Question

among H2O,H2O,NH3,PH3 which has highest proton affinity?

Answer 1

The size of N atom is less than the size of the P atom.So the electron density on the N atom is more than that on the P atom.Hence it becomes a stronger base i.e. it extracts H+ easily which makes it have a higher proton affinity than PH3.
Not Sure Though

Answer 2

H2O will have highest proton affinity because of its highest dipole moment towards O

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