How to calculate formal charges of [NCO]- 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 NCO atoms. Based on the formal charges present on each individual atom, we will also determine the overall formal charge on NCO Lewis structure.

So, let’s start reading!

Name of the molecule Isocyanate   
Chemical formula[NCO] 
The formal charge on N-atom0
The formal charge on C-atom0
The formal charge on O-atom-1
The overall formal charge on [NCO]   -1

How to calculate the formal charges on NCO 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 calculated 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).

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

most stable NCO- lewis structure

The above structure is composed of a total of 16 valence electrons. A carbon (C) atom is present at the center. It is triple-bonded to a nitrogen (N) atom on one side and single-bonded to an oxygen (O) atom on the other side.

The central C-atom does not possess any lone pair of electrons, while the outer N and O-atoms contain 1 and 3 lone pairs, respectively.

So let’s see how we can use the NCO Lewis structure and the formal charge formula given above to determine the formal charges present on each bonded atom.

For nitrogen atom

calculating formal charge on nitrogen atom in NCO-

  • Valence electrons of nitrogen = It is present in Group V A = 5 valence electrons
  • Bonding electrons =1 triple bond = 3(2) = 6 electrons
  • Non-bonding electrons = 1 lone pair = 2 electrons
  • Formal charge = 5 – 2 – 6/2 = 5 – 2 – 3 = 5 – 5 = 0

 The formal charge on the N-atom in [NCO] is 0.

For carbon atom

calculating formal charge on carbon atom in NCO-

  • Valence electrons of carbon = It is present in Group IV A = 4 valence electrons
  • Bonding electrons = 1 triple bond + 1 single bond = 6 + 2 = 8 electrons
  • Non-bonding electrons = no lone pairs = 0 electrons
  • Formal charge = 4 – 0 – 8/2 = 4 – 0 – 4 = 4 – 4 = 0

 The formal charge on the C-atom in [NCO] is 0.

For oxygen atom

calculating formal charge on oxygen atom in NCO-

  • Valence electrons of oxygen = It is present in Group VI A = 6 valence electrons
  • Bonding electrons = 1 single bond = 2 electrons
  • Non-bonding electrons = 3 lone pairs = 3(2) = 6 electrons
  • Formal charge = 6 – 6 – 2/2 = 6 – 6 – 1 = 6 – 7 = -1

 The formal charge on the O-atom in [NCO] is -1.

As per the above calculation, zero formal charges are present on the nitrogen and carbon atoms. Contrarily, the single-bonded O-atom carries a -1 formal charge which is also the charge present on NCO overall.

NCO- lewis structure with formal charge

The NCO Lewis structure is thus enclosed in square brackets, and a -1 formal charge is placed at the top right corner, as shown below.

NCO- formal charge

You may also note that two other resonance forms are possible for representing NCO Lewis structure. The formal charges present on each resonance form are as shown below.  

NCO- resonance structure with formal charge

A -1 overall formal charge is present on each of the above resonance structures. Resonance form 1 is the most stable and thus the most preferred Lewis representation of NCO because, in this, the -1 formal charge is located on the most electronegative O-atom.

Resonance structure 3 is the least stable due to the highest formal charges present in it. However, the actual NCO structure is a hybrid of the three resonance forms.

Also, check –

FAQ

How can you calculate NCO formal charges?

The formal charges present on the bonded atoms in NCO 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 present on N-atom in NCO?

The N-atom carries zero or no formal charges in NCO Lewis structure.

What is the formal charge present on C-atom in NCO?

The C-atom carries zero or no formal charges in NCO Lewis structure.

What is the formal charge present on O-atom in NCO?

The O-atom carries a -1 formal charge in NCO Lewis structure. 

What is the overall charge present on NCO?

A -1 formal charge is present on NCO Lewis structure overall. It is thus a polyatomic monovalent anion.

Summary

  • The best possible Lewis structure of a 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].
  • NCO comprises three different resonance structures.
  • In the most stable resonance structure of NCO, the central C-atom and a triple-bonded N-atom carry zero or no formal charges.
  • A -1 formal charge is present on the most electronegative, single-bonded O-atom, which is also the charge present on the (NCO) ion overall.
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