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Covalent molecules formed by heteroatoms bound to have some ionic character. The ionic character is due to shifting of the electron pair towards A or B in the molecule AB . Hence, atoms acquire small and equal charge but opposite in sign. Such a bond which has some ionic character is described as a polar covalent bond. Polar covalent molecules can exhibit a dipole moment. The dipole moment is equal to the product of charge separation, q and the bond length, d for the bond. The unit of dipole moment is Debye. One Debye is equal to $10^{-18}$ esu cm.
The dipole moment is a vector quantity. It has both magnitude and direction. Hence, the dipole moment of molecules depends upon the relative orientation of the bond dipole, but not the polarity of bonds alone. The symmetrical structure shows a zero dipole moment. Thus, a dipole moment help to predict the geometry of the molecules. Dipole moment values can be used to distinguish between cis- and trans-isomers; ortho-, meta- and para-forms of a substance, etc. The percentage of ionic character of a bond can be calculated by the application of the following formula:
$
\% \text { ionic character }=\frac{\text { Experimental value dipole moment }}{\text { Theoretical value of dipole moment }} \times 100
$

ii. A diatomic molecule has a dipole moment of 1.2 D . If the bond length is $1.0 \times 10^{-8} cm$, what fraction of charge does exist on each atom? (1)
iii. The dipole moment of $NF _3$ is very much less that of $NH _3$. Why? (2)
OR
A covalent molecule, $x-y$, is found to have a dipole moment of $1.5 \times 10^{-29} cm$ and a bond length 150 pm . What will be the percentage of ionic character of the bond? (2)