To calculate the formal charge of an atom in a molecule, use the following formula:
Formal Charge (FC)=Valence Electrons (VE)−Non-bonded Electrons (NBE)−Number of Bonds (B)\text{Formal Charge (FC)}=\text{Valence Electrons (VE)}-\text{Non- bonded Electrons (NBE)}-\text{Number of Bonds (B)}Formal Charge (FC)=Valence Electrons (VE)−Non-bonded Electrons (NBE)−Number of Bonds (B)
Where:
- Valence Electrons (VE) is the number of electrons the neutral atom has, based on its group number in the periodic table.
- Non-bonded Electrons (NBE) are the electrons in lone pairs on the atom (each lone pair counts as 2 electrons).
- Number of Bonds (B) is the number of bonds the atom forms (each bond counts as 1 electron for this calculation, equivalent to half the bonding electrons).
Step-by-step method:
- Determine the number of valence electrons for the neutral atom (e.g., 6 for oxygen, 4 for carbon).
- Count all the lone pair electrons on the atom.
- Count the number of bonds the atom forms.
- Apply the formula:
FC=VE−NBE−B\text{FC}=\text{VE}-\text{NBE}-\text{B}FC=VE−NBE−B
This assumes bonding electrons are shared equally between atoms.
Example:
For nitrogen in NH₃ (ammonia):
- VE = 5 (nitrogen group number)
- NBE = 2 electrons (1 lone pair)
- B = 3 bonds
So,
FC=5−2−3=0\text{FC}=5-2-3=0FC=5−2−3=0
The formal charge on nitrogen is zero
. This method helps identify the most stable Lewis structure by minimizing formal charges and placing negative charges on more electronegative atoms. Note that formal charge is a bookkeeping tool and does not always reflect actual charge distribution
