State of hybridization

 

This butterfly rule is applicable  only for simple molecules like

 BeF2,  BF3, H2O,  NH3,   H2S .......  (ep-rule) 

CH4,  C2H6, C2H2  ...................   (pi-rule)

Here,   ep   = electron pair   =   valence electron +  shared electron

                                                                       2

For N atom of Ammonia,   valence electron = 5

                                            shared electron  =  3  (shared from  3 H atoms)

Electron pair (ep) = (5+3)/2  = 4 pair =  sp3 hybridization

Thus, N-atom  of ammonia is sp3 hybridized