Formal Charge (HL) (DP IB Chemistry)
Revision Note
Formal Charge
A limitation of the model of covalent bonding is that when drawing Lewis formulas for molecules, it is sometimes possible to come up with more than one structure while still obeying the octet rule
This leads to the problem of deciding which structure is appropriate and is consistent with other information such as spectroscopic data on bond lengths and electron density
One approach to determining which is the preferred structure is to determine the formal charge (FC) of all the atoms present in the molecule
It is a kind of electronic book keeping involving the bonding, non-bonding and valence electrons
Formal charge is described as the charge assigned to an atom in a molecule, assuming that all the electrons in the bonds are shared equally between atoms, regardless of differences in electronegativity
The formula for calculating FC is
FC= (number of valence electrons) - ½(number of bonding electrons) - (number of non-bonding electrons)
or
FC= V - ½B - N
The Lewis formula which is preferred is the one which:
the difference in FC of the atoms is closest to zero
has negative charges located on the most electronegative atoms
The process of drawing a Lewis formula has been covered previously, but here is a reminder of how to draw the Lewis formula of tetrachloromethane, CCl4,
Diagram to show the Lewis formula of carbon tetrachloride
Steps in drawing the Lewis formula for CCl4
To work our the formal charge of the C and Cl atoms in the structure simply apply the FC formula:
FC for carbon = (4) - ½(8) - 0 = 0
FC for chlorine = (7) - ½(2) - 6 = 0
Notice that formal charge is calculated for one of each type of atom and does not count the total number of atoms in the molecule
Worked Example
What is the formal charge on boron in the BH4- ion?
Answer
Boron is a group 13 element, so has 3 valence electrons. Hydrogen has one valence electron and the charge on the ion is -1, so there are 8 electrons in the diagram. The Lewis formula is therefore:
Lewis formula of BH4-
The number of bonded electrons is 8 and the number of non-bonded electrons is zero. So the formal charge on B is:
FC (B) = (3) - ½(8) - 0 = -1
It is possible to draw three resonance structures for sulfur dioxide, SO2:
The three resonance structures of sulfur dioxide
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 FC on the first structure is as follows:
FC on sulfur = (6) - ½(8) -(2) = 0
FC on oxygen = (6) - ½(4) -(4) = 0
Difference in FC = ΔFC = FCmax- FCmin = 0
The FC on the second (and third) structures is as follows:
FC on sulfur = (6) - ½(6) -(2) = +1
FC on left side oxygen = (6) - ½(2) -(6) = -1
FC on right side oxygen = (6) - ½(4) -(4) = 0
Difference in FC = ΔFC = FCmax- FCmin = 2
Worked Example
What is the formal charge on the two resonance structures shown?
Resonance structures of carbon dioxide
Deduce which is the preferred structure.
Answer
Structure I
FC on carbon = (4) - ½(8) -(0) = 0
FC on oxygen = (6) - ½(4) -(4) = 0
Difference in FC = ΔFC = FCmax- FCmin = 0
Structure II
FC on carbon = (4) - ½(8) -(0) = 0
FC on left oxygen = (6) - ½(6) -(2) = +1
FC on right oxygen = (6) - ½(2) -(6) = -1
Difference in FC = ΔFC = FCmax- FCmin = 2
Structure I is the preferred structure as the difference is zero
Examiner Tips and Tricks
The term Lewis structure and Lewis formula mean the same thing.
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