Question

(a) When talking about atoms in a Lewis structure, what is meant by the term formal charge? (b) Does the formal charge of an atom represent the actual charge on that atom? Explain. (c) How does the formal charge of an atom in a Lewis structure differ from the oxidation number of the atom?

Short answer

(a) Formal charge is a concept used to assess the stability of a Lewis structure by calculating the "charge" of an atom in a molecule, determined by valence electrons, unbonded electrons, and a fraction of bonded electrons. (b) The formal charge of an atom does not represent the actual charge on that atom, as it is a bookkeeping method rather than an actual depiction of electron distribution. (c) Formal charge is used for stability and resonance structures in Lewis structures, while oxidation number analyzes changes in oxidation states in redox reactions. They are related concepts with distinct applications.

Step-by-step solution

(a) Definition of Formal Charge

Formal charge is a concept used in Lewis structures to determine the "charge" of an atom in a molecule. It is calculated by taking the number of valence electrons in the neutral atom, subtracting the number of unbonded (lone pair) electrons, and then subtracting half the number of bonded (shared) electrons. Formal Charge = (Valence electrons in neutral atom) - (Unbonded electrons) - 1/2(Bonded electrons)

(b) Formal Charge vs. Actual Charge

The formal charge of an atom does not represent the actual charge on the atom. The formal charge is a bookkeeping method used to determine which Lewis structure is most stable and likely to occur. The actual charge on an atom is the result of the atom gaining or losing electrons to form ions or sharing electrons in a polar covalent bond. In a Lewis structure, formal charges are used to evaluate the electron distribution in a molecule, but they do not necessarily depict the true electron distribution in the actual molecule.

(c) Formal Charge vs. Oxidation Number

The formal charge and oxidation number are two different ways of evaluating the electron distribution in a molecule. The formal charge is, as explained earlier, used to determine the stability of a Lewis structure. On the other hand, oxidation number is a concept used in redox (reduction-oxidation) reactions to determine the change in the oxidation state of an atom. The oxidation number is calculated by assigning electrons to an atom using certain rules, such as the more electronegative atom in a bond receiving both shared electrons. Formal charge, unlike oxidation number, does not completely assign bonding electrons to only one atom in a bond. In summary, formal charge is a concept used to determine stability and resonance structures in Lewis structures, while oxidation number is utilized in redox reactions to analyze changes in the oxidation state of atoms within a molecule or ion. They are related concepts, but are not the same thing and have distinct applications.