Question

Why do nitrogen show tetra covalency in some cases ?

Answer 1

Consider the neutral ammonia molecule, `NH_3`. Is this capable of further reaction?

Explanation:

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 `N-H` bonds] rather than 7 electrons.) Likewise, there are 9 electrons around oxygen (including core), and, therefore, this bears a negative charge.

Of course we can have tetra-alkyl ammonium salts as well, `NR_4^+`.

The nitrogen containing species is now a quaternary ammonium salt, with a formal positive charge on nitrogen.