Assertion (A) : The shape of $N{H}_3$is tetrahedral.

Reason (R) : ‘N’ undergoes $s{p}^3$hybridization in $N{H}_3$.

According to VSPER theory the shape of $N{H}_3$ should be tetra hydral but it distorted to pyramidal Due to one lone pair of electrons present on the nitrogen atom.  But nitrogen undergoes $s{p}^3$ hybridization out of 4 pairs 3 are the bond pairs, one is lone pair.