C22- Lewis structure

C₂^2- (acetylide) has two carbon atoms.

In the C₂^2- Lewis structure, there is a triple bond between the two carbon atoms, and on each carbon atom, there is one lone pair.

Also, there is a negative (-1) charge on each carbon atom.

Alternative method: Lewis structure of C₂^2-

Rough sketch

• First, determine the total number of valence electrons

In the periodic table, carbon lies in group 14. Hence, carbon has four valence electrons.

Since C₂^2- has two carbon atoms, so…

Valence electrons of two carbon atoms = 4 × 2 = 8

Now the C₂^2- has a negative (-2) charge, so we have to add two more electrons.

So the total valence electrons = 8 + 2 = 10

Learn how to find: Carbon 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, there are only two atoms and both atoms are carbon, so we can assume any one as the central atom.

Let’s assume that the central atom is left carbon.

• And finally, draw the rough sketch

Lone pair

Here, we have a total of 5 electron pairs. And one C — C bond is already marked. So we have to only mark the remaining four 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.

Always start to mark the lone pairs from outside atoms. Here, the outside atom is right carbon.

So for right carbon, there are three lone pairs, and for left carbon, there is one lone pair.

Mark the lone pairs on the sketch as follows:

Formal charge

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 – 2 – ½ (2) = +1

For right carbon atom, formal charge = 4 – 6 – ½ (2) = -3

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.

Convert a lone pair of the right carbon atom to make a new C — C bond with the left carbon atom as follows:

In the above structure, you can see that the central atom (left carbon) doesn’t form an octet. Hence, the octet rule is not satisfied.

Therefore, again convert a lone pair of the right carbon atom to make a new C — O bond with the left carbon atom. Also, we have to again check whether there are charges on atoms or not.

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 – 2 – ½ (6) = -1

For right carbon atom, formal charge = 4 – 2 – ½ (6) = -1

Here, both carbon atoms have charges, so mark them on the sketch as follows:

Final structure

The final structure of C₂^2- has two carbon atoms connected by a triple covalent bond. In this layout, each carbon atom satisfies the octet rule by sharing three pairs of electrons and maintaining one lone pair of its own. This configuration is the most stable because it allows both atoms to reach a full valence shell while accounting for the two extra electrons present in the ion. As a result, this specific electronic distribution serves as the definitive and most accurate Lewis representation of the acetylide ion.

To properly represent this as a polyatomic ion, the entire Lewis structure is enclosed within square brackets. The overall charge of 2- is then written as a superscript outside the brackets at the top right, indicating that the structure possesses two additional electrons beyond the valence count of the two neutral carbon atoms.

Next: H₂SO₃ Lewis structure

External video

• C2 2- Lewis Structure: How to Draw the Lewis Structure for C2 2- (Acetylide anion) – YouTube • Wayne Breslyn

External links

• https://www.thegeoexchange.org/chemistry/bonding/Lewis-Structures/C22–lewis-structure.html
• https://www.chemistryscl.com/general/carbide-C2-2–ion-lewis-structure/index.php