C2H2 Lewis structure

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C₂H₂ (acetylene or ethyne) has two carbon atoms and two hydrogen atoms.

In the C₂H₂ Lewis structure, 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.

Steps

Use these steps to correctly draw the C₂H₂ Lewis structure:

#1 First draw a rough sketch
#2 Mark lone pairs on the atoms
#3 Calculate and mark formal charges on the atoms, if required
#4 Convert lone pairs of the atoms, and minimize formal charges
#5 Repeat step 4 if needed, until all charges are minimized, to get a stable Lewis structure

Let’s discuss each step in more detail.

#1 First draw a rough sketch

• First, determine the total number of valence electrons

In the periodic table, carbon lies in group 14, and hydrogen lies in group 1.

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

Learn how to find: Carbon valence electrons and Hydrogen valence electrons

• 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 on the atoms

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 Calculate and mark formal charges on the atoms, if required

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 Convert lone pairs of the atoms, and minimize formal 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 Repeating step 4 to get a stable Lewis structure

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. The outside atom (left carbon) also forms an octet, and both hydrogens form a duet. Hence, the octet rule and duet rule are 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

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External links

• https://geometryofmolecules.com/c2h2-lewis-structure-molecular-geometry-hybridization-bond-angle/
• https://techiescientist.com/c2h2-lewis-structure/
• https://www.chemistryscl.com/general/C2H2-lewis-structure/index.php
• https://www.thegeoexchange.org/chemistry/bonding/Lewis-Structures/C2H2-Lewis-structure.html
• https://lambdageeks.com/hcch-lewis-structure/