Why do nitrogen show tetra covalency in some cases ?
Consider the neutral ammonia molecule,
The ammonia molecule is a neutral species; nevertheless it can undergo acid-base reaction by virtue of the lone pair on nitrogen:
#NH_3 + H_2O rightleftharpoons NH_4^+ + ""^(-)OH#
We say that the nitrogen atom has become quaternized; i.e. the lone pair has formed a coordinate covalent bond to a proton (the nitrogen now bears a formal positive charge because it is associated with 6 electrons [2 inner core and 4 from the
Of course we can have tetra-alkyl ammonium salts as well,
The nitrogen containing species is now a quaternary ammonium salt, with a formal positive charge on nitrogen.