Valence Electrons and Lewis Dot Structures – HNCS- Example #38

A simple method for writing Lewis Dot 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  isothiocyanic acid  (HNCS).               

                                  

Step 1: Connect the atoms with single bonds. 

Fig.1: Connect the atoms of isothiocyanic acid (HNCS) with single bonds

 

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

Where n in this case is 3 since isothiocyanic acid has one hydrogen atom.

Where V = 1*1 + 1*5 + 1*4 + 1*6 = 16, V is the number of valence electrons of the atom.

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

 

Step 3 & 4: The Lewis resonance structures of  HNCS  areas follows:

Fig.2 : Electron dot structures of isothiocyanic acid (HNCS)

 Structure #3 is the most stable resonance Lewis structure since there is an octet of electrons around each atom and there is no charge separation. Structure #1 is more stable than Structure #2 since the negative charge is on N (3.04)  which is more electronegative than S (2.58).