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Q)

# Which of the following ionic species has the greatest protons affinity to form stable compound?

(a) $I^-$

(b) $HS^-$

(c) $NH^-_2$

(d) $F^-$

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A)
Toolbox:
• Strongest proton affinity will be for species which is the strongest base or whose conjugate acid is the weakest.

Answer: $NH^-_2$

The conjugate of $NH^-_2$ is $NH_3$ which is the weakest acid out of HI, $H_2S$, $NH_3$ and HF.

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Solution :
Proton affinity decreases in moving across the period from left to right due to increase in charge, within a group the proton affinities decreases from top to bottom. Nitrogen family > Oxygen family > Halogens
Strongest base would have the highest proton affinity i.e, $NH_2^-$
Hence Answer is $NH_2^-$