Which of the following ionic species has the greatest proton affinity to form stable compound?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → F−
Note 1: The given answer is as per the official NTA Answer Key. However, the correct answer should be Option (2) NH2−, as explained below.
Proton affinity is the tendency of a species to take up a proton, so it is greatest for the strongest base. Each anion forms its conjugate acid:
\( \displaystyle \ce{NH2- + H+ -> NH3}\qquad \ce{F- + H+ -> HF} \)
\( \displaystyle \ce{HS- + H+ -> H2S}\qquad \ce{I- + H+ -> HI} \)
The weaker the conjugate acid, the stronger the base. \( \ce{NH3} \) is a far weaker acid than HF, \( \ce{H2S} \) or HI, so \( \ce{NH2-} \) is the strongest base and has the greatest proton affinity; the order is \( \ce{NH2-} \) > \( \ce{F-} \) > \( \ce{HS-} \) > \( \ce{I-} \).
Option (1) HS− — Incorrect. \( \ce{H2S} \) is a much stronger acid than \( \ce{NH3} \), so \( \ce{HS-} \) is a weaker base than \( \ce{NH2-} \).
Option (2) NH2− — Correct as per NCERT. The conjugate base of the very weak acid \( \ce{NH3} \); it is the strongest base of the four.
Option (3) F− — The answer given by NTA, but incorrect. HF forms the strongest bond to hydrogen of these, but HF is a far stronger acid than \( \ce{NH3} \), so \( \ce{F-} \) is a weaker base than \( \ce{NH2-} \).
Option (4) I− — Incorrect. HI is a very strong acid, so \( \ce{I-} \) is the weakest base and has the least proton affinity.
Note 2: The question is from beyond the NCERT textbook.