Identify Correct Properties and Trends of Group 15 Hydrides

To determine the correct statements regarding the properties and periodic trends of Group 15 hydrides ($\text{EH}_3$, where $\text{E} = \text{N, P, As, Sb, Bi}$), we analyze each statement individually:

1. Statement A: Reducing nature increases from $\text{NH}_3$ to $\text{BiH}_3$

   • As we move down Group 15 from Nitrogen ($\text{N}$) to Bismuth ($\text{Bi}$), the atomic size of the central atom increases ($\text{r}_{\text{N}} < \text{r}_{\text{P}} < \text{r}_{\text{As}} < \text{r}_{\text{Sb}} < \text{r}_{\text{Bi}}$).
   • This increase in size leads to an increase in the $\text{E-H}$ bond length and a significant decrease in the $\text{E-H}$ bond dissociation energy.
   • A weaker $\text{E-H}$ bond allows hydrogen to be liberated more easily, thereby increasing the reducing character. Thus, $\text{NH}_3$ is the weakest reducing agent, and $\text{BiH}_3$ is the strongest.
   • Statement A is correct.

2. Statement B: Tendency to donate lone pair of electrons decreases from $\text{NH}_3$ to $\text{BiH}_3$

   • The basic character of group 15 hydrides depends on their ability to donate a lone pair of electrons (Lewis basicity).
   • Down the group, as the size of the central atom increases, the same lone pair of electrons is dispersed over a larger volume, resulting in a lower electron density on the central atom.
   • Hence, the availability of the lone pair for donation decreases, leading to a decrease in basic strength from $\text{NH}_3$ to $\text{BiH}_3$ ($\text{NH}_3 > \text{PH}_3 > \text{AsH}_3 > \text{SbH}_3 > \text{BiH}_3$).
   • Statement B is correct.

3. Statement C: The stability of hydrides decreases from $\text{NH}_3$ to $\text{BiH}_3$

   • Thermal stability of the hydrides is directly related to the strength of the $\text{E-H}$ bond.
   • Since the $\text{E-H}$ bond strength decreases down the group due to larger atomic radii and poorer orbital overlap, the thermal stability decreases significantly from $\text{NH}_3$ to $\text{BiH}_3$.
   • Statement C is correct.

4. Statement D: $\text{HEH}$ bond angle decreases from $\text{NH}_3$ to $\text{SbH}_3$

   • The electronegativity of the central element decreases down the group ($\text{N} > \text{P} > \text{As} > \text{Sb}$).
   • High electronegativity of Nitrogen pulls the $\text{E-H}$ bonding electron pairs closer to the central atom, causing strong repulsions between bond pairs and expanding the $\text{H-E-H}$ bond angle.
   • As electronegativity decreases down the group, the bond pairs move further away from the central atom, reducing bond pair–bond pair repulsions. Consequently, the bond angle decreases: $\text{NH}_3 \;(107.8^\circ) > \text{PH}_3 \;(93.6^\circ) > \text{AsH}_3 \;(91.8^\circ) > \text{SbH}_3 \;(91.3^\circ)$
   • Statement D is correct.

Since statements A, B, C, and D are all correct, the correct choice is A, B, C and D.