9.8
Bond Polarity, Dipole Moment, and Percent Ionic Character
Bond Polarity
The absolute value of the difference in electronegativity (ΔEN) of two bonded atoms provides a rough measure of the polarity to be expec…
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.
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Q1: How does electronegativity difference determine bond type?
The electronegativity difference between bonded atoms determines whether a bond is nonpolar covalent, polar covalent, or ionic. When the difference is zero, the bond is nonpolar covalent. A small difference produces a polar covalent bond, while a large difference results in an ionic bond due to complete electron transfer. The periodic table positions of atoms provide the best guide to predicting bond character.
Q2: What is a dipole moment and how is it calculated?
A dipole moment (µ) quantifies the separation of charge in a polar bond. It is calculated as the product of the charge magnitude (Q) and the distance between charges (r). The formula is µ = Q × r, with results expressed in debye units, where one debye equals 3.34 × 10⁻³⁰ C·m. Dipole moment vectors point from the less electronegative atom toward the more electronegative atom.
Q3: How do you determine if a bond is ionic or polar covalent using percent ionic character?
Percent ionic character is calculated by dividing a bond's measured dipole moment by the dipole moment assuming complete electron transfer, then multiplying by 100. Bonds with greater than 50% ionic character are classified as ionic. For example, hydrogen fluoride has 41% ionic character, making it polar covalent since it falls below the 50% threshold.
Q4: Why is carbon dioxide nonpolar despite having polar bonds?
Carbon dioxide is nonpolar because its molecular geometry causes bond dipole moments to cancel. The linear structure places two polar C=O bonds on opposite sides of the carbon atom. Since the bond moments point in opposite directions, their vector sum equals zero, resulting in no net dipole moment for the molecule overall.
Q5: What makes water a polar molecule?
Water is polar because its bent molecular geometry prevents bond dipole moments from canceling. The two O-H bonds are polar due to oxygen's higher electronegativity, and the bent shape caused by lone pairs on oxygen means the bond moments do not point in opposite directions. This results in a net dipole moment, making water a polar molecule.
Q6: How does molecular structure affect overall polarity?
Molecular polarity depends on both individual bond polarity and molecular geometry. For diatomic molecules, bond dipole moment determines polarity. For polyatomic molecules, bond dipoles must be added as vectors in three-dimensional space. If bond moments cancel due to symmetrical arrangement, the molecule is nonpolar; if they do not cancel, the molecule is polar.
Q7: What role does bond length play in determining bond type?
Bond length, along with electronegativity difference, helps determine bond type. Shorter bond lengths typically correspond to stronger attractions and greater electron density concentration. In ionic bonds, electron transfer results in different bond characteristics than in covalent bonds. Understanding bond energies and bond lengths helps predict how atoms will interact and what type of bond will form.