Formal Charge (HL) (DP IB Chemistry): Revision Note
Formal charge
Sometimes, more than one valid Lewis formula can be drawn for a molecule
This can even occur when following the octet rule
Formal charge (FC) is used to decide which structure is most likely
Formal charge:
Assigns a charge to each atom in a structure
Assumes electrons in bonds are shared equally, regardless of electronegativity
It is a simple method that compares:
The number of valence electrons (V) an atom brings
The number of bonding electrons (B) an atom shares
The number of non-bonding (lone pair) electrons (N) an atom holds
The formula for calculating formal charge is:
FC= (number of valence electrons) - ½(number of bonding electrons) - (number of non-bonding electrons)
or
FC= V - ½B - N
Worked Example
What is the formal charge on boron in the BH4- ion?
Answer:
Boron is a group 13 element, so it has 3 valence electrons
Each hydrogen contributes one electron
The 1- charge on the ion adds 1 electron
Total valence electrons = 3 + (4 x 1) + 1 = 8
Therefore, the Lewis formula is:
The number of bonded electrons is 8
The number of non-bonded electrons is zero
So, the formal charge on boron is:
FC (B) = (3) - (½ x 8) - 0 = -1
Formal charge and resonance
Formal charge is especially useful when comparing resonance structures
Resonance structures are different Lewis formulas that can be drawn for the same species
The preferred Lewis formula is usually the one where:
The difference in formal charges is closest to zero
Any negative charges are placed on the most electronegative atom
Example: sulfur dioxide
It is possible to draw three resonance structures for sulfur dioxide, SO2:
The first structure is an illustration of the expansion of the octet as the sulfur has 10 electrons around it
Formal charge can be used to decide which of the Lewis formulas is preferred
The formal charge on the first structure is:
FC on sulfur = (6) - (½ x 8) - (2) = 0
FC on oxygen = (6) - (½ x 4) - (4) = 0
The difference in formal charge is:
ΔFC = FCmax- FCmin
ΔFC = 0 - 0 = 0
The formal charge on the second (and third) structures is:
FC on sulfur = (6) - (½ x 6) - (2) = +1
FC on left side oxygen = (6) - (½ x 2) - (6) = -1
FC on right side oxygen = (6) - (½ x 4) - (4) = 0
The difference in formal charge is:
ΔFC = FCmax- FCmin
ΔFC = (+1) - (-1) = 2
The first structure is the preferred structure because it has the difference in formal charge closest to zero
Worked Example
What is the formal charge on the two resonance structures shown?
Determine which resonance structure is preferred based on formal charge.
Answer:
Structure I
Formal charge calculations:
Carbon = (4) - (½ x 8) - (0) = 0
Oxygen = (6) - (½ x 4) - (4) = 0
Difference in formal charge calculation:
ΔFC = FCmax- FCmin
ΔFC = 0 - 0 = 0
Structure II
Formal charge calculations:
Carbon = (4) - (½ x 8) - (0) = 0
Left hand oxygen = (6) - (½ x 6) - (2) = +1
Right hand oxygen = (6) - (½ x 2) - (6) = -1
Difference in formal charge calculation:
ΔFC = FCmax- FCmin
ΔFC = (+1) - (-1) = 2
Structure I is the preferred structure as the difference in formal charge is closest to zero
You've read 0 of your 5 free revision notes this week
Unlock more, it's free!
Did this page help you?
