I was working with ammonia ($\ce{NH3}$) and phosphine($\ce{PH3}$). Both have lone pair electron. But $\ce{NH3}$ is stronger base (Lewis base) than $\ce{PH3}$.
I was told by my instructor that it is because of size and polarity of ($\ce{NH3}$). He said for the polarity of nitrogen, it draws electron density toward itself and for this excess electron density it donates partial electron density via coordinate covalent bond.
But how much electron density is too much and when a molecule would donate partial electron density?( to be exact) [ would better if mention other cases or general case also]
And also can't phosphine do the same?
