C2H2 (acetylene or ethyne) has two carbon atoms and two hydrogen atoms. In the lewis structure of C2H2, there is a triple bond between the two carbon atoms, and each carbon is attached with one hydrogen atom, and none of the atoms has a lone pair.
Here’s how you can draw the C2H2 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 and hydrogen has one valence electron.
Since C2H2 has two carbon atoms and two hydrogen atoms, so…
Valence electrons of two carbon atoms = 4 × 2 = 8
Valence electrons of two hydrogen atoms = 1 × 2 = 2
And the total valence electrons = 8 + 2 = 10
- Second, find the total electron pairs
We have a total of 10 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 = 10 ÷ 2 = 5
- 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 there are only two atoms remaining and both atoms are carbon, 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 hydrogens on either side.
- And finally, draw the rough sketch
#2 Mark Lone Pairs
Here, we have a total of 5 electron pairs. And three bonds are already marked. So we have to only mark the remaining two electron pairs as lone pairs on the sketch.
Also remember that carbon is a period 2 element, so it can not keep more than 8 electrons in its 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 left carbon. But no need to mark on hydrogen, because each hydrogen has already two electrons.
So for left carbon, there are two lone pairs, and for right carbon, there is zero lone pair because all two 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 – 4 – ½ (4) = -2
For right carbon atom, formal charge = 4 – 0 – ½ (4) = +2
For each hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0
Here, both carbon atoms have charges, so mark them on the sketch as follows:
The above structure is not a stable lewis structure because both carbon atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.
#4 Minimize Charges
Convert a lone pair of the left carbon atom to make a new C — C bond with the right carbon atom as follows:
#5 Minimize Charges Again
Since there are charges on both carbon atoms, again convert a lone pair of the left carbon atom to make a new C — C 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 C2H2.
Next: CH2O Lewis Structure