Molecular Polarity and Dipole Moments

A polar molecule has an uneven distribution of electric charge, with a partial negative end and a partial positive end, and the size of this separation is measured by the dipole moment μ = Q × r, expressed in debye (D). In brief: water (1.85 D) is polar because its bent shape does not cancel its two O–H bond dipoles, while carbon dioxide (0 D) is nonpolar although each C=O bond is polar, because its linear shape makes the dipoles cancel.

Polarity determines the way molecules attract each other, how they dissolve and how they respond to electric fields and microwaves. To decide whether a molecule is polar you need two pieces of information: the polarity of its bonds and its shape. This article explains both, gives dipole moments for common molecules and shows how to estimate the ionic character of a bond.

What makes a bond polar?

When two different atoms share electrons, the atom with the higher electronegativity attracts the shared electrons more strongly. This gives it a partial negative charge (δ−) and the other atom a partial positive charge (δ+). The larger the difference in electronegativity (Δχ), the more polar the bond. As a rough guide:

• Δχ below about 0.4: nonpolar covalent (C–H is 2.55 − 2.20 = 0.35).
• Δχ between about 0.4 and 1.7: polar covalent (O–H is 3.44 − 2.20 = 1.24).
• Δχ above about 1.7: mainly ionic (Na–Cl is 3.16 − 0.93 = 2.23).

These limits are only conventions and the transition is gradual. The bond dipole is drawn as an arrow pointing from δ+ to δ−, with a cross at the positive end.

What is a dipole moment?

The dipole moment is the product of the magnitude of the partial charge Q and the distance r between the charges:

μ = Q × r

The SI unit is the coulomb-meter, but the debye is more convenient: 1 D = 3.336 × 10⁻³⁰ C·m. This is the dipole formed by one electron charge and a positive charge, 0.208 Å apart. The dipole moment is a vector, and for a molecule with several polar bonds the total is the vector sum of all the bond dipoles.

How do you decide whether a molecule is polar?

1. Draw the Lewis structure and determine the molecular shape with VSEPR.
2. Decide whether each bond is polar and draw its dipole arrow.
3. Check whether the arrows cancel because of symmetry.
4. If they cancel, the molecule is nonpolar; if they do not, it is polar.

Molecules with identical atoms around the central atom in a symmetrical shape (linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, square planar) are nonpolar. Lone pairs on the central atom usually break the symmetry, so bent, pyramidal, seesaw and T-shaped molecules are polar.

Dipole moments of common molecules

Interesting pairs are NH₃ and NF₃. In ammonia the N–H dipoles and the lone pair point the same way, giving 1.47 D. In NF₃ the N–F bond dipoles point toward fluorine, opposite to the lone pair, so they cancel partly and the moment is only 0.23 D.

How do you estimate percent ionic character?

The percent ionic character of a bond is the ratio of the measured dipole moment to the moment that would result if a full electron charge were separated over the bond length.

Worked example: hydrogen chloride

1. The H–Cl bond length is 127.4 pm and the measured dipole moment is 1.08 D.
2. A full charge separation would give μ = e × r = 1.602 × 10⁻¹⁹ C × 1.274 × 10⁻¹⁰ m = 2.04 × 10⁻²⁹ C·m.
3. In debye, 2.04 × 10⁻²⁹ / 3.336 × 10⁻³⁰ = 6.12 D.
4. Percent ionic character = 1.08 / 6.12 × 100 = 18 %.

This means that the bond has an average partial charge of about +0.18 on hydrogen and −0.18 on chlorine, so it is mostly covalent.

What are the consequences of polarity?

• Solubility: polar molecules dissolve in polar solvents such as water, and nonpolar molecules in nonpolar solvents.
• Boiling points: dipole–dipole attractions raise the boiling points of polar compounds compared with nonpolar compounds of similar mass.
• Behavior in an electric field: a stream of water from a tap is deflected by a charged rod, while a stream of a nonpolar liquid such as hexane is not.
• Microwave heating: polar molecules such as water rotate to follow the oscillating field of a microwave oven, and the rotation heats the food.
• Chemical reactivity: the positive end of a polar bond is attacked by nucleophiles, and the negative end by electrophiles.

Who introduced the dipole moment?

Peter Debye developed the theory of molecular dipole moments from 1912 and showed how to determine them from the dielectric constant of a substance as a function of temperature. The unit is named after him. He was awarded the Nobel Prize in Chemistry in 1936 for his work on molecular structure, including the study of dipole moments and of diffraction of X-rays and electrons in gases. Dipole moments were important evidence for the shapes of molecules, for example in showing that water is bent and not linear.

Which mistakes are common?

• Judging polarity by bonds alone: CO₂ and CCl₄ have polar bonds but are nonpolar molecules.
• Ignoring lone pairs: they are the reason water and ammonia are polar.
• Forgetting that the symmetry has to be complete: in CH₂Cl₂ the bond dipoles differ and do not cancel.
• Mixing up polarity with ionic compounds: an ionic compound is not a polar molecule but a lattice of ions.
• Mixing up the direction of the dipole arrow: by chemistry convention it points toward the more electronegative atom.