How to calculate formal charges of thiocyanate [SCN]- 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 bonded atoms in the different resonance structures of the thiocyanate [SCN] ion and also in its best possible Lewis structure. We will also determine the overall charge present on [SCN]

So for all this interesting information, continue reading!

Name of the molecular ion Thiocyanate
Chemical formula[SCN]
The formal charge on the S-atom
0
The formal charge on C-atom0
The formal charge on N-atom-1
The overall formal charge on [SCN]-1

How to calculate the formal charges on SCN 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).

The most preferred Lewis structure of the thiocyanate [SCN] ion is given below.

best stable scn- lewis structure

The above Lewis structure displays a total of 16 valence electrons. A carbon (C) atom at the center is bonded to a sulfur (S) and a nitrogen (N) atom at the sides via double covalent bonds. No lone pair of electrons is present at the central C-atom. However, each of the outer S and N-atoms contains 2 lone pairs of electrons each in SCN Lewis structure.

Now let us use this Lewis structure and the formula given above to determine the formal charges on three bonded atoms in SCN.

For the carbon atom

calculating formal charge on carbon atom in scn-

  • Valence electrons of Carbon = It is present in Group IV A = 4 valence electrons
  • Bonding electrons = 2 double bonds = 2(4) = 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 central C-atom in SCN is 0.  

For the sulfur atom

calculating formal charge on sulfur atom in scn-

  • Valence electrons of Sulfur = 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 S-atom in SCN is 0.  

For the nitrogen atom

calculating formal charge on nitrogen atom in scn-

  • Valence electrons of Nitrogen = It is present in Group V A = 5 valence electrons
  • Bonding electrons = 1 double bond = 4 electrons
  • Non-bonding electrons = 2 lone pairs = 2(2) = 4 electrons
  • Formal charge = 5 – 4 –4 / 2 = 5 – 4 – 2 = 5–6 = -1

The formal charge on the N-atom in SCN is -1.

The above calculation shows that zero formal charges are present on the carbon and sulfur atoms in SCN Lewis structure.

However, a -1 formal charge is present on the nitrogen atom, which is also the charge present on the thiocyanate ion overall.

scn- lewis structure with formal chargeThe SCN Lewis structure is thus enclosed in square brackets, and a -1 formal charge is placed at the top right corner, as shown below.

overall formal charge on scn- lewis structure

Another interesting fact is that in addition to this Lewis structure, two other resonance forms are possible for representing the Lewis structure of SCN.

resoanance structure of scn-

Let us once again use the formal charge formula and determine the formal charges in the other two resonance forms.

For resonance form 2

2nd resonance form of scn-

For the carbon atom

calculating formal charge on carbon in 2nd resonance form of SCN-

  • Valence electrons of Carbon = It is present in Group IV A = 4 valence electrons
  • Bonding electrons = 1 single bond + 1 triple bond = 2 + 3(2) = 8 electrons
  • Non-bonding electrons = no lone pairs = 0 electrons

∴ Formal charge on the Carbon atom = 4 – 0– 8/2 = 4 – 0 – 4 = 4 – 4 = 0

For the sulfur atom

calculating formal charge on sulfur in 2nd resonance form of SCN-

  • Valence electrons of Sulfur = 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 on the Sulfur atom = 6 – 6–2/2 = 6 – 6 – 1 = 6  – 7 = -1 

For the nitrogen atom

calculating formal charge on nitrogen in 2nd resonance form of SCN-

  • 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 on the Nitrogen atom = 5 – 2 – 6 / 2 = 5 – 2 – 3 = 5 – 5 = 0

overall charge in 2nd form of scn- resonanceFor resonance form 3

scn- 3rd form of resonance

For the carbon atom

calculating formal charge on carbon in 3rd resonance form of SCN-

  • Valence electrons of Carbon = It is present in Group IV A = 4 valence electrons
  • Bonding electrons = 2 single bonds = 2(2) = 4 electrons
  • Non-bonding electrons = no lone pairs = 0 electrons

∴ Formal charge on the Carbon atom = 4 – 0–4 / 2 = 4 – 0 – 2 = 4 – 2 = +2

For the sulfur atom

calculating formal charge on sulfur in 3rd resonance form of SCN-

  • Valence electrons of Sulfur = 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 on the Sulfur atom = 6 – 6–2 / 2 = 6 – 6 – 1= 6 –7 = -1 

For the nitrogen atom

calculating formal charge on nitrogen in 3rd resonance form of SCN-

  • Valence electrons of Nitrogen = It is present in Group V A = 5 valence electrons
  • Bonding electrons = 1 single bond = 2 electrons
  • Non-bonding electrons = 3 lone pairs = 3(2) = 6 electrons

∴ Formal charge on the Nitrogen atom = 5 – 6–2 / 2 = 5 – 6 – 1 = 5 – 7 = -2

overall charge in 3rd form of scn- resonance

The above formal charge calculation reveals that SCN resonance form 2 is stable as only one of the three bonded atoms carries a -1 formal charge in it. The resonance form 3 is the least stable as all three bonded atoms carry high formal charges. However, the resonance form 1 is the most stable and, thus, the most preferred Lewis representation of SCN because the -1 formal charge is present on the most electronegative N-atom in this case.

overall charge in scn-

Also, check –

FAQ

How can you calculate [SCN]  formal charges?

The formal charges present on the bonded atoms in [SCN]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

How many resonance forms are possible for drawing the Lewis structure of SCN?

The SCN Lewis structure can be represented by the following three resonance forms.

How many resonance forms are possible for drawing the Lewis structure of SCN-

Which of the three resonance forms is the most stable Lewis structure of SCN and why?

The SCN resonance form in which the central S-atom is double bonded to a C-atom on one side to an N-atom on the other side is the most preferred Lewis representation.

It is also the most stable because the formal charges are as minimized in it as possible. The -1 formal charge is present on the most electronegative N-atom, which is also the charge present on the ion.

most stable structure of scn- with formal charge

In the most stable SCN Lewis structure, what is the formal charge present on C-atom?

Zero or no formal charge is present on C-atom.

What is the formal charge on N-atom in SCN?

-1 formal charge is present on N-atom in the most stable SCN Lewis structure.

What is the formal charge on S-atom in SCN?

Zero or no formal charge is present on S-atom in SCN.  

Which two out of the three bonded atoms carry the same formal charges in SCN?

Both C-atom and S-atom carry zero formal charges in SCN.

What is the overall charge on SCN?

-1 overall charge is present on thiocyanate [SCN] ion.

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].
  • In the most stable SCN Lewis structure, zero formal charges are present on the C-atom and S-atom, respectively.
  • -1 formal charge is present on the N-atom, which is also the overall formal charge present on the SCN ion overall.
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