its an easy ques....
as u knw that , NH3 as well as PH3 has a lone pair ........but we also have to consider drago's rule , which states tht the hydrides of all the p block elements less than 2.5 can't hybridise.....the electronegativity of N is 3.0 whereas the electro negativity of P is 2.45 .....so the Phosphorus in PH3 does nt hybridise .....and so ,its lone pair is in sterio chemically inert s- orbital due to which the lone pair is not easy to donate and hence cannot be treated as a good lewis base.....whereas in case of N , in NH3 the lone pair is in sp3 hybridised orbital n can b easily donated by nitrogen........
remem. greater the s- character greater will b the penetration power i.e more energy is required to remove the electron from that orbital which has greater s- character than the orbital which has lower s- chac. ....
at last u can say , tht in PH3 the l.p is in almost s -orbital and in case of NH3 , it is in sp3 hybridised orbital having s-charater = 33% only
