Answer the question on basis of passage given below:
Nitrogen differs from the rest of the members of its group due to its small size, high electronegativity, high ionisation enthalpy and non-availability of d-orbitals. Nitrogen has unique ability to form \(p\pi-p\pi\) multiple bonds with itself and with other elements having small size and high electronegativity (e.g., C, O). Heavier elements of this group do not form \(p\pi-p\pi\) bonds as their atomic orbitals are so large and diffused that they cannot have effective overlapping.
Nitrogen family show reducing characters. Least basic amongst the following is:
(A) \(SbH_3\)
(B) \(BiH_3\)
(C) \(NH_3\)
(D) \(PH_3\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) Only
The basic character of the group 15 hydrides comes from the lone pair on the central atom. In \( \ce{NH3} \) the lone pair sits in a small orbital on the small N atom, so it is readily donated to a proton. Down the group the atom becomes larger, the lone pair is spread over a bigger volume (lower electron density) and is donated less readily, so the basicity decreases:
\( \ce{NH3} \) > \( \ce{PH3} \) > \( \ce{AsH3} \) > \( \ce{SbH3} \) ≥ \( \ce{BiH3} \)
(A) SbH3 — Incorrect. Very weakly basic, but still placed above \( \ce{BiH3} \) in the order.
(B) BiH3 — Correct. Bi is the largest atom of the group, so its lone pair is the least available; \( \ce{BiH3} \) is the least basic.
(C) NH3 — Incorrect. \( \ce{NH3} \) is the most basic hydride of the group.
(D) PH3 — Incorrect. \( \ce{PH3} \) is much less basic than \( \ce{NH3} \), but more basic than \( \ce{SbH3} \) and \( \ce{BiH3} \).
The same order explains the reducing character mentioned in the question: as the E–H bond weakens down the group, the hydrides give up hydrogen more easily, so BiH3 is the strongest reducing agent and the least basic.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Reactivity towards Hydrogen – “… The stability of hydrides decreases from NH3 to BiH3 which can be observed from their bond dissociation enthalpy. Consequently, the reducing character of the hydrides increases. Ammonia is only a mild reducing agent while BiH3 is the strongest reducing agent amongst all the hydrides. Basicity also decreases in the order NH3 > PH3 > AsH3 > SbH3 ≥ BiH3.”