Arrange the following in increasing order of their basic strength:A.…

The correct answer is Option (2) → D < E < B < C < A

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 \( \ce{BiH3} \).

• (D) \( \ce{BiH3} \) — first (least basic). Bi is the largest atom of the group, so its lone pair is the most diffuse and the least available.

• (E) \( \ce{SbH3} \) — second. Sb is smaller than Bi, so \( \ce{SbH3} \) is as basic as or a little more basic than \( \ce{BiH3} \).

• (B) \( \ce{AsH3} \) — third. As is smaller than Sb, so its lone pair is more available.

• (C) \( \ce{PH3} \) — fourth. P is smaller still; \( \ce{PH3} \) is a weak base and forms \( \ce{PH4^+} \) only with strong acids such as HI.

• (A) \( \ce{NH3} \) — fifth (most basic). N is the smallest atom, so its lone pair is the most concentrated and readily accepts a proton:

      \( \displaystyle \ce{NH3 + H2O <=> NH4^+ + OH^-} \)

Shortcut: The increasing order must end with \( \ce{NH3} \) (A); only Option (2) 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: