Formal charges of CH3, CH3- and CH3+ 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.

This article is very interesting because, in this, we will calculate the formal charges present on three related chemical entities, i.e., methyl (CH3) free radical, methylium [CH3] + cation, and methanide [CH3] anion. Based on the formal charge calculation, we will determine the overall charge present on each and its stability.

So, without any further delay, dive into the article, and let’s start reading!

How to calculate the formal charges on CH3, CH3+, and CH3 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).

Now let us apply the formula given above to calculate the formal charges present on CH3, CH3+, and CH3 atoms using the most preferred Lewis representation of each, one by one.

For methyl (CH3)

Name of the molecule Methyl 
Chemical formulaCH3
The formal charge on C-atom0
The formal charge on each H-atom0
The overall formal charge on CH3   0

The most preferred Lewis structure of CH3 is as shown below.

best stable lewis structure of CH3

The above Lewis structure displays a total of 7 valence electrons. The central C-atom is bonded to three H-atoms via single covalent bonds. One electron is present unbonded on the central C-atom. This denotes that it is a free radical, an unstable, highly reactive molecule due to the presence of an odd number of unpaired electrons.

Now let’s apply the formal charge formula to the atoms present in this structure.

For the central carbon atom

calculating formal charge on carbon atom in CH3

  • Valence electrons of carbon = It is present in Group IV A = 4 valence electrons
  • Bonding electrons = 3 single bonds = 3 (2) = 6 electrons
  • Non-bonding electrons = 1 unpaired electron
  • Formal charge on the carbon atom = 4– 1 – 6/2 = 4– 1 – 3= 4 – 4 = 0

 The formal charge on the central C-atom in CH3 is 0.

For each hydrogen atom

calculating formal charge on hydrogen atom in CH3

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

 The formal charge on each H-atom in CH3 is also 0.

As the above calculation shows that zero formal charges are present on each bonded atom in the CH3 Lewis structure.

CH3 lewis structure with formal charge

Therefore, the overall charge present on the free radical (CH3) is also 0.

CH3 formal charge

For methylium [CH3]+

Name of the molecule Methylium 
Chemical formula[CH3]+
The formal charge on C-atom+1
The formal charge on each H-atom0
The overall formal charge on CH3+   +1

The most preferred Lewis structure of [CH3]+ is as shown below.

best stable ch3+ lewis structure

It consists of a total of 6 valence electrons. The carbon atom at the center is single-bonded to three H-atoms, one on each side. There is no lone pair or unbonded electrons on the central C-atom in this case.

Now let’s once again calculate the formal charges.

For the central carbon atom

calculating formal charge on Carbon atom in CH3+

  • Valence electrons of carbon = It is present in Group IV A = 4 valence electrons
  • Bonding electrons = 3 single bonds = 3 (2) = 6 electrons
  • Non-bonding electrons = no lone pair = 0 electrons
  • Formal charge on the carbon atom = 4– 0 – 6/2 = 4– 0 – 3= 4 – 3 = +1

 The formal charge on the central C-atom in [CH3]+ is +1.

For each hydrogen atom

calculating formal charge on Hydrogen atom in CH3+

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

 The formal charge on each H-atom in [CH3]+ is 0.

As per the calculation shown above, a +1 formal charge is present on the central C-atom, while the single-bonded H-atoms carry zero formal charges. Thus, the overall formal charge on CH3+ is +1.

CH3+ lewis structure with formal charge

The CH3+ Lewis structure is consequently enclosed in square brackets, and a +1 formal charge is placed at the top right corner, as shown below.

CH3+ formal charge

For methanide [CH3]

Name of the molecule Methanide   
Chemical formula[CH3]
The formal charge on C-atom-1
The formal charge on each H-atom0
The overall formal charge on CH3   -1

The most preferred Lewis structure of [CH3] is given below.

best stable ch3- lewis structure

It displays a total of 8 valence electrons. The central C-atom is single-bonded to three H-atoms, and it also possesses a lone pair of electrons. So, let’s find out what are the formal charges present on the bonded atoms in the above Lewis structure.

For the central carbon atom

calculating formal charge on carbon atom in CH3-

  • Valence electrons of carbon = It is present in Group IV A = 4 valence electrons
  • Bonding electrons = 3 single bonds = 3 (2) = 6 electrons
  • Non-bonding electrons = 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 central C-atom in [CH3] is -1.

For each hydrogen atom

calculating formal charge on hydrogen atom in CH3-

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

 The formal charge on each H-atom in [CH3] is 0.

According to the above calculation, the central C-atom carries a -1 formal charge, while no formal charges are present on the single-bonded H-atoms.

CH3- lewis structure with formal charge

Therefore, the overall charge present on CH3 Lewis structure is -1.

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

CH3- formal charge

Now, as we have seen that no formal charges are present in the CH3 Lewis structure; however, a +1 and a -1 formal charge is present on CH3+ and CH3 Lewis structures, respectively. Thus, as per the formal charge concept, CH3 should ideally be more stable than CH3+ and CH3 Lewis structures.

However, in terms of chemical reactivity, the presence of a single unbonded electron and an incomplete octet of the central C-atom makes CH3 a highly reactive and unstable molecule.

Conversely, CH3 is more stable than CH3+ as the central C-atom has a complete octet (8 valence electrons) in CH3 as opposed to a total of 6 valence electrons around the central C-atom in CH3+.

In conclusion, the stability increases in the order CH3 < CH3+ < CH3. 

Also, check –

FAQ

How can you calculate CH3, CH3+, and CH3formal charges?

The formal charges present on the bonded atoms in CH3, CH3+, and CH3,  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 C-atom in CH3?

The central C-atom carries zero formal charges in the CH3 Lewis structure.

What is the formal charge present on H-atoms in CH3?

All three H-atoms carry zero formal charges in CH3.

What is the formal charge present on the central C-atom in CH3+?

The central C-atom carries a +1 formal charge in the CH3+ Lewis structure.

What is the formal charge present on H-atoms in CH3+ and CH3?

In both CH3+ and CH3, all three single-bonded H-atoms carry zero or no formal charges.

What is the formal charge present on the central C-atom in CH3?

The central C-atom carries a -1 formal charge in CH3 Lewis structure.

Do the central C-atoms carry the same charge in each of CH3, CH3+, and CH3?

No. The central C-atom carries zero or no formal charges in the CH3 Lewis structure. Contrarily, a +1 and a -1 formal charge is present on the central C-atom in CH3+ and CH3, respectively.

Do all H-atoms carry the same formal charges in each of CH3, CH3+, and CH3

Yes, no formal charge is present on any H-atom in either of the CH3, CH3+, and CH3 Lewis structures.

What is the overall charge present on CH3, CH3+, and CH3Lewis structures? 

The overall charge present on:

  • CH3 is zero.
  • CH3+ is +1.
  • CH3 is -1.

Summary

  • The best possible Lewis representation of a molecule or molecular ion is the one in which the formal charges are as close to zero as possible.
  • The formal charge formula is [ V.E – N.E – B.E/2].
  • In CH3, the central C-atom, as well as each single-bonded H-atom, carries zero formal charges. It is a highly reactive and unstable free radicalThe overall charge present on CH3 is 0.
  • In CH3+, no formal charges are present on all three H-atoms. However, the central C-atom carries a +1 formal charge which is also the charge present on the ion overall. The overall charge present on CH3+ is +1. It is thus a monovalent carbocation.
  • In CH3, the central C-atom carries a -1 formal charge, while no formal charges are present on any of the three H-atoms. The overall charge present on CH3 is -1. It is thus a monovalent carbanion.
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