The dipole moment of phosphine is 0.58 D which is less than 1.42 D for NH 3. PH 3 has a lone pair and does have a trigonal planar geometry - for it is not symmetrical. It must be polar since it is not symmetrical.

In contrast, the dipole moments of amines decrease with substitution, starting with ammonia, which has a dipole moment of 1.47 D. It is a vector quantity and its direction is always given from less electronegative atom to more electronegative atom. PH 3 is called phosphine and it quite toxic and flammable. The dipole moment is 0.58 D, which increases with substitution of methyl groups in the series: CH3PH2, 1.10 D (CH3)2PH, 1.23 D (CH3)3P, 1.19 D. It is the product of the charges and the distance between partial charges. A molecule may be nonpolar either when there is an equal sharing of electrons between the two atoms of a diatomic molecule or because of the symmetrical arrangement of polar bonds in a more complex molecule.Hint: The size of the P atom is greater than the N atom, down the group basicity of the atom decreases when the size of the atom increases and the electron density decreases. Hint: Dipole moment is a measure of polarity of a bond.

Ph3 dipole moment