How to calculate formal charges of Cyanide [CN]- ion 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.

In this article, we will calculate the formal charges present on the bonded atoms in the cyanide [CN] ion and also the overall charge present on it.

So, without any further delay, let’s start reading!

Name of the molecular ion  Cyanide or Nitrile
Chemical formula[CN]
The formal charge on C-atom-1
The formal charge on N-atom0
The overall formal charge on [CN]   -1

How to calculate the formal charges on CN 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 Cyanide ion [CN].

The most preferred Lewis representation of Cyanide ion [CN] is as shown below.

best stable cn- lewis structure

It consists of a total of 10 valence electrons. Cyanide ion [CN]–  contains two atoms, one Carbon (C) and one Nitrogen (N) atom. The Carbon (C) and Nitrogen (N) atoms are connected to each other via a triple covalent bond.

Each of the two triple-bonded atoms contains one lone pair of electrons in the CN Lewis structure.

It is the best possible Lewis structure of [CN] 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 [CN] Lewis structure.

For Carbon atom

calculating formal charges on carbon in CN-

  • Valence electrons of Carbon = It is present in Group IV-A = 4 valence electrons
  • Bonding electrons around Carbon = 1 triple bond = 3(2) = 6 electrons
  • Non-bonding electrons on Carbon = 1 lone pair = 2 electrons
  • Formal charge on the Carbon atom = 4 – 2 – 6/2 = 4 – 2 – 3 = 4 – 5  = -1

The formal charge on the C-atom in [CN] is -1.

For Nitrogen atom

calculating formal charges on nitrogen in CN-

  • Valence electrons of Nitrogen = It is present in Group V-A = 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 [CN] is 0.

This calculation shows no zero formal charges is present on the nitrogen atom, while the Carbon atom carries a -1 formal charge, which is also the charge present on the cyanide ion [CN] overall, as shown below.   

cn- lewis structure with formal charge

The CN Lewis structure is finally enclosed in square brackets, and a -1 formal charge is placed at the top right corner.

cn- formal charge

Also, check –

FAQ

How can you calculate [CN]  formal charges?

The formal charges present on the bonded atoms in [CN]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 [CN]?

The central Carbon (C) atom carries -1 formal charge in [CN].

What is the formal charge on N-atom in [CN]?

Zero or no formal charge is present on the  Nitrogen (N) atom in [CN].

What is the overall formal charge on [CN]?

The overall formal charge on [CN]is –1.

Summary

  • The best possible Lewis structure of a molecule or molecular ion 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 [CN], a -1 formal charge is present on the C-atom.
  • The triple-bonded N-atom has zero formal charges in [CN].
  • The overall formal charge on [CN] is thus 0 + (-1) = -1.
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