CH3CN (acetonitrile) has two carbon atoms, three hydrogen atoms, and one nitrogen atom. In the lewis structure of CH3CN, there is a single bond between the two carbon atoms. The left carbon is attached with three hydrogen atoms, and the right carbon makes a triple bond with one nitrogen atom. And on the nitrogen atom, there is one lone pair.
Here’s how you can draw the CH3CN 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, hydrogen has one valence electron, and nitrogen has five valence electrons.
Since CH3CN has two carbon atoms, three hydrogen atoms, and one nitrogen atom, so…
Valence electrons of two carbon atoms = 4 × 2 = 8
Valence electrons of three hydrogen atoms = 1 × 3 = 3
Valence electrons of one nitrogen atom = 5 × 1 = 5
And the total valence electrons = 8 + 3 + 5 = 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
Here hydrogen can not be the central atom. Because the central atom is bonded with at least two other atoms, and hydrogen has only one electron in its last shell, so it can not make more than one bond.
Now we have to choose the central atom from carbon and nitrogen. Place the least electronegative atom at the center.
Since carbon is less electronegative than nitrogen, assume that the central atom is carbon.
Here, there are two carbon atoms, so we can assume any one as the central atom.
Let’s assume that the central atom is right carbon.
Therefore, place carbons in the center and hydrogen and nitrogen on either side.
- And finally, draw the rough sketch
#2 Mark Lone Pairs
Here, we have a total of 8 electron pairs. And five bonds are already marked. So we have to only mark the remaining three electron pairs as lone pairs on the sketch.
Also remember that both (carbon and nitrogen) are the period 2 elements, so they can not keep more than 8 electrons in their last shell. And hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell.
Always start to mark the lone pairs from outside atoms. Here, the outside atoms are hydrogens and nitrogen. But no need to mark on hydrogen, because each hydrogen has already two electrons.
So for nitrogen, there are three lone pairs, and for carbon, there is zero lone pair because all three 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 left carbon atom, formal charge = 4 – 0 – ½ (8) = 0
For right carbon atom, formal charge = 4 – 0 – ½ (4) = +2
For each hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0
For nitrogen atom, formal charge = 5 – 6 – ½ (2) = -2
Here, both carbon and nitrogen atoms have charges, so mark them on the sketch as follows:
The above structure is not a stable lewis structure because both carbon and nitrogen atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.
#4 Minimize Charges
Convert a lone pair of the nitrogen atom to make a new C — N bond with the right carbon atom as follows:
#5 Minimize Charges Again
Since there are charges on carbon and nitrogen atoms, again convert a lone pair of the nitrogen atom to make a new C — N bond with the right carbon atom as follows:
In the above structure, you can see that the central atom (right carbon) forms an octet. Hence, the octet rule is satisfied.
Also, the above structure is more stable than the previous structures. Therefore, this structure is the stable lewis structure of CH3CN.
Next: IF4– Lewis Structure