CNO– (fulminate) has one carbon atom, one nitrogen atom, and one oxygen atom. In the lewis structure of CNO–, there is a triple bond between nitrogen and carbon atom, and a single bond between nitrogen and oxygen atom. The carbon atom has one lone pair, and the oxygen atom has three lone pairs.
Also, there is a negative (-1) charge on the carbon atom and the oxygen atom, and a positive (+1) charge on the nitrogen atom.
Here’s how you can draw the CNO– lewis structure step by step.
Step #1: draw sketch
Step #2: mark lone pairs
Step #3: mark charges
Step #4: minimize charges
Step #5: minimize charges again (if there are)
Let’s break down each step in detail.
#1 Draw Sketch
- First, determine the total number of valence electrons
Hence, carbon has four valence electrons, nitrogen has five valence electrons, and oxygen has six valence electrons.
Since CNO– has one carbon atom, one nitrogen atom, and one oxygen atom, so…
Valence electrons of one carbon atom = 4 × 1 = 4
Valence electrons of one nitrogen atom = 5 × 1 = 5
Valence electrons of one oxygen atom = 6 × 1 = 6
Now the CNO– has a negative (-1) charge, so we have to add one more electron.
So the total valence electrons = 4 + 5 + 6 + 1 = 16
- Second, find the total electron pairs
We have a total of 16 valence electrons. And when we divide this value by two, we get the value of total electron pairs.
Total electron pairs = total valence electrons ÷ 2
So the total electron pairs = 16 ÷ 2 = 8
- Third, determine the central atom
We have to place the least electronegative atom at the center.
Since carbon is less electronegative than nitrogen and oxygen, the central atom should be carbon, right?
But if we place carbon in the center and nitrogen and oxygen outside, then it will become OCN– (cyanate), which is a different compound.
So here, we have to choose the central atom from nitrogen and oxygen.
Since nitrogen is less electronegative than oxygen, assume that the central atom is nitrogen.
Therefore, place nitrogen in the center and carbon and oxygen on either side.
- And finally, draw the rough sketch
#2 Mark Lone Pairs
Here, we have a total of 8 electron pairs. And two bonds are already marked. So we have to only mark the remaining six electron pairs as lone pairs on the sketch.
Also remember that all three (carbon, nitrogen, and oxygen) are the period 2 elements, so they can not keep more than 8 electrons in their last shell.
Always start to mark the lone pairs from outside atoms. Here, the outside atoms are carbon and oxygen.
So for carbon and oxygen, there are three lone pairs, and for nitrogen, there is zero lone pair because all six electron pairs are over.
Mark the lone pairs on the sketch as follows:
#3 Mark Charges
Use the following formula to calculate the formal charges on atoms:
Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons
For carbon atom, formal charge = 4 – 6 – ½ (2) = -3
For nitrogen atom, formal charge = 5 – 0 – ½ (4) = +3
For oxygen atom, formal charge = 6 – 6 – ½ (2) = -1
Here, all three atoms have charges, so mark them on the sketch as follows:
The above structure is not a stable lewis structure because all three atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.
#4 Minimize Charges
Here we have to convert a lone pair of the carbon atom (instead of the oxygen atom). Because carbon is less electronegative than oxygen. And so, it can give more valence electrons to share them.
Or remember this way, in CNO– and similar type structures, we have to make sure that the negative charge comes on the most electronegative element. In this case, the most electronegative element is oxygen.
Therefore, convert a lone pair of the carbon atom to make a new C — N bond with the nitrogen atom as follows:
#5 Minimize Charges Again
Since there are charges on the atoms, again convert a lone pair of the carbon atom to make a new C — N bond with the nitrogen atom as follows:
In the above structure, you can see that the central atom (nitrogen) forms an octet. Hence, the octet rule is satisfied.
Now there is still a negative (-1) charge on the carbon atom and the oxygen atom, and a positive (+1) charge on the nitrogen atom.
This is okay, because the structure with a negative charge on the most electronegative atom is the best lewis structure. And in this case, the most electronegative element is oxygen.
Also, the above structure is more stable than the previous structures. Therefore, this structure is the most stable lewis structure of CNO–.
And since the CNO– has a negative (-1) charge, mention that charge on the lewis structure by drawing brackets as follows:
Next: BrO2– Lewis Structure