Question

Which of the following correctly represents the order of basic strength among Group 15 hydrides?

• A. BiH$_3$>SbH$_3$>AsH$_3$>PH$_3$>NH$_3$
• B. NH$_3$>PH$_3$>AsH$_3$>SbH$_3$>BiH$_3$
• C. NH$_3$<PH$_3$<AsH$_3$<SbH$_3$<BiH$_3$
• D. PH$_3$>NH$_3$>AsH$_3$>SbH$_3$>BiH$_3$

Hint:

Basic strength of Group 15 hydrides decreases down the group due to decreasing availability of the lone pair on the central atom.

Answer 1

The Correct Option is B

The basic strength of Group 15 hydrides decreases down the group. This is because the availability of the lone pair for donation (which determines basicity) decreases as the size of the central atom increases, leading to poorer overlap with the proton (H$^+$). Thus, the correct order is: \[ \text{NH}_3>\text{PH}_3>\text{AsH}_3>\text{SbH}_3>\text{BiH}_3 \]

• (A) Reverse order — incorrect.
• (B) Correct trend — correct.
• (C) Incorrect as it suggests increasing basicity down the group.
• (D) NH$_3$ is more basic than PH$_3$, so this order is wrong.