpolar.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of each polar bond as a tug-of-war arrow pointing toward the greedier atom. Now imagine all the arrows tied together at the center. If the arrows pull in perfectly opposite directions, they cancel and the whole thing doesn't move — that's a nonpolar molecule like CO₂ (two equal arrows pointing apart). But if the arrows point in directions that don't balance — like the two arrows in bent water both pulling toward oxygen — the molecule ends up with a "tug" in one direction, a net dipole. Symmetric molecules are like a perfect tug-of-war stalemate; lopsided ones have a clear winner. (The analogy treats dipoles as force vectors, which is exactly how the vector sum works.)

Key takeaways

  • Bond dipole points toward the more electronegative atom (δ⁻).
  • Molecular polarity = vector sum of bond dipoles (+ lone pairs).
  • Nonpolar despite polar bonds: CO₂ (linear), BF₃ (trigonal planar), CH₄/CCl₄ (tetrahedral), SF₆ (octahedral), PCl₅ (trigonal bipyramidal).
  • Polar: H₂O (bent), NH₃ (trigonal pyramidal), HCl, CHCl₃, SO₂.
  • Symmetric geometry + identical outer atoms → dipoles cancel → nonpolar.
  • Lone pairs on the central atom often create polarity (H₂O, NH₃).
  • "Like dissolves like" is a direct consequence of molecular polarity.
  • Bond dipole points toward the more electronegative atom.
  • Molecular polarity = vector sum of bond dipoles (+ lone-pair contributions).
  • Symmetric geometries cancel dipoles → nonpolar (CO₂, BF₃, CH₄, SF₆).
  • Asymmetric geometries → polar (H₂O, NH₃, CHCl₃).
  • Lone pairs help create a net dipole in bent/pyramidal molecules.
  • Polarity explains solubility ("like dissolves like") and intermolecular forces.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Define a bond dipole and relate it to electronegativity differences.
  • Determine whether a molecule is polar or nonpolar from its geometry and bond dipoles.
  • Explain why some molecules with polar bonds (CO₂, BF₃, CH₄) are nonpolar overall.
  • Predict the direction of a molecule's net dipole.

Sources & references

  1. OpenStax, *Chemistry 2e*, "7.6 Molecular Structure and Polarity."
  2. OpenStax, *Chemistry 2e*, "7.2 Covalent Bonding."
  3. OpenStax, *Chemistry 2e*, "Ch. 7 Introduction."

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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