The #PH_3# molecule can be represented in a tetrahedral arrangement similar to #NH_3#. How does one #PH_3# molecule interact with other #PH_3# molecules?
For the ammonia molecule, there is a large difference in electronegativity between the hydrogen and nitrogen nuclei, and the nitrogen atom polarizes electron density towards itself,
On the other hand, there is not a great of electronegativity difference between the phosphorus atom, and the hydrogen atom. The
And the consequence? Because of the reduced intermolecular interaction, the boiling point of phosphine is
And thus dispersion forces only operate in phosphine.