Proton accepting tendency is known as the strength of basicity. In R−
..
N
H2,N has lone pair of electron which intensify due to electron releasing R -group and increase the tendency to donate lone pair of electrons to H+ Secondly as the size of the ion increases there is less attraction for H+ to form weaker bonds with H - atom and are less basic. The order of the given series: RNH2>NH3>HS−>I−