HCN Lewis Structure in 6 Steps (With Images)

So you have seen the above image by now, right?

Let me explain the above image in short.

HCN lewis structure has a Carbon atom (C) at the center which is surrounded by one Hydrogen atom (H) and one Oxygen atom (O). There is a triple bond between the Carbon (C) & Nitrogen (N) atom and a single bond between Carbon (C) & Hydrogen (H) atom. There is 1 lone pair on the Nitrogen atom (N).

If you haven’t understood anything from the above image of HCN lewis structure, then just stick with me and you will get the detailed step by step explanation on drawing a lewis structure of HCN.

So let’s move to the steps of drawing the lewis structure of HCN.

Steps of drawing HCN lewis structure

Step 1: Find the total valence electrons in HCN molecule

In order to find the total valence electrons in HCN molecule, first of all you should know the valence electrons present in hydrogen atom, carbon atom as well as nitrogen atom.
(Valence electrons are the electrons that are present in the outermost orbit of any atom.)

Here, I’ll tell you how you can easily find the valence electrons of hydrogen, carbon as well as nitrogen using a periodic table.

Total valence electrons in HCN molecule

→ Valence electrons given by hydrogen atom:

Hydrogen is group 1 element on the periodic table. ^[1] Hence the valence electron present in hydrogen is 1.

You can see that only 1 valence electron is present in the hydrogen atom as shown in the above image.

→ Valence electrons given by carbon atom:

Carbon is group 14 element on the periodic table. ^[2] Hence the valence electrons present in carbon is 4.

You can see the 4 valence electrons present in the carbon atom as shown in the above image.

→ Valence electrons given by nitrogen atom:

Nitrogen is a group 15 element on the periodic table. ^[3] Hence the valence electrons present in nitrogen is 5.

You can see the 5 valence electrons present in the nitrogen atom as shown in the above image.

Hence, 

Total valence electrons in HCN molecule = valence electron given by 1 hydrogen atom + valence electrons given by 1 carbon atom + valence electrons given by 1 nitrogen atom = 1 + 4 + 5 = 10.

Step 2: Select the central atom

For selecting the center atom, you have to remember that the atom which is less electronegative remains at the center.

(Remember: If hydrogen is present in the given molecule, then always put hydrogen outside.)

Now here the given molecule is HCN and it contains hydrogen atom (H), carbon atom (C) and nitrogen atom (N).

So as per the rule we have to keep hydrogen outside.

Now, you can see the electronegativity values of carbon atom (C) and nitrogen atom (N) in the above periodic table.

If we compare the electronegativity values of carbon (C) and nitrogen (N) then the carbon atom is less electronegative.

So here the carbon atom (C) is the center atom and the nitrogen atom (N) is the outside atom.

Step 3: Connect each atoms by putting an electron pair between them

Now in the HCN molecule, you have to put the electron pairs between the carbon (C) & hydrogen (H) atom and between the carbon (C) & nitrogen (N) atom.

This indicates that these atoms are chemically bonded with each other in a HCN molecule.

Step 4: Make the outer atoms stable

Now in this step, you have to check the stability of the outer atoms.

Here in the sketch of HCN molecule, you can see that the outer atoms are hydrogen atom and nitrogen atom.

These hydrogen and nitrogen atoms are forming a duplet and octet respectively and hence they are stable. 

Also, in step 1 we have calculated the total number of valence electrons present in the HCN molecule.

The HCN molecule has a total 10 valence electrons and all these valence electrons are used in the above sketch of HCN.

Hence there are no remaining electron pairs to be kept on the central atom. 

So now let’s proceed to the next step.

Step 5: Check the octet on the central atom. If it does not have octet, then shift the lone pair to form a double bond or triple bond.

In this step, you have to check whether the central carbon atom (C) is stable or not.

In order to check the stability of the central carbon (C) atom, we have to check whether it is forming an octet or not.

Unfortunately, the carbon atom is not forming an octet here. Carbon has only 4 electrons and it is unstable.

Now to make this carbon atom stable, you have to shift the electron pair from the outer nitrogen atom so that the carbon atom can have 8 electrons (i.e octet).

But after shifting one electron pair, the carbon atom is still not forming an octet as it has only 6 electrons. 

So again we have to shift one more electron pair from the nitrogen atom.

After shifting this electron pair, the central carbon atom will get 2 more electrons and thus its total electrons will become 8.

You can see from the above picture that the carbon atom is forming an octet.

And hence the carbon atom is stable.

Now let’s proceed to the final step to check whether the lewis structure of HCN is stable or not.

Step 6: Check the stability of lewis structure

Now you have come to the final step in which you have to check the stability of lewis structure of HCN.

The stability of lewis structure can be checked by using a concept of formal charge.

In short, now you have to find the formal charge on hydrogen (H) atom, carbon (C) atom as well as nitrogen (N) atoms present in the HCN molecule.

For calculating the formal charge, you have to use the following formula;

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

You can see the number of bonding electrons and nonbonding electrons for each atom of HCN molecule in the image given below.

For Hydrogen (H) atom:
Valence electron = 1 (because hydrogen is in group 1)
Bonding electrons = 2
Nonbonding electrons = 0

For Carbon (C) atom:
Valence electrons = 4 (because carbon is in group 14)
Bonding electrons = 8
Nonbonding electrons = 0

For Nitrogen (N) atom:
Valence electrons = 5 (because nitrogen is in group 15)
Bonding electrons = 6
Nonbonding electrons = 2

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

From the above calculations of formal charge, you can see that the hydrogen (H) atom, carbon (C) atom as well as nitrogen (N) atom have a “zero” formal charge.

This indicates that the above lewis structure of HCN is stable and there is no further change in the above structure of HCN.

In the above lewis dot structure of HCN, you can also represent each bonding electron pair (:) as a single bond (|). By doing so, you will get the following lewis structure of HCN.

I hope you have completely understood all the above steps.

For more practice and better understanding, you can try other lewis structures listed below.

Try (or at least See) these lewis structures for better understanding:

N2 lewis structure	O2 lewis structure
CO lewis structure	CH4 lewis structure
BF3 lewis structure	NO3- lewis structure