Which of the following substances has the highest proton affinity :-

(A) H₂O

(B) H₂S

(C) NH₃

(D)PH₃

In order to find the highest proton affinity among the given compounds, you need to know which element amongst Oxygen, Sulphur, Nitrogen and Phosphorus is the most electronegative. The more electronegative will be an atom, the more will be its affinity towards a proton. As among the given elements, oxygen has the highest electronegativity, therefore the highest proton affinity will be for H₂O.