$Arrange N{H}_3, N{F}_3, B{F}_3 in the increa\sin g order of dipole moment.$

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Please find below the solution to the asked query:
15(b)
The correct order is :BF₃ < NF₃ < NH₃    


BF₃  has symmetrical structure in which the three B-F bonds are oriented at an angle of 120° to one another. Also the three bonds lie in one plane and the dipole moments of these bonds cancel one another giving net dipole moment equal to zero.

In the two molecules, nitrogen is sp³ hybridized with 3 sigma bonds and 1 lone pair of electrons. So, the structure of the molecules is tetrahedral with bond angles being approximately being 109^o.  Since F is the most electronegative element among the elements of the two compounds, the dipole should be more for NF₃ but it is not the case so.

In NH₃ molecule, the direction of lone pair of electrons is in the same direction of the 3 N-H dipoles, thus the electron donation form hydrogen atoms to nitrogen atom and the withdrawal of it by lone pair of electrons are all in the same direction. So, they all add up to give a larger dipole moment.

In NF₃ molecule, the direction of N-F dipoles is opposite to that of lone pair of electrons. So, the electron withdrawal from nitrogen atom by the lone pair of electrons and fluorine atoms partially cancel each other to give a smaller dipole moment.

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