Bond Polarity, Dipole Moment, and Percent Ionic Character

The type of chemical bonds, whether it is nonpolar covalent, polar covalent, or ionic, is largely determined by the difference in the electronegativity between the bonding atoms and their bond length.

In bromine, the bond between the two bromine atoms is nonpolar since the difference in electronegativity between them is zero.

In hydrogen bromide, the bond is polar covalent since the more electronegative bromine attracts the electron density away from the less electronegative hydrogen atom.

In potassium bromide, the bond is an ionic bond due to the larger difference in electronegativity, resulting in complete electron transfer from potassium to bromide.

When two equally, but oppositely charged particles are separated by a distance a dipole, is formed. The quantitative measure of the dipole is called the dipole moment represented by the letter µ, which is the product of the magnitude of the charges, Q, reported in coulombs, and the distance between the charges, r, reported in meters.

The dipole moment is given in debye units, where one debye equals 3.34 × 10⁻³⁰ C·m.

Whether a compound is polar covalent or ionic can be determined by calculating the percent ionic character, which is the ratio of a bond’s measured dipole moment to the dipole moment assuming a complete electron transfer. Bonds with more than 50% ionic character are considered ionic.

Consider hydrogen fluoride, where the hydrogen and fluorine atoms are separated by a distance of 92 pm.

If the bond is ionic, the dipole moment is calculated assuming a complete electron transfer. The charge of an electron is multiplied by the distance between the atoms and the obtained value is divided by one debye, which yields the dipole moment of 4.41 D.

However, the experimentally measured dipole moment of hydrogen fluoride is 1.18 D. Therefore, to determine the percent ionic character of hydrogen fluoride, the measured value of 1.18 D is divided by 4.41 D, which gives a percent ionic character of 41%.

Since the percent ionic character of hydrogen fluoride is less than 50%, it is a polar covalent bond.