Lewis Structure of HCN (With 6 Simple Steps to Draw!)

I’m super excited to teach you the lewis structure of HCN in just 6 simple steps.

Infact, I’ve also given the step-by-step images for drawing the lewis dot structure of HCN molecule.

So, if you are ready to go with these 6 simple steps, then let’s dive right into it!

Lewis structure of HCN contains a single bond between a Carbon (C) & Hydrogen (H) atom and a triple bond between the Carbon (C) and Nitrogen (N) atom. The Carbon atom (C) is at the center and it is surrounded by Hydrogen (H) and Nitrogen atom (N). The Nitrogen atom has 1 lone pair.

Let’s draw and understand this lewis dot structure step by step.

(Note: Take a pen and paper with you and try to draw this lewis structure along with me. I am sure you will definitely learn how to draw lewis structure of HCN).

6 Steps to Draw the Lewis Structure of HCN

Step #1: Calculate the total number of valence electrons

Here, the given molecule is HCN. In order to draw the lewis structure of HCN, first of all you have to find the total number of valence electrons present in the HCN molecule.
(Valence electrons are the number of electrons present in the outermost shell of an atom).

So, let’s calculate this first.

Calculation of valence electrons in HCN

• For Hydrogen:

Hydrogen is a group 1 element on the periodic table. ^[1]

Hence, the valence electrons present in hydrogen is 1 (see below image).

• For Carbon:

Carbon is a group 14 element on the periodic table. ^[2]

Hence, the valence electrons present in carbon is 4 (see below image).

• For Nitrogen:

Nitrogen is a group 15 element on the periodic table. ^[3]

Hence, the valence electrons present in nitrogen is 5 (see below image).

Hence in a HCN molecule, 

Valence electron given by Hydrogen (H) atom = 1
Valence electrons given by Carbon (C) atom = 4
Valence electrons given by Nitrogen (N) atom = 5
So, total number of Valence electrons in HCN molecule = 1 + 4 + 5 = 10

Step #2: Select the center atom (H is always outside)

While selecting the atom, always put the least electronegative atom at the center.

(Remember: Fluorine is the most electronegative element on the periodic table and the electronegativity decreases as we move right to left in the periodic table as well as top to bottom in the periodic table). ^[4]

Here in the HCN molecule, hydrogen (H) will always remain outside as per the rule. Now, if we compare the carbon atom (C) and nitrogen atom (N), then carbon is less electronegative than nitrogen.

So, carbon should be placed in the center and the nitrogen atom will surround it.

Step #3: Put two electrons between the atoms to represent a chemical bond

Now in the above sketch of HCN molecule, put the two electrons (i.e electron pair) between the hydrogen atom, carbon atom and oxygen atom to represent a chemical bond between them.

These pairs of electrons present between the Hydrogen (H), Carbon (C) and Nitrogen (N) atoms form a chemical bond, which bonds the hydrogen, carbon and nitrogen atoms with each other in a HCN molecule.

Step #4: Complete the octet (or duplet) on outside atoms. If the valence electrons are left, then put the valence electrons pair on the central atom

Don’t worry, I’ll explain!

In the Lewis structure of HCN, the outer atoms are hydrogen atom and nitrogen atom.

Hydrogen already has a duplet (see below image).

So now, you have to complete the octet on nitrogen atom (because nitrogen requires 8 electrons to have a complete outer shell).

Now, you can see in the above image that the nitrogen atom forms an octet.

Also, all the 10 valence electrons of HCN molecule (as calculated in step #1) are used in the above structure. So there are no remaining electron pairs.

Hence there is no change in the above sketch of HCN.

Let’s move to the next step.

Step #5: Check whether the central atom has octet or not. If it does not have an octet, then move the electron pair from the outer atom to form a double bond or triple bond

In this step, we have to check whether the central atom (i.e carbon) has an octet or not. 

In simple words, we have to check whether the central Carbon (C) atom is having 8 electrons or not.

As you can see from the above image, the central atom (i.e carbon) has only 4 electrons. So it does not fulfill the octet rule.

Now, in order to fulfill the octet of carbon atom, we have to move the electron pair from the outer atom (i.e nitrogen atom) to form a double bond.

Still, the octet of carbon atom is not fulfilled as it has only 6 electrons.

So again moving another electron pair from the nitrogen atom, we will get the following structure.

Now you can see from the above image that the central atom (i.e carbon), is having 8 electrons. So it fulfills the octet rule and the carbon atom is stable.

Step #6: Final step – Check the stability of lewis structure by calculating the formal charge on each atom

Now, you have come to the final step and here you have to check the formal charge on hydrogen atom, carbon atom as well as nitrogen atom.

For that, you need to remember the formula of formal charge;

Formal charge = Valence electrons – Nonbonding electrons – (Bonding electrons)/2

• For Hydrogen:
  Valence electron = 1 (as it is in group 1)
  Nonbonding electrons = 0
  Bonding electrons = 2
• For Carbon:
  Valence electrons = 4 (as it is in group 14)
  Nonbonding electrons = 0
  Bonding electrons = 8
• For Nitrogen:
  Valence electron = 5 (as it is in group 15)
  Nonbonding electrons = 2
  Bonding electrons = 6

Formal charge	=	Valence electrons	–	Nonbonding electrons	–	(Bonding electrons)/2
H	=	1	–	0	–	2/2	=	0
C	=	4	–	0	–	8/2	=	0
N	=	5	–	2	–	6/2	=	0

So you can see above that the formal charges on hydrogen, carbon as well as nitrogen are “zero”.

Hence, there will not be any change in the above structure and the above lewis structure of HCN is the final stable structure only.

Each electron pair (:) in the lewis dot structure of HCN represents the single bond ( | ). So the above lewis dot structure of HCN can also be represented as shown below.

Related lewis structures for your practice:
Lewis structure of H2O
Lewis structure of N2
Lewis structure of O2
Lewis structure of CO
Lewis structure of CH4

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