Chemical Bonding & Molecular Structure

Concept:Dipole moment depends on molecular geometry and bond polarity; symmetric molecules have zero dipole moment.Explanation:• $\mathrm{BF}_3$ is trigonal planar and symmetric, so its net dipole moment is zero.• $\mathrm{NH}_4^+$ is tetrahedral with all bonds identical and symmetric; despite the positive charge, the molecule has zero dipole moment.• $\mathrm{NF}_3$ is pyramidal with a lone pair on nitrogen. The bond dipoles point toward fluorine (more electronegative), while the lone pair contributes a dipole in the opposite direction. This partial cancellation results in a smaller net dipole compared to $\mathrm{NH}_3$.• $\mathrm{NH}_3$ is also pyramidal with a lone pair. Here, the bond dipoles point toward nitrogen, and the lone pair dipole adds to the net moment, giving a larger dipole than $\mathrm{NF}_3$.Thus, the order is: $\mathrm{BF}_3 = \mathrm{NH}_4^+ < \mathrm{NF}_3 < \mathrm{NH}_3$.Answer:Option A.