How to calculate formal charges of NO2- 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 nitrite [NO2] ion using its best possible Lewis representation. In addition, we will also discuss the formal charges present in the different resonance structures of NO2.

So, what are you waiting for? Dive into the article, and let’s start reading!

 Name of the molecule Nitrite Chemical formula [NO2]– The formal charge on N-atom 0 The formal charge on each single bonded O-atom -1 The formal charge on the double-bonded O-atom 0 The overall formal charge on [NO2]– -1

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How to calculate the formal charges on NO2– atoms?

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

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 following two resonance structures are possible for representing NO2. Each resonance structure is a way of representing the Lewis structure of a molecule or molecular ion.

Each of the above resonance structures consists of a total of 18 valence electrons. The central nitrogen (N) atom is single-bonded to an oxygen (O) atom on one side and double-bonded to another O-atom on the other side, respectively.

The single-bonded O-atom has three lone pairs of electrons. The double-bonded O-atom carries two lone pairs, while one lone pair of electrons is present on the central N-atom.

Now let’s see how we can use resonance structure 1 and apply the formal charge formula given above to determine NO2 formal charges.

For nitrogen atom

• Valence electrons of nitrogen = It is present in Group V A = 5 valence electrons
• Bonding electrons =1 double bond + 1 single bond = 4 + 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 NO2 is 0.

For the single-bonded oxygen atom

• 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 single-bonded O-atom in NO2 is -1.

For the double-bonded oxygen atom

• Valence electrons of oxygen = It is present in Group VI A = 6 valence electrons
• Bonding electrons = 1 double bond = 4 electrons
• Non-bonding electrons = 2 lone pairs = 2(2) = 4 electrons
• Formal charge = 6 – 4 – 4/2 = 6 – 4 – 2 = 6 – 6 = 0

The formal charge on the double-bonded O-atom in NO2 is 0.

The above calculation shows that zero formal charges are present on the central N-atom as well as on the double-bonded O-atom. Contrarily, the single-bonded O-atom carries a -1 formal charge which is also the charge present on the nitrite (NO2) ion overall.

Identical formal charges are present on all three bonded atoms in resonance structure 2.

The pi-bonded electrons in the N=O double bond, as well as the un-bonded/ lone pairs of electrons, keep revolving from one point to another on the molecular ion, with a consequent shift in formal charges. The actual structure of NO2 is thus a hybrid of the two equivalent resonance structures.

As a final step, the NO2 Lewis structure is enclosed in square brackets, and a -1 formal charge is placed on the top right corner, as shown below.

Also, check –

FAQ

How can you calculate NO2– formal charges?

The formal charges present on the bonded atoms in NO2 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 the central N-atom in NO2–?

The central N-atom carries zero or no formal charges in NO2 Lewis structure.

What is the formal charge present on the single-bonded O-atom in NO2–?

The single bonded O-atom carries a -1 formal charge in NO2.

What is the formal charge present on the double-bonded O-atom in NO2–?

The double-bonded O-atom carries zero or no formal charges in NO2.

Do both the O-atoms carry the same formal charge in NO2–?

No, the single-bonded O-atom carries a -1 formal charge, while the double-bonded O-atom carries zero formal charges in NO2 Lewis structure.

What is the overall charge present on NO2–?

-1 formal charge is present on NO2 ion overall.

How many resonance structures are possible for representing NO2–?

Two distinct resonance structures are possible for representing NO2, as shown below.

Which one, out of the two possible resonance structures, is the most preferred Lewis representation of NO2–?

Both NO2 resonance structures are equivalent, as each of the three bonded atoms carries identical formal charges in each resonance structure. Therefore, both structures are equally stable.

Conversely the most preferred NO2 Lewis structure is a hybrid of the two resonance structures where the electrons continuously keep revolving from one position to another.

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].
• The nitrite [NO2] ion consists of two distinct resonance structures.
• In each resonance form, the central N-atom and the double-bonded O-atom carry zero or no formal charges.
• -1 formal charge is present on the single-bonded O-atom in NO2 which is also the charge present on the ion overall.
• The nitrite [NO2] ion is a monovalent anion with an overall -1 formal charge.
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