Dipole moment of a bond is a vector and physical quantity to calculate the percentage ionic character in a covalent bond. It is expressed as:

Dipole moment $\left(\mathrm{\mu }\right)=\underset{\mathrm{\delta }\times \mathrm{d}}{\to }$ where, $\delta$ is dipole moment and $d$ is the bond length. It is usually expressed in terms of C.G.S. unit known as Debye (D) $1\mathrm{D}={10}^{-18}$ esu cm. In S.I. unit it is expressed in coulomb metre. Resultant dipole moment $\left({\mathrm{\mu }}_{\mathrm{R}}\right)$of two bond moments $\left({\mathrm{\mu }}_1 \mathrm{and} {\mathrm{\mu }}_2\right)$ acting at an angle $\mathrm{\theta }$, is given by:

${\mathrm{\mu }}_{\mathrm{R}}=\sqrt{{{\mathrm{\mu }}_1}^2+{{\mathrm{\mu }}_2}^2+2{\mathrm{\mu }}_1{\mathrm{\mu }}_2\cos \left(\mathrm{\theta }\right)}$

$\text{ If }{\mathrm{\mu }}_1={\mathrm{\mu }}_2,\text{ Also if }\mathrm{cos\theta }=-1,\text{ ie, }\mathrm{\theta }=180°,\text{ then }\mathrm{\mu }=0$ (molecule is non-polar)

If $\mu \neq 0$ molecule is polar. Dipole moment plays an important role in deciding the stability order of alkanes,i.e., a more stable alkane has less dipole moment. The dipole moment of a molecule can predict the geometrical and position isomers as well as orientations in benzene nucleus and polarity of molecule.

Which species is polar?