A simple and automatic procedure for writing Lewis structures is given in a previous
article entitled “Lewis
Structures and the Octet Rule”. Relevant worked examples were given in the following
articles: Examples #1,
#2,
#3
, #4,
#5,
#6,
#7,
#8, #9, #10, #11, #12,
#13, #14, #15, #16, #17, #18, #19, #20, #21, #22, #23, #24, #25, #26, #27, #28, #29, #30, #31, #32, #33, #34 and #35.
Let us consider
the case of methanediimine CN2H2 (HNCNH)
Step 1: Connect the atoms with
single bonds.
 |
|
Fig.1:
Connect the methanediimine atoms 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 CN2H2 has 3 non-hydrogen atoms.
Where V = 5*2 +
4 + 1*2 = 16
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 electron
dot structures of CN2H2 are
as follows:
 |
| Fig. 2: Resonance Structures of methanediimine |
Structure #2 is the most stable Lewis
structure because there is an octet of electrons around each atom
and there is no charge separation.
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