Molecular polarity

The polarity of a molecule is measured through its dipole moment μ: imagine charges +q and -q separated through a distance d then by definition μ=qd. A practical unit for the μ at the molecular level is the Debye (D), equivalent to 3.336×10^-30 C m.

Polarity is a measure of charge separation within bonds and can frequently be associated to the electro-negativity variations among the atoms, as can be seen in the following series of μ/D values: HF (6.4); HCl (3.6); HBr (2.7); HI (1.4). Though,There are, reasons why a unique correlation of dipole moments with electronegativity variations is not feasible, as follows.

1. Lone-pair electrons also have dipole moments. Sometimes these might be reinforces the moment resulting from electronegativity difference (like in H₂O and NH₃, that both have very large net dipoles); in other cases the two contributions may opposite to each other, as occurs in CO where the net dipole is very small.
2. In polyatomic molecules the dipoles related with each bond add vectorially to give a resulting that depends on the bond angles. In highly symmetrical molecules like BF₃, CF₄, PF₅ or SF₆ the net dipole moment is zero although the individual bonds may be strongly polar. With an entire dipole moment. Larger dipoles lead to stronger intermolecular forces even though other factors (example hydrogen bonding) can also be involved. For molecular species like LiF(g) the dipole moment is almost that predicted for an ionic Li⁺F^- charge distribution. Such types of ionic species do not, though, condense to create molecular solids or liquids but rather ones with usual ionic structures in which each anion is surrounded by various cations and vice versa.

Application of an electric field to a substance causes a partial alignment of polar molecules; so molecular dipole moments contribute to the dielectric constant of a solid or liquid, one of the very significant physical properties determining the behaviour of a solvent. The dielectric constant arises from the molecular polarizability in nonpolar substances, and is usually much smaller than with polar molecules.