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

The basic strength of group 15 hydrides decreases down the group. The basic strength of these compounds is related to the availability of the lone pair of electrons on the central atom for donation, for the formation of a new bond.
• NH3: It is the smallest among the group 15 hydrides and the lone pair is highly available and accessible. Therefore it is the strongest base.
• PH3, AsH3, and BiH3: As you move down the group the atoms get larger (P, As and Bi). Due to the increased size of the central atom, the lone pair becomes more diffuse and is held more weakly, making them less effective at forming a bond with a proton. Therefore basicity decreases with increasing size of the atom.
Thus, the order of increasing basic strength is BiH3<AsH3<PH3 <NH3.