Q.Why does NH3 act as a Lewis base ?

Ammonia, $NH_3$, is a pyramidal molecule ($sp^3$ hybridised nitrogen with one lone pair, HNH angle about $107°$, close to the ideal tetrahedral angle because only $sp^3$ hybrid orbitals are used for both bonding and the lone pair) manufactured industrially by the Haber process, $N_2 + 3H_2 \rightleftharpoons 2NH_3$, favoured by high pressure (about 200 atm), a moderate temperature (about 700 K) with an iron catalyst promoted by molybdenum, and rapid removal of $NH_3$ to shift the equilibrium forward. Its lone pair on nitrogen makes it an excellent Lewis base and ligand, so it readily donates electrons to $H^+$ (forming $NH_4^+$) or to metal ions, e.g. $Cu^{2+} + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+}$ (deep blue complex). $NH_3$ is strongly hydrogen-bonded because nitrogen is small and highly electronegative, giving it an unusually high boiling point compared with $PH_3$, which cannot hydrogen-bond since phosphorus is larger …