Answer the question on the basis of the passage given below:
There are some deposits of nitrates and phosphates in the earth's crust. Nitrates are more soluble in water. Nitrates are difficult to reduce under laboratory conditions, but microbes do it easily. Ammonia forms a large number of complexes with transition metals. Hybridization easily explains the ease of sigma donation of \(NH_3\) and \(PH_3\).
Phosphine is an inflammable gas, it is prepared from white phosphorus.
\(NH_3\) is a stronger base than \(PH_3\) because:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Nitrogen is smaller in size than phosphorus
Both \( \ce{NH3} \) and \( \ce{PH3} \) act as bases by donating the lone pair on the central atom. Nitrogen is much smaller than phosphorus, so the lone pair on N is concentrated in a small volume (high electron density) and is readily donated to a proton. On the larger P atom the lone pair is spread over a bigger volume and is less available. Basicity therefore decreases down the group: \( \ce{NH3} \) > \( \ce{PH3} \) > \( \ce{AsH3} \) > \( \ce{SbH3} \) ≥ \( \ce{BiH3} \).
Why the other options are incorrect
Option (1) Phosphorus is smaller in size than nitrogen — Incorrect. Phosphorus (period 3) is larger than nitrogen (period 2), not smaller.
Option (3) PH3 does not have lone pair of electron — Incorrect. \( \ce{PH3} \) does have a lone pair on phosphorus; it is weakly basic and forms \( \ce{PH4^{+}} \) salts with HI.
Option (4) NH3 does not have lone pair of electron — Incorrect. \( \ce{NH3} \) has one lone pair on nitrogen; without it ammonia could not act as a base at all.
Note 1: 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 – …
Note 2: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 02-06 S3 31-05 S1 | – |