Answer the question on basis of passage given below:
P block elements are placed in groups 13 to 18 of the periodic table. Their valence shell electronic configuration is \(ns^2\, \ np^{1-6}\). Group 16 of the p-block elements are known as the group of chalcogens having \(ns^2\, \ np^4\) as their general electronic configuration. They exhibit number of oxidation states but the stability of \(-2\) oxidation state decreases down the group. They are sometimes also known as group of chalcogens as the name is derived from the Greek word for brass and points to the association of sulphur and its congeners with copper. Their ionization enthalpy decreases down the group. Oxygen shows anomalous behaviour due to its small size and high electronegativity.
Given below are the hydrides formed by group 15 elements. Choose the correct decreasing order of basicity of the following hydrides
A. \(AsH_3\)
B. \(PH_3\)
C. \(NH_3\)
D. \(BiH_3\)
E. \(SbH_3\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → C, B, A, E, D
The hydrides of group 15 (EH3) act as bases through the lone pair on the central atom. Down the group the atom becomes larger, so the lone pair is spread over a larger volume and is less available for donation. The basic strength therefore decreases from \( \ce{NH3} \) to \( \ce{BiH3} \).
(C) \( \ce{NH3} \) — first (most basic). N is the smallest atom, so its lone pair is the most concentrated and readily accepts a proton:
\( \displaystyle \ce{NH3 + H2O <=> NH4^+ + OH^-} \)
(B) \( \ce{PH3} \) — second. P is larger than N; \( \ce{PH3} \) is a weak base and forms \( \ce{PH4^+} \) only with strong acids such as HI.
(A) \( \ce{AsH3} \) — third. As is larger than P, so its lone pair is less available.
(E) \( \ce{SbH3} \) — fourth. Sb is larger still.
(D) \( \ce{BiH3} \) — fifth (least basic). Bi is the largest atom of the group, so its lone pair is the most diffuse; \( \ce{SbH3} \) is as basic as or a little more basic than \( \ce{BiH3} \).
Shortcut: The decreasing order must start with \( \ce{NH3} \) (C); only Option (1) does.
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):
Chemical Properties – “… (i) Reactivity towards hydrogen: All the elements of Group 15 form hydrides of the type EH3 where E = N, P, As, Sb or Bi. … 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.”
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 |
|---|---|---|---|---|
| – | – | – | 02-07 S3 13-06 S3 12-06 S3 09-06 S3 | – |