When semicarbazide reacts with a ketone (or aldehyde) to form semicarbazone. Only one nitrogen atom of semicarbazide acts as a nucleophile and attack the carbonyl carbon of the ketone. The product fo the reaction consequently is `R_(2)C=N- NH-CONH_(2)` rather than `R_(2)C=NCONH-NH_2`. what factor account for the fact that two nitrogen atoms of semicarbazide are relatively non nucleophilic ?