Formal Charge-
Formal Charge is a hypothetical charge assigned to an atom in a molecule, assuming that the electrons in a chemical bond are equally shared between atoms, regardless of their electronegativity.
* It helps to identify the most stable Lewis structure for a molecule or ion.
Why is Formal Charge Important?
* Determines the most stable resonance structure.
* Useful in predicting molecular structure and bonding.
Formal Charge Formula
Where:
V = Number of valence electrons in the free atom
L = Number of lone pair electrons on the atom
B = Number of bonding (shared) electrons
Step-by-Step Method to Calculate Formal Charge
* Draw the correct Lewis structure of the molecule or ion.
* Count valence electrons (V) for each
* Count lone pair electrons (L) on each atom.
* Count bonding electrons (B) shared with other atoms.
* Use the formal charge formula for each atom.
*Sum of all formal charges should equal the overall charge of the molecule/ion.
Example : Formal Charge on Each Atom in NO₃⁻ (Nitrate Ion)
Structure has many resonance:
Take one resonance form:
O = N - O⁻
|
O
N: V = 5, L = 0, B = 8 (4 bonds)
O (double bond): V = 6, L = 4, B = 4
O (single bond with -1 charge): V = 6, L = 6, B = 2
Total charge = -1, matches given.
So, this is stable structure.
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