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

The order given in my book is

$$\ce{NH3} > \ce{SbH3} > \ce{AsH3} > \ce{PH3}$$

Phosphorous is more electronegative than arsenic and antimony. Then shouldn't the dipole moment be more in case of $$\ce{PH3}?$$

And why does the anomaly of $$\ce{NH3}$$ 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