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Formal Charges
In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used t…
Some molecules or polyatomic ions can be represented by multiple Lewis structures, but how to decide which one is the dominant structure?
By calculating the formal charges of the atoms, the Lewis structure closest to the actual structure of the molecule can be determined.
Each atom is assigned a hypothetical charge called the formal charge, which would be the charge on the atom if all other atoms in the molecule had the same electronegativity. It assumes that each bonding electron is equally shared by the two atoms.
Consider hydrogen chloride. To determine the formal charge on each atom, first add the number of nonbonding electrons to half the number of bonding electrons and then subtract the obtained value from the number of valence electrons.
An aggregate of all formal charges in a molecule or ion is equal to the net charge of the molecule or ion.
For example, nitrous oxide can be represented by three possible Lewis structures – one with two double bonds, one with a triple bond between the nitrogen atoms, and one with a triple bond between the nitrogen and oxygen — all that satisfy the octet. The best Lewis structure is identified by formal charge calculations.
Nitrogen has five valence electrons, and oxygen has six valence electrons. The calculation based on the number of nonbonding electrons and half the number of bonding electrons gives the formal charge for each structure. Because nitrous oxide is a neutral molecule, the sum of all formal charges needs to be zero.
Generally, the formal charges on the individual atoms in a dominant Lewis structure are closest to zero. Hence, the third structure with a higher formal charge can be ignored.
Additionally, a negative formal charge, if present, should be carried by the most electronegative atom.
Since oxygen is more electronegative than nitrogen, the second structure, with the negative formal charge on the oxygen, is identified as the dominant structure for nitrous oxide.
Formal charges are not the actual charges of molecules or atoms, but a bookkeeping convention. The actual charge of the molecule depends on several factors, including the difference in electronegativity between the constituent atoms.
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Q1: What is formal charge and how is it calculated?
Formal charge is a hypothetical charge assigned to an atom in a molecule, calculated by subtracting nonbonding electrons and half the bonding electrons from the valence electrons of a neutral atom. It assumes bonding electrons are equally shared between atoms. The formula is: formal charge = valence electrons − nonbonding electrons − (1/2 × bonding electrons). Formal charge is a bookkeeping convention, not an actual charge on the atom.
Q2: Why do formal charges help identify the dominant Lewis structure?
Formal charges help predict the most appropriate Lewis structure when multiple valid structures exist. The dominant structure typically has formal charges closest to zero, with negative charges on more electronegative atoms. By comparing formal charge distributions across possible structures, you can determine which arrangement best represents the actual molecular geometry and electron distribution.
Q3: How do you verify formal charge calculations for an entire molecule or ion?
Sum all individual formal charges in the structure. For a neutral molecule, the total must equal zero. For an ion, the sum must equal the ion's charge. This verification confirms your calculations are correct and that the Lewis structure accurately represents the species. If the sum doesn't match the expected charge, recalculate the formal charges for each atom.
Q4: What guidelines help choose between Lewis structures with non-zero formal charges?
When formal charges cannot be zero, prefer structures with the smallest non-zero charges. Adjacent formal charges should be zero or opposite in sign. If multiple structures have similar charge distributions, choose the one placing negative formal charges on more electronegative atoms. These guidelines help identify the most stable and realistic molecular structure.
Q5: Why is the formal charge on an atom different from its actual charge?
Formal charge assumes equal electron sharing between bonded atoms, which doesn't reflect reality due to electronegativity differences. Actual charge depends on several factors, including electronegativity differences between atoms and the degree of electron transfer. Formal charge is purely a bookkeeping tool to help predict molecular structure, not a measure of true atomic charge.
Q6: How do formal charges explain why less electronegative atoms occupy central positions?
Formal charge calculations demonstrate that placing the less electronegative atom in the center typically minimizes formal charges and produces the most stable structure. For example, in carbon dioxide, carbon in the center with two double bonds yields all zero formal charges, while oxygen in the center creates large formal charges. This principle helps predict molecular geometry and stability.
Q7: Can formal charges be used to distinguish between resonance structures?
Yes, formal charge analysis helps identify which resonance forms are most significant. Structures with lower formal charges and charges on more electronegative atoms are more important contributors to the actual structure. This helps explain electron delocalization and predicts which resonance form dominates the molecule's properties and reactivity.