HCN Lewis Structure, Molecular Geometry, Hybridization, MO Diagram, and Polarity


Hydrogen Cyanide is a very toxic acid and is famous for causing irritation in the eyes and respiratory system if any human inhales HCN in substantial quantity.

The compound is a colorless substance that is available in liquid or gaseous form.

HCN has a very strong and pungent smell which is not favorable for humans. The smell can be categorized as being that of bitter almonds

It is considered to be a dangerous and poisonous substance that is stored carefully to avoid any leaks or combustion because the storage containers if exposed to extreme heat might cause explosions.

The molecular weight of HCN is 27.025 g/mol.

The boiling point of the compound is 78.1 deg F and the melting point is 7.9 deg F.

Below are the reactions or methods which lead to the creation of this compound:

When methane reacts with ammonia and oxygen we get hydrogen cyanide and water. This reaction is completed when Platinum is added as a catalyst.

2 CH4    +    2NH3    +     3O2      ——->      2HCN    +     6H2O

There are other methods to create HCN too but they need outer push or energy to form this compound, for example, reactor walls.

Did you know that HCN is present in nature as well? There are different places from where we can obtain HCN.
We can obtain HCN from the pits of fruits.

Some research authors have said that we can find HCN in neurons.

HCN is very harmful if inhaled. The compound is distributed so fast in our bodies that the effects can be seen instantly. However, if inhaled in a small amount, the compound can get metabolized by the human body. Crazy enough, isn’t it?

Well, now enough of the basic introduction, let us move ahead and look at the Lewis structure of the compound and see how the bonds are formed in HCN.


Lewis Structure of HCN

Some compounds have a very unique and different Lewis structure and HCN is one of those. Thus to understand the Lewis structure in-depth let’s go step by step in understanding the concept.

First of all, to remind you Lewis’s structure is a pictorial representation of different bonds and lone pair of electrons between two or more atoms of a compound.

Step 1: The foremost step of creating a Lewis structure is finding the valence electrons.

Here we have to find the valence electrons of all three atoms, hydrogen, carbon, and nitrogen.

The number of valence electron is only 1 in Hydrogen because it is an exception atom which doesn’t follow the octet rule and thus doesn’t need 8 electrons to fill its octet but needs only 1.

Similarly, the valence electrons of Carbon are 4 and that of Nitrogen is 5.

The atomic number of Carbon is 6 so 2 electrons are filled in ‘s’ orbital and the rest 4 are in the outer orbital that is why the valence number of electrons in carbon is 4.

For Nitrogen, its atomic number is 7, so after 2 electrons occupy ‘s’ orbital, the rest 5 are in the outer orbital so the valence number of electrons is 5.

Now to find the total number of valence electrons we will add up the valence electrons of all three atoms:

=1+4+5 = 10 valence electrons.

Step 2: Now we will draw the Lewis dot structure of the compound. See the diagram below:

Now you can see that the central atom here is Carbon because it is easy for Carbon to become stable as it is the least electronegative of all.

However, hydrogen is the least electronegative but it cant be a central atom because it has only one spare electron.

The other two atoms H and N are attached to C by a single bond.

To make the representation clean we have to show the remaining lone pair of electrons on the atoms too after the initial bonds are made.

Here, after two electrons carbon share with hydrogen and nitrogen each, it is left with 2 more electrons in the outer shell.

The octet of hydrogen is complete so there are no lone pairs on it.

And for nitrogen, after sharing one electron with carbon it is left with 4 electrons which means there are 2 lone pairs of electrons on it.

Step 3: Balancing the charges on the compound.

So a lot of lone pairs will only make the compound unstable in nature. Thus, there will be additional bond pairing between carbon and nitrogen.

So as carbon has two electrons left thus it can make 2 more bonds with nitrogen, leaving nitrogen with only one pair of lone electrons.

This is the most stable Lewis structure that can be made for HCN.

HCN lewis structure


We hope that you got a clear idea of how the bonds between HCN are made. Now let’s move to see the hybridization of the compound.


Hybridization of HCN

The hybridization of HCN in sp.

It is important to find the hybridization of any compound because it gives us an insight into how the electrons are distributed in different orbitals.

There is a simple formula that can be used to determine the hybridization of HCN easily,

= GA + [VE – V – C]/2


GA = group of atoms attached to the central atom

VE = valence electrons on the central atom

V = valency of central atom

C = any charge on the molecule

Here, GA is 2, VE is 4, Valency of Carbon is 4 and there is no charge present on the molecule.

Now, putting these values in the formula,

= 2 + [4 – 4 – 0]/2

= 2

Therefore, the hybridization is sp.

HCN hybridization


Molecular Geometry of HCN

Why find molecular geometry of HCN?

Though we have seen the Lewis structure of HCN we need to see how the 3D representation of the compound looks like.

And to find that we need to find the molecular geometry of the compound. The easiest way to find the molecular geometry of any compound is with the help of the VSEPR theory.

According to the VSEPR chart shown below if we put the atoms of this compound in the general formula we find that the shape of HCN is Linear.

VSEPR chart

This is because A refers to the central atom and X is the other neighboring atoms which is 2 in the case of HCN, giving us the formula of AX2.

HCN geometry


HCN MO (Molecular Orbital) Diagram

What is an MO Diagram?

HCN MO Diagram

MO diagram is nothing but a description of how the chemical bonds are formed in any compound. The diagram is a representation of different energy levels and why a compound exists in nature or why some compounds don’t exist at all.

With the help of this theory, we can learn more about the internal structures, bond sharing, and different energy of orbitals of a compound.

In the case of HCN, let us look at how the atomic orbitals fuse to make molecular orbitals.

Electronic configuration of C is 2s2 2p2, electronic configuration of H is 1s1, and electronic configuration of N is 2s2 2p3

Here, one sp orbital of C fuses with 1s orbital of H.

And the other sp orbital of C fuses with one of the p orbitals of Nitrogen. The px orbitals of both C and N form sigma bonds while the Py and Pz orbitals form perpendicular Pi bonds.


Polarity of HCN

HCN polarity

Now let us look at whether the compound is polar or nonpolar in nature. Let us first find out what is the electronegativity of each atom here.

Carbon has an electronegativity of 2.55, for hydrogen, the electronegativity is 2.2 and for nitrogen, the electronegativity is 3.04.

As you can see that there is not a vast difference between the electronegativity of carbon and nitrogen but still this small difference gives strong results.

Nitrogen here in this compound will try to pull the electrons of Carbon towards itself. Due to this, there will be some negative charge on the nitrogen atom, making this compound slightly polar in nature.

You must also refer to  HCN polarity.

Hence, we can say that this compound has some polarity.

Now as there are repulsions in the atoms we can say easily that there is some bond angle between the atoms too. The shape of this compound is linear the bond angle can be easily identified to be 180 degrees.



This was a very interesting compound to study. The properties and bond formation are quite amazing here.
Now let us quickly look at what we have learned so far.

  • HCN is a highly toxic substance that has a bitter almond-like smell.
  • There is one bond between H and C and three bonds between C and nitrogen. There is one lone pair of
  • electrons on the nitrogen atom.
  • The compound has sp hybridization.
  • The molecular geometry of HCN is linear.
  • The compound is polar in nature.

We hope that you have understood the Lewis structure, hybridization, and molecular geometry of this compound.
In case of any questions or doubts, you can reach out to us.

Thank you for reading!

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