Question

Why does $N{{H}_{3}}$​ forms hydrogen bond but $P{{H}_{3}}$ ​does not?

Answer 1

We know that Hydrogen bonding can occur in those molecules where hydrogen atom is covalently bonded to highly electronegative atoms such as Fluorine, Oxygen or Nitrogen because they tend to withdraw electron density of the covalent bond with Hydrogen atom, making itself completely electron deficient. Hence H atoms tend to act like a proton which gets attracted towards lone pairs easily.

Complete step by step solution:
We should begin with the conditions for Hydrogen bonding to occur:-
-H atom should be attached to a highly electronegative atom (such as N, O or F).
-These electronegative atoms must have at least 1 lone pair.
Now let us discuss about $N{{H}_{3}}$ and $P{{H}_{3}}$:-
-$N{{H}_{3}}$: The ‘N’ atom of $N{{H}_{3}}$ is small in size and has highest electronegativity in Group – 15 (also known as pnictogens) of periodic table. The electronegative value of nitrogen is 3.09 and that for hydrogen is 2.21. So there exists a high electronegativity difference between in the N-H bond due to which nitrogen becomes partially negatively charged whereas hydrogen becomes partially positively charged. This negatively charged N atom interacts with the H atom of neighboring $N{{H}_{3}}$molecule which is known as hydrogen bonding.
-$P{{H}_{3}}$: The ‘P’ atom of $P{{H}_{3}}$is comparatively larger in size than ‘N’ and has electronegativity almost near to hydrogen atom. The electronegative value of phosphorus is 2.19 and that for hydrogen is 2.21. So there exists a very negligible electronegativity difference between in the P-H bond due to which there is no distribution of electron cloud as positively and negatively charged. So $P{{H}_{3}}$ does not show any hydrogen bonding.
-Therefore, it can be concluded that $N{{H}_{3}}$​ forms hydrogen bond while$P{{H}_{3}}$ ​ does not.

Note: -Electronegativity can be defined as the measurement of the tendency of an atom to attract a shared pair of electrons towards itself.
-$N{{H}_{3}}$has a higher boiling point than$P{{H}_{3}}$ due to ammonia being more non- polar than$P{{H}_{3}}$ as well as it shows hydrogen bond interaction.