Question

Amongst NH3, PH3, AsH3, SbH3 and BiH3

(i) which one is thermally least stable ?

(ii) which one has the highest melting point ?

(iii) which one is the most basic ?

(iv) which one has the smallest bond angle ?​

Answer 1

Observe the periodic table I have attached carefully.

(i) Observe how all these hydrides are of elements present in group-15 of the periodic table. As we go down the group, the number of shells increases and the possibility for thermal decomposition at lower temperatures increases. Also, the strength of Hydrogen bond decreases and the hydrides become more and more unstable.

So, Bismuth Hydride will be thermally least stable.

ii) The highest melting point will be of NH3 or ammonia of about -77.73 degrees C.

iii) Ammonia is the most basic. As when we go down group-15, the lone pair, which can accept H+ ions, accepts H+ ions with increasing difficulty, the lone pair becomes less nucleophilic and less basic.

iv)Bismuth Hydride will have the smallest bond angle, as bond pair-bond pair repulsion decreases as we go down group-15 as electron density reduces, resulting in smaller and smaller bond angle.