Answer the question on basis of passage given below:
All the members of Group 15 elements form covalent hydrides with hydrogen as \(NH_3, PH_3, AsH_3, SbH_3, BiH_3\). Group 16 elements form covalent hydrides with hydrogen as \(H_2O, H_2S, H_2Se, H_2Te, H_2Po\). Group 17 elements form covalent hydrides with hydrogen as \(HF, HCl, HBr, HI\). Based on these facts answer the following question.
Which of the following is the strongest Lewis base?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(NH_3\)
A Lewis base is an electron-pair donor. In \( \ce{NH3} \) the nitrogen atom carries one lone pair, and since nitrogen is less electronegative than oxygen or chlorine, this lone pair is held less tightly and is most readily donated. That is why ammonia forms complexes such as \( \ce{[Cu(NH3)4]^{2+}} \) and \( \ce{[Ag(NH3)2]^{+}} \) with metal ions.
Why the other options are incorrect
Option (1) \(H_2O\) — Incorrect. Water does have lone pairs, but oxygen is more electronegative than nitrogen and holds them more firmly, so \( \ce{H2O} \) is a weaker Lewis base than \( \ce{NH3} \).
Option (2) \(HCl\) — Incorrect. HCl is a proton donor (an acid); the lone pairs on the highly electronegative chlorine are hardly available for donation.
Option (4) \(H_2S\) — Incorrect. The lone pairs of sulphur sit in large, diffuse orbitals and \( \ce{H2S} \) behaves mainly as a weak acid, so it is a much weaker base than \( \ce{NH3} \).
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):
Ammonia – “… The presence of a lone pair of electrons on the nitrogen atom of the ammonia molecule makes it a Lewis base. It donates the electron pair and forms linkage with metal ions and the formation of such complex compounds finds applications in detection of metal ions such as Cu2+, Ag+.”