Lewis structure of HCN

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The Lewis structure of HCN contains one single bond and one triple bond, with carbon in the center, and hydrogen and nitrogen on either side. There is one lone pair on the nitrogen atom, and hydrogen atom and carbon atom do not have any lone pair.

Steps

By using the following steps, you can easily draw the Lewis structure of HCN:

#1 Draw skeleton
#2 Show chemical bond
#3 Mark lone pairs
#4 Complete octet on central atom
#5 Calculate formal charge and check stability

Let’s one by one discuss each step in detail.

#1 Draw skeleton

In this step, first calculate the total number of valence electrons. And then, decide the central atom.

• Let’s calculate the total number of valence electrons

We know that… hydrogen is a group 1 element, carbon is a group 14 element, and nitrogen is a group 15 element. Hence, hydrogen has one valence electron, carbon has four valence electrons, and nitrogen has five valence electrons.

Now HCN has one hydrogen atom, one carbon atom, and one nitrogen atom.

So the total number of valence electrons = valence electrons of hydrogen atom + valence electrons of carbon atom + valence electrons of nitrogen atom

Therefore, the total number of valence electrons = 1 + 4 + 5 = 10

• Now decide the central atom

We can not assume hydrogen as the central atom, because the central atom is bonded with at least two other atoms. And hydrogen has only one electron in its last shell, so it can not make more than one bond.

Therefore, choose the central atom from carbon and nitrogen.

The atom with the least electronegative value is placed at the center. By looking at the periodic table, we get the electronegativity values for carbon and nitrogen as follows:

Electronegativity value of carbon = 2.55
Electronegativity value of nitrogen = 3.04

Obviously, carbon is less electronegative than nitrogen. Hence, assume that carbon is the central atom.

So now, put carbon in the center and hydrogen and nitrogen on either side. And draw the rough skeleton structure for the Lewis structure of HCN something like this:

Also read: How to draw Lewis structure of SO₂ (6 steps)

#2 Show chemical bond

Place two electrons between the atoms to show a chemical bond. Since carbon is surrounded by hydrogen and nitrogen, use four electrons to show two chemical bonds as follows:

#3 Mark lone pairs

As calculated earlier, we have a total of 10 valence electrons. And in the above structure, we have already used four valence electrons. Hence, six valence electrons are remaining.

Two valence electrons represent one lone pair. So six valence electrons = three lone pairs.

Note that hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell. And both (carbon and nitrogen) are the period 2 elements, so they can not keep more than 8 electrons in their last shell.

Also, make sure that you start marking these lone pairs on outside atoms first. And then, on the central atom.

The outside atoms are hydrogen and nitrogen. But hydrogen can not keep more than 2 electrons in its last shell. Hence, don’t mark the lone pairs on hydrogen.

So nitrogen will get three lone pairs. And the central atom (carbon) will not get any lone pair, because all three lone pairs are used.

Now draw the Lewis structure of HCN something like this:

In the above structure, you can see that the octet is completed on outside atoms. But, the central atom (carbon) doesn’t form an octet.

So in the next step, we have to complete the octet on the central atom.

Also read: How to draw Lewis structure of N₂ (5 steps)

#4 Complete octet on central atom

Remember that carbon is a period 2 element, so it can not keep more than 8 electrons in its last shell.

Now carbon already has four valence electrons. Hence, carbon needs four more valence electrons to complete its octet.

So convert two lone pairs from the nitrogen atom to make a new bond with the carbon atom. And then, the Lewis structure of HCN looks something like this:

In the above structure, you can see that the octet is completed on the central atom (carbon), and also on the outside atoms. Therefore, the octet rule is satisfied.

After completing the octet, one last thing we need to do is, calculate the formal charge and check the stability of the above structure.

Also read: How to draw Lewis structure of NO₃^– (5 steps)

#5 Calculate formal charge and check stability

The following formula is used to calculate the formal charges on atoms:

Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons

Collect the data from the above structure and then, write it down below as follows:

• For hydrogen atom

Valence electrons = 1
Nonbonding electrons = 0
Bonding electrons = 2

Formal charge = 1 – 0 – ½ (2) = 0

• For carbon atom

Valence electrons = 4
Nonbonding electrons = 0
Bonding electrons = 8

Formal charge = 4 – 0 – ½ (8) = 0

• For nitrogen atom

Valence electrons = 5
Nonbonding electrons = 2
Bonding electrons = 6

Formal charge = 5 – 2 – ½ (6) = 0

Mention the formal charges of atoms on the structure. So the Lewis structure of HCN looks something like this:

In the above structure, you can see that the formal charges of all atoms are zero. Therefore, this is the stable Lewis structure of HCN.

And each horizontal line drawn in the above structure represents a pair of bonding valence electrons.

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Related

• Lewis structure of SO₂
• Lewis structure of N₂
• Lewis structure of NO₃^–
• Lewis structure of CH₄
• Lewis structure of CH₂O

External video

• HCN Lewis Structure: How to Draw the Lewis Structure for HCN – Wayne Breslyn

External links

• HCN Lewis Structure in 6 Steps (With Images) – Pediabay
• HCN Lewis Structure & Molecular Geometry – What’s Insight
• HCN Lewis structure, Molecular geometry, Bond angle, Shape – Topblogtenz
• HCN Lewis Structure, Molecular Geometry, Shape, and Polarity – Geometry of Molecules
• Molecular Geometry of Hydrogen Cyanide (HCN) – Chemistry Learner
• HCN Lewis Structure, Geometry – Kemicalinfo
• Chemical Bonding: HCN Lewis Structure – The Geoexchange
• HCN Lewis Structure, Molecular Geometry, Hybridization, MO Diagram, and Polarity – Techiescientist
• Lewis Structure and Hybridization of HCN (hydrocyanic acid, hydrogen cyanide) – ChemistNate
• HCN Lewis Structure, Molecular Geometry, Hybridization, Polar or Nonpolar – Studocu
• How to draw HCN Lewis Structure? – Science Education and Tutorials
• What is the Lewis structure of HCN? – Socratic
• What is the Lewis structure of HCN? – Homework.Study.com
• Draw the Lewis structure for HCN – Vaia
• Is HCN Lewis structure polar or nonpolar – Proprep
• The Lewis Dot Structure for HCN – MakeTheBrainHappy
• Is this a correct Lewis structure of HCN? – Brainly
• Which is the correct Lewis electron dot structure for hydrogen cyanide, HCN? – Numerade
• Two Possible Lewis Structures For The Molecule HCN – Chegg
• What would the Lewis structure look like for HCN? – Laurence Lavelle
• Draw the Lewis structure for HCN – Quizlet
• Determine the Lewis Dot Structure for hydrocyanic acid:HCN – Pearson
• What is the central atom of HCN and why? – Quora