# Why does the dipole moment order of group 15 hydrides increase down the group?

The order given in my book is NH3>SbH3>AsH3>PH3 Phosphorous is more electronegative than As and Sb then shouldn't the dipole moment be more in case of PH3 . And why does the anomaly of NH3 occur?

• Geometry/symmetry also has a role in determining the dipole, for example, if the molecules were planar as is $\ce{BF3}$ the dipole would be zero. – porphyrin Aug 16 at 16:56
• @porphyrin could you convert this into a proper answer for this question – Stan Aug 16 at 17:59