Final answer:
The only true statement regarding formal charge is that the sum of the formal charges of each atom in a molecule or ion equals the overall charge of the molecule or ion. Option C is the correct answer.
Step-by-step explanation:
The concept of formal charge plays a crucial role in understanding the distribution of electrons in a molecular structure. It is important when we seek to write the most stable Lewis structures for molecules and ions, especially when multiple structures are possible due to resonance. The formal charge on an atom is calculated by taking the number of valence electrons in the free atom, subtracting the number of nonbonding electrons, and then subtracting the number of bonding electrons (divided equally for the bonds the atom participates in).
Considering the statements given:
- The formal charge of each individual atom in a molecule or ion is not an actual atomic charge that can be experimentally determined. Instead, it is a theoretical construct used for bookkeeping during structure prediction.
- The formal charge of each individual atom can vary among different resonance forms. Different resonance forms will distribute electrons differently, thus altering the formal charges of the involved atoms.
- The sum of the formal charges in a molecule or ion must indeed equal the overall charge of that molecule or ion. This is a consistency rule that must be satisfied in all valid Lewis structures.
Based on this information, we can confidently say that statement 3 is correct, making the correct answer choice "c. 3 only".