Question

Why does $P{H}_3$ have a lower boiling point than $N{H}_3$?

Answer 1

Hint: Think of the major difference between the structures between $P{H}_3$ and $N{H}_3$.
Also, try and think of the differences in chemical properties between Phosphorus and Nitrogen and think of how that might affect the polarity of the compound.

Complete step-by-step answer:
Let us first analyse each of these compounds one at a time and have absolute clarity about their chemical properties, so as to ensure we do not make silly mistakes later on.
Phosphine (IUPAC name: phosphane) is the compound with the chemical formula $P{H}_3$
It is a colourless, flammable, toxic gas and is classed as a pnictogen hydride. Pure phosphine is odourless, but technical grade samples have a highly unpleasant odour like garlic or rotting fish, due to the presence of substituted phosphine and di-phosphane ($P_2{H}_4$).
With traces of $P_2{H}_4$ present, $P{H}_3$ is spontaneously flammable in air (pyrophoric), burning with a luminous flame.

Phosphines are also a group of organophosphorus compounds with the formula $R_{3}P$ (R = organic derivative).
Organophosphines are important in catalysts where they are complex to various metal ions; complexes derived from a chiral phosphine can catalyse reactions to give chiral, enantioenriched products.
Now that we have covered $P{H}_3$ in detail, let us now look at $N{H}_3$.
Ammonia is a compound of nitrogen and hydrogen with the formula $N{H}_3$. A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a characteristic pungent smell.
It is a common nitrogenous waste, particularly among aquatic organisms, and it contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers.
Ammonia, either directly or indirectly, is also a building block for the synthesis of many pharmaceutical products and is used in many commercial cleaning products.
Having now put down a few chemical properties of both these elements, let us now look at this particular case and try to explain why the boiling point of phosphine is lower than that of ammonia.
Experimentally, we find out that the boiling points of the given chemical compounds is:
Ammonia [$N{H}_3$] - -33°C

Phosphine [$P{H}_3$] - -83°C.

The reason for this phenomenon to occur even though there is usually a trend of boiling points increasing from the top to bottom of a group is as follows:

$N{H}_3$ is a polar molecule with strong Hydrogen bonding.

This hydrogen bonding is the strongest of all the bonds and results in greater attraction between $N{H}_3$ molecule than in $P{H}_3$ molecules which has weak dispersion forces.

Hence, through this analysis we have proven why $P{H}_3$ has a lower boiling point than $N{H}_3$.

Note: This question requires the thorough knowledge of the properties of group V elements and that of their hydrides for us to be able to come up with an acceptable solution. This knowledge would also involve knowing about the many exceptions that are present in the chemical properties of these elements including but not limited to the polarisation of the ammonia molecule.