How to calculate formal charges of Hydrocyanic acid (HCN) with lewis structure?

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In covalently bonded molecules, formal charge is the charge assigned to an atom based on the assumption that the bonded electrons are equally shared between concerning atoms, regardless of their electronegativity.

The overall formal charge present on a molecule is a measure of its stability.

The fewer the formal charges present on the bonded atoms in a molecule (close to zero), the greater the stability of its Lewis structure.

Hydrogen Cyanide (HCN) is a colorless-to-bluish volatile liquid that rapidly evaporates into a gas and has the potential to cause immediate fatal toxicity upon inhalation.

In this article, we will calculate the formal charges present on the bonded atoms in the HCN Lewis structure and also the overall charge present on the molecule. 

 So, continue reading!

Name of the molecule Hydrocyanic acid
Chemical formulaHCN
The formal charge on the central C-atom0
The formal charge on the N-atom0
The formal charge on the H-atom0
The overall formal charge on HCN0

How to calculate the formal charges on HCN atoms?

The formal charges can be calculated using the formula given below:

formal charge formula

The formal charge of an atom = [valence electrons of an atom – non-bonding electrons – ½ (bonding electrons)]

  • The valence electrons (V.E) of an atom are the total number of electrons present in its valence shell. Valence electrons can be determined by locating the position of the elemental atom in the Periodic Table.
  • Non-bonding electrons (N.E) are the number of lone pairs present on the atom. (1 lone pair means 2 nonbonding electrons).
  • Bonding electrons (B.E) are the total electrons shared with the atom via covalent chemical bonds. (1 single bond means 2 bonding electrons).

Now let us use this formula to calculate the formal charges in the most preferred Lewis structure of Hydrogen Cyanide (HCN).

The most preferred Lewis representation of HCN is as shown below.

most stable hcn lewis structure

It consists of a total of 10 valence electrons. A Carbon (C) atom is present at the center. It is bonded to one atom of Hydrogen (H) via a single covalent bond on one side, and on the other side, the central C-atom is bonded to an atom of Nitrogen (N) via a triple covalent bond.

No lone pair of electrons is present at the central C-atom and outer H-atom, while the N-atom carries a single electron pair in the HCN Lewis structure, respectively.

It is the best possible Lewis structure of HCN because the formal charges are minimized in it, and thus, it is the most stable.

Let’s find out how we can determine the formal charges present on each atom in the HCN Lewis structure.

For the central Carbon atom

calculating formal charge on carbon atom in HCN

  • Valence electrons of Carbon = It is present in Group IVA = 4 valence electrons
  • Bonding electrons around Carbon = 1 single bond + 1 triple bond = 2 + 3(2) = 8 electrons
  • Non-bonding electrons on Carbon = no lone pair = 0 electrons
  • Formal charge on the Carbon atom = 4 – 0 – 8/2 = 4 – 0 – 4 = 4 – 4 = 0

The formal charge on the central C-atom in HCN is 0.

 For Nitrogen atom

calculating formal charge on nitrogen atom in HCN

  • Valence electrons of Nitrogen = It is present in Group VA = 5 valence electrons
  • Bonding electrons around Nitrogen = 1 triple bond = 3(2) = 6 electrons
  • Non-bonding electrons on Nitrogen = 1 lone pair = 2 electrons
  • Formal charge on the Nitrogen atom = 5 – 2 – 6/2 = 5 – 2 – 3 = 5 – 5 = 0

The formal charge on the N-atom in HCN is 0.

For Hydrogen atom

calculating formal charge on hydrogen atom in HCN

  • Valence electrons of Hydrogen = It is present in Group I A = 1 valence electrons
  • Bonding electrons around Hydrogen = 1 single bond = 2 electrons
  • Non-bonding electrons on Hydrogen = no lone pair = 0 electrons
  • Formal charge on the Hydrogen atom = 1 – 0 – 2/2 = 1 – 0 – 1 =1 – 1 =0

The formal charge on the H-atom in HCN is 0.

This calculation shows zero formal charges are present on all three bonded atoms in the HCN Lewis structure, i.e., the central C-atom, the outer N-atom, and the H-atom, as shown below.   

hcn lewis structure with formal charge

As a result, there is no overall charge on the HCN Lewis structure, and thus it is a neutral molecule.

HCN Formal charge

Also, check –

FAQ

How can you calculate HCN formal charges?

The formal charges present on the bonded atoms in HCN  can be calculated using the formula given below:

V.E – N.E – B.E/2

Where –

⇒ V.E = valence electrons of an atom

⇒ N.E = non-bonding electrons, i.e., lone pairs

⇒ B.E = bonding electrons

What is the formal charge on C-atom in HCN?

The central Carbon (C) atom carries zero or no formal charge in HCN.

What is the formal charge on N-atoms in HCN?

Zero or no formal charge is present on the triple-bonded (N≡C) nitrogen atom in HCN.

What is the overall formal charge on HCN?

The overall formal charge on HCN is 0.

What is the formal charge on the H-atom in HCN?

Zero or no formal charge is present on single-bonded (H-C) hydrogen atoms in HCN.

Summary

  • The best possible Lewis structure of a molecule is the one in which the bonded atoms carry formal charges as close to zero as possible.
  • The formal charge formula is [ V.E – N.E – B.E/2].
  • In HCN, zero formal charges are present on the central C-atom.
  • The outer H and N-atoms also have zero formal charges in the HCN Lewis structure.
  • The overall formal charge on HCN is 0.
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