1.

What do you mean by formal charge ? Explain its significance with the help of suitable example.

Answer»

Formal charge is the charge assigned to an atom in a molecule, assuming that all electrons are shared equally between atoms, regardless of their relative electronegativities.

Structure (I) :

Number of atomTotal number of electrons in free atom (V.E.)Total number of non-bonding electrons(N.E.)Total number of shared electrons in bond(B.E.)

Formal charge
F.C=(V.E)-(N.E)-\(\frac{1}{2}\)(B.E.)

1644F.C = 6-4-\(\frac{1}{2}\)(4) = 0
2408

F.C = 4-0-\(\frac{1}{2}\)(8) = 0

3644

F.C = 6-4-\(\frac{1}{2}\)(4) = 0

Structure (II) :

Number of atomTotal number of electrons in free atom (V.E.)Total number of non-bonding electrons(N.E.)Total number of shared electrons in bond(B.E.)

Formal charge
F.C=(V.E)-(N.E)-\(\frac{1}{2}\)(B.E.)

1626

F.C = 6-2-\(\frac{1}{2}\)(6) = ±1

2408

F.C = 4-0-\(\frac{1}{2}\)(8) = 0

3662

F.C = 6-6-\(\frac{1}{2}\)(2) = -1

Structure (III) :

Number of atomTotal number of electrons in free atom (V.E.)Total number of non-bonding electrons(N.E.)Total number of shared electrons in bond(B.E.)

Formal charge
F.C=(V.E)-(N.E)-\(\frac{1}{2}\)(B.E.)

1662

F.C = 6-6-\(\frac{1}{2}\)(2) = -1

2408

F.C = 4-0-\(\frac{1}{2}\)(8) = 0

3626

F.C = 6-2-\(\frac{1}{2}\)(6) = +1

While determining the best Lewis structure per molecule, the structure is chosen such that the formal charge is as close to zero as possible. 

The structure having the lowest formal charge has the lowest energy.

In structure (I),

The formal charge on each atom is 0 while in structures (II) and (III) formal charge on carbon is 0 while oxygens have formal charge -1 or +1. 

Hence, 

The possible structure with the lowest energy will be structure (I). 

Thus,

Formal charges help in the selection of the lowest energy structure from a number of possible Lewis structures for a given species.


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