3.2 Dipole Moment | Chemical Bonding | Chemistry 11

A dipole moment ( $μ$ ) is a measure of the separation of positive and negative electrical charges within a system. It is a measure of a system's overall polarity. For a chemical bond, it is the product of the magnitude of the separated charges and the distance between them.

The dipole moment is a vector quantity, possessing both magnitude and direction. It is conventionally represented by an arrow pointing from the positive charge to the negative charge.

Mathematical Formula

The dipole moment is calculated using the following formula:

$μ = q \cdot r$

Where:

$μ$ = Dipole moment
$q$ = Magnitude of the positive or negative charge
$r$ = Distance of separation between the charges

Figure 3.2.1: Dipole moment representation showing positive and negative charges with separation distance

Units of Dipole Moment

The dipole moment is commonly expressed in debyes (D), a non-SI unit. The SI unit is the Coulomb-meter (C·m).

The conversion between these units is:

$1 D = 3.335 \times 1 0^{- 30} C \cdot m$

Dipole Moment and Electronegativity

The difference in electronegativity between two bonded atoms is the primary factor that determines the bond's polarity and, consequently, its dipole moment.

Large Electronegativity Difference: A significant difference in electronegativity between two atoms leads to a large separation of charge. This results in a highly polar bond (more ionic character) and a large dipole moment.
Small Electronegativity Difference: A small difference in electronegativity results in a less polar or covalent bond, with a smaller or zero dipole moment. If the electronegativity difference is zero (as in diatomic molecules like $O_{2}$ or $N_{2}$ ), the bond is nonpolar, and the dipole moment is zero.

The overall polarity of a molecule depends not only on the polarity of its individual bonds but also on its geometry, which determines if the individual bond dipoles cancel each other out. For example, $C O_{2}$ has polar bonds but a net dipole moment of zero due to its linear shape.

Applications of Dipole Moment

Percentage Ionic Character: Dipole moment values can be used to calculate the ionic character of a covalent bond using the formula: $% Ionic Character = \frac{μ _{o b ser v e d}}{μ _{i o ni c}} \times 100$
Molecular Geometry: It helps in distinguishing between cis and trans isomers and determining the shapes of molecules like $H_{2} O$ (bent) vs $C O_{2}$ (linear).
Comparison of Polarity: It allows for the comparison of molecules like $N H_{3}$ and $N F_{3}$ .

The dipole moment is a vector quantity, possessing both magnitude and direction. It is conventionally represented by an arrow pointing from the positive charge to the negative charge.

Mathematical Formula

The dipole moment is calculated using the following formula:

$μ = q \cdot r$

Where:

$μ$ = Dipole moment
$q$ = Magnitude of the positive or negative charge
$r$ = Distance of separation between the charges

Units of Dipole Moment

The dipole moment is commonly expressed in debyes (D), a non-SI unit. The SI unit is the Coulomb-meter (C·m).

The conversion between these units is:

$1 D = 3.335 \times 1 0^{- 30} C \cdot m$

Dipole Moment and Electronegativity

The difference in electronegativity between two bonded atoms is the primary factor that determines the bond's polarity and, consequently, its dipole moment.

Large Electronegativity Difference: A significant difference in electronegativity between two atoms leads to a large separation of charge. This results in a highly polar bond (more ionic character) and a large dipole moment.
Small Electronegativity Difference: A small difference in electronegativity results in a less polar or covalent bond, with a smaller or zero dipole moment. If the electronegativity difference is zero (as in diatomic molecules like $O_{2}$ or $N_{2}$ ), the bond is nonpolar, and the dipole moment is zero.

Applications of Dipole Moment

Percentage Ionic Character: Dipole moment values can be used to calculate the ionic character of a covalent bond using the formula: $% Ionic Character = \frac{μ _{o b ser v e d}}{μ _{i o ni c}} \times 100$
Molecular Geometry: It helps in distinguishing between cis and trans isomers and determining the shapes of molecules like $H_{2} O$ (bent) vs $C O_{2}$ (linear).
Comparison of Polarity: It allows for the comparison of molecules like $N H_{3}$ and $N F_{3}$ .