CN– (cyanide) has one carbon atom and one nitrogen atom.
In the CN– Lewis structure, there is a triple bond between the carbon and nitrogen atom, and on both carbon and nitrogen atoms, there is one lone pair.
Also, there is a negative (-1) charge on the carbon atom.
Steps
Here’s how you can draw the CN– Lewis structure step by step:
#1 Draw a rough sketch
#2 Mark lone pairs on the atoms
#3 Mark charges on the atoms
#4 Minimize charges by converting lone pairs
#5 If central atom doesn’t form octet, convert lone pair and mark charges again
Let’s break down each step in detail.
#1 Draw a rough sketch
- First, determine the total number of valence electrons
In the periodic table, carbon lies in group 14, and nitrogen lies in group 15.
Hence, carbon has four valence electrons and nitrogen has five valence electrons.
Since CN– has one carbon atom and one nitrogen atom, so…
Valence electrons of one carbon atom = 4 × 1 = 4
Valence electrons of one nitrogen atom = 5 × 1 = 5
Now the CN– has a negative (-1) charge, so we have to add one more electron.
So the total valence electrons = 4 + 5 + 1 = 10
Learn how to find: Carbon valence electrons and Nitrogen 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
We have to place the least electronegative atom at the center.
Since carbon is less electronegative than nitrogen, assume that the central atom is carbon.
- And finally, draw the rough sketch
#2 Mark lone pairs on the atoms
Here, we have a total of 5 electron pairs. And one C — N bond is already marked. So we have to only mark the remaining four 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.
Always start to mark the lone pairs from outside atoms. Here, the outside atom is nitrogen.
So for nitrogen, there are three lone pairs, and for carbon, there is one lone pair.
Mark the lone pairs on the sketch as follows:
#3 Mark charges on the atoms
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 – 2 – ½ (2) = +1
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 by converting lone pairs
Convert a lone pair of the nitrogen atom to make a new C — N bond with the carbon atom as follows:
In the above structure, you can see that the central atom (carbon) doesn’t form an octet. Hence, the octet rule is not satisfied.
#5 Convert lone pair and mark charges again
Therefore, again convert a lone pair of the nitrogen atom to make a new C — N bond with the 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 carbon atom, formal charge = 4 – 2 – ½ (6) = -1
For nitrogen atom, formal charge = 5 – 2 – ½ (6) = 0
Here, the carbon atom has a charge, so mark it on the sketch as follows:
In the above structure, you can see that the central atom (carbon) forms an octet. And the outside atom (nitrogen) also forms an octet. Hence, the octet rule is satisfied.
Now there is still a negative (-1) charge on the carbon atom. But we can not convert a lone pair to a bond because carbon can not keep more than 8 electrons in its last shell.
The formal charges on atoms are closer to zero. Also, the above structure is more stable than the previous structures. Therefore, this structure is the most stable Lewis structure of CN–.
And since the CN– has a negative (-1) charge, mention that charge on the Lewis structure by drawing brackets as follows:
Next: PF3 Lewis structure
External links
- https://techiescientist.com/cn-lewis-structure/
- https://geometryofmolecules.com/cn-cyanide-ion-lewis-structure-molecular-geometry-polarity/
- https://www.thegeoexchange.org/chemistry/bonding/Lewis-Structures/CN-lewis-structure.html
- https://lambdageeks.com/cn-lewis-structure/
- https://topblogtenz.com/cyanide-cn-lewis-structure-molecular-orbital-bond-orders/
Deep
Learnool.com was founded by Deep Rana, who is a mechanical engineer by profession and a blogger by passion. He has a good conceptual knowledge on different educational topics and he provides the same on this website. He loves to learn something new everyday and believes that the best utilization of free time is developing a new skill.