Question

Which is the correct order of dipole moments of $BF _{3}, NF _{3}$ and $NH _{3} ?$

• A. $NH _{3}> BF _{3}> NF _{3}$
• B. $BF _{3}> NF _{3}> NH _{3}$
• C. $NH _{3}> NF _{3}> BF _{3}$
• D. $NF _{3}> NH _{3}> BF _{3}$

Answer 1

Answer: (C)

$BF _{3}$ has zero dipole moment.



In $NH _{3}, N$ is more electronegative than $H$. So, nitrogen pulls the electron from H towards itself and the direction of moment is same due to $N - H$ bond as that of the lone pair of electrons on nitrogen.



In $NF _{3}, F$ is more electronegative than $N$. All $F$ atoms pull the electrons towards themselves and resultant dipole moment is opposite to the direction of that of the lone pair at $N$ atom. Hence, net dipole moment decreases.



Therefore, dipole moment of $NH _{3}$ is greater than $NF _{3}$. Hence, order of dipole moment is

$ NH _{3}> NF _{3}> BF _{3} $