Lewis Dot Structures|How to draw Lewis Structure – Ex. #34: NCN-2

A simple and general procedure for writing Lewis structures is given in a previous article entitled “Lewis Structures and the Octet Rule”.  
Several worked examples relevant to this procedure were given in previous posts please see the Sitemap - Table of Contents (Lewis Electron Dot Structures).

Let us consider the case of the cyanamide ion NCN^-2   and draw the Lewis electron dot structures:

 

Step 1: Connect the atoms with single bonds.

Fig. 1: Connecting the atoms of the NCN^-2 with single bonds according to Step 1 of the method

 Step 2: Calculate the # of electrons in π bonds (multiple bonds) using  formula (1): 

Where n in this case is 3 since NCN^-2 consists of  3 atoms.

Where V = 5 + 4 + 5 – (-2) = 16, V is the number of the valence electrons of the ion.

Therefore, P = 6n + 2 – V = 6 *3 + 2 – 16 = 4   Therefore, there are either 2 double bonds or 1 triple  bond.

 

Step 3 & 4: The Lewis dot structures of NCN^-2 are as follows:

Fig. 2: Lewis resonance structures of the cyanamide NCN^-2 ion

The ion represented by the electron dot structure #1 is the most stable comparing to the #2 and #3 because of less charge separation. The cyanamide ion is isoelectronic and hence isostructural with the CO₂ molecule.