Two possible Lewis electron-dot diagrams for fulminic acid, HCNO, are shown below.
Explain why the diagram on the left is the better representation for the bonding in fulminic acid. Justify your choice based on formal charges.
The most stable resonance form of a molecule has the least formal charges.
To calculate formal charges we can use the following formula.
Valence Charge = # of valence e's - (number of bonds + number of lone electrons).
Therefore let's calculate formal charges for the molecule on the left:
C : 4-(4+0) = 0
N: 5-(4+0) = +1
O: 6- (1+6) = -1
Therefore, the molecule on the left has 2 charges, plus and minus one.
Let's now focus on that on the right:
C : 4-(3+2) = -1
N: 5-(4+0) = +1
O: 6- (2+4) = 0
Both molecules have the same formal charges. So, what do we do now? We have to see what atoms carry what formal charges. On the left, oxygen has a negative one charge while on the right carbon has a negative one charge. Since, oxygen is more electronegative than carbon, it is much more favorable for it to have a negative charge. Therefore, molecule on the left is the better representation.
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