What does "formal" mean (e.g. in formal charge)?

Question

From Advanced Organic Chemistry by David Lewis:

The simple Lewis covalent bond model involves (in the formal sense, at least) the contribution of one electron to the bonding electron pair from each partner ... There are compounds known whose bonding is not completely explained by the simple Lewis covalent model. In such cases, Lewis proposed that one of the two partners in the bond formally contributes both electrons to the bond. Instead, modern applications of Lewis theory use the concept of formal charge to indicate that both electrons in a covalent bond formally originate on only one of the two atoms

Clearly, it seems that the author has some meaning in mind for "formal" besides it just being the first word in "formal charge." I can't quite figure out what it is, however, despite some online research. So what does "formal" mean here?

Answer 1

One of meanings of formal described in the Oxford Dictionary is Theoretical as opposed to practical. So formal charges are in effect theoretical (by a calculation) not the real charge distribution in a molecule (see below).

Linus Pauling does a good job in explaining the naming reasoning in his classical book, "The Nature Of The Chemical Bond and The Structure Of Molecules and Crystals" (1940, pg. 7). After showing a number of Lewis structures,

These formal charges are, as indicated by their name, to be considered as conventional in significance; they do not show in general the actual distribution of electric charges among the atoms in a molecule or complex ion. Thus in the ammonium ion the unit positive charge of the complex is not to be considered as residing exclusively on the nitrogen atom; as a consequence of the partial ionic character of the N—H bonds, discussed in the following chapter, part of the excess positive charge can be considered to be transferred to each of the hydrogen atoms.

For the sake of completion, students also get confused by other related terms. This article is a good read with a historical account. Valence, Oxidation Number, and Formal Charge: Three Related but Fundamentally Different Concepts by Gerard Parkin (https://doi.org/10.1021/ed083p791)

Valence: Number of electrons that an atom uses in bonding. Oxidation number: The charge remaining on an atom when all ligands are removed heterolytically in their closed form, with the electrons being transferred to the more electronegative partner; homonuclear bonds do not contribute to the oxidation number. Formal charge: The charge remaining on an atom when all ligands are removed homolytically. Coordination number: The number of atoms bonded to the atom of interest. Valence Number of electrons that an atom uses in bonding.

Answer 2

Formal is relating to a form. It is IIRC a standard grammatical construct.

Specific domains often use their own meanings of terms, overriding the general language meaning. In context of chemistry :

Formal is (strictly) fitting enumerated requirements for some form as a kind of convention to follow. Regardless of how much or if the content (as the complement to the form) fits the observed reality.

Oxidation states are formal atom charges if all more electronegative atoms fully attract bond electrons to themselves. But they do not have such charges.

IF $\ce{N}$ from $\ce{NH3}$ provided electron pair to $\ce{H+}$ to form $\ce{NH4+}$, there is formal charge +1 on $\ce{N}$ atom. But there is not such a charge, but smaller one.

$\ce{3 H2 + O2 -> 3 H3O}$ is formally a correct chemical equation.

The formal requirements for the equation form (as the complement to equation context) are:

• using conventional atomic symbols
• using conventional specific symbols for interactions
• matching charges and atom count inventories on both equation sides.

But that does not mean that such an equation does happen nor that mentioned compounds even exist.

As those formal requirements have the content requirements complement.

Content requirements have their own set of rules, based on known chemical properties and behavior of compounds.

If you create some official document, the document is formally correct, if it strictly matches enumerated formal requirements = requirements for the form of the document. But its content may not be regulated by these formal requirements and can be still nonsense.

BTW, formal can also mean related to formic acid.