Formal charges of CH3, CH3- and CH3+ with Lewis structure?
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:
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 formula
CH3
The formal charge on C-atom
0
The formal charge on each H-atom
0
The overall formal charge on CH3
0
The most preferred Lewis structure of CH3 is as shown below.
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.
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
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.
Therefore, the overall charge present on the free radical (CH3) is also 0.
For methylium [CH3]+
Name of the molecule
Methylium
Chemical formula
[CH3]+
The formal charge on C-atom
+1
The formal charge on each H-atom
0
The overall formal charge on CH3+
+1
The most preferred Lewis structure of [CH3]+ is as shown below.
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
Valence electrons of carbon = It is present in Group IV A = 4 valence 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
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.
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.
For methanide [CH3]–
Name of the molecule
Methanide
Chemical formula
[CH3]–
The formal charge on C-atom
-1
The formal charge on each H-atom
0
The overall formal charge on CH3–
-1
The most preferred Lewis structure of [CH3]– is given below.
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
Valence electrons of carbon = It is present in Group IV A = 4 valence 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
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.
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.
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–.
How can you calculate CH3, CH3+, and CH3– formal 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 CH3– Lewis 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 radical. The 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.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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