Question

Arrange the following in the order of increasing basic strength:
(A). NH₃
(B). PH₃
(C). AsH₃
(D). BiH₃
Choose the correct answer from the options given below:

• A. (D), (C), (B), (A)
• B. (A), (B), (C), (D)
• C. (C), (B), (A), (D)
• D. (D), (B), (C), (A)

Hint:

As you go down a group, the atomic size increases, which makes the lone pair less readily available for bonding, and the basic strength decreases down the group in group 15 elements.

Answer 1

The Correct Option is D

Group 15 hydrides exhibit decreasing basic strength down the group. This basicity is determined by the availability of the central atom's lone pair of electrons for bond formation.
• NH3: As the smallest group 15 hydride, ammonia has a highly available and accessible lone pair, making it the strongest base.
• PH3, AsH3, and BiH3: Moving down the group, the central atoms (P, As, Bi) increase in size. This larger atomic size causes the lone pair to become more diffuse and weakly held, reducing its effectiveness in bonding with a proton. Consequently, basicity diminishes with increasing atomic size.
Therefore, the increasing order of basic strength is BiH3<AsH3<PH3 <NH3.