# Which of the following represents the decreasing order of the basicity of the hydrides of Group-15 p-block elements?

$(A)\; \text{NH}_3\; \gt \text{AsH}_3\; \gt \text{BiH}_3 \gt \text{PH}_3$ $(B)\; \text{PH}_3\; \gt \text{AsH}_3\; \gt \text{BiH}_3 \gt \text{NH}_3$ $(C)\; \text{BiH}_3\; \gt \text{AsH}_3\; \gt \text{NH}_3 \gt \text{PH}_3$ $(D)\; \text{NH}_3\; \gt \text{PH}_3\; \gt \text{AsH}_3 \gt \text{BiH}_3$

Answer: $(D)\; \text{NH}_3\; \gt \text{PH}_3\; \gt \text{AsH}_3 \gt \text{BiH}_3$
Nitrogen is a small size atom; Lone pair electrons are concentrated in a small region. As we move down the group, the size of the central atom increases.
Therefore, the electron cloud diffuses over a larger volume and as a result the basic strength or the electron donor capacity decreases.