Lone pair is a valence e- pair which is not shared with another atom
and is sometimes called a
non-bonding pair.For ex. NH3 has 1 lone pair H2O has 2 lone pair.
Lone pairs can make a contribution to a molecule's dipole moment. NH3 has a dipole moment of 1.47 D. As the electronegativity
of nitrogen (3.04) is greater than that of hydrogen (2.2) the result is
that the N-H bonds are polar with a net negative charge on the nitrogen
atom and a smaller net positive charge on the hydrogen atoms. There is
also a dipole associated with the lone pair and this reinforces the
contribution made by the polar covalent N-H bonds to ammonia's dipole moment. In contrast to NH
3,NF3 has a much lower dipole moment of 0.24 D. Fluorine is more electronegative than nitrogen and the polarity
of the N-F bonds is opposite to that of the N-H bonds in ammonia, so
that the dipole due to the lone pair opposes the N-F bond dipoles,
resulting in a low molecular dipole moment.
Lone
pair electrons have effect on the net dipole moment of a molecule. A
dipole moment is the product of the magnitude of the partial charges on the
molecule and the distance in which they are separated. Lone pair electrons
affect both the partial charges and the distance between partial charges.
Thanks & regard
Aarti Gupta
askiitains Faculty 
