Which equation represents the formal charge of an atom?

The expression for formal charge is derived from the concept of electron accounting in a molecule. The correct equation for calculating the formal charge of an atom is:

Formal charge = valence - Nnonbonding - 1/2 Nbonding.

In this equation, "valence" refers to the number of valence electrons an atom has in its neutral state, while Nnonbonding represents the number of non-bonding (lone pair) electrons that are localized on that atom. The term 1/2 Nbonding accounts for the bonding electrons shared between two atoms in a bond — since each bond involves electrons contributed equally by both bonded atoms, only half of these bonding electrons effectively belong to the atom in question when calculating formal charge.

This method provides a way to evaluate how many electrons an atom effectively "owns" in a molecule, which can help in predicting the molecule's reactivity and stability. Understanding formal charge is crucial for analyzing Lewis structures and ensuring that the sum of the formal charges equals the overall charge of the molecule.

The other options do not accurately represent the formal charge calculation. They lack the proper accounting of both non-bonding and bonding electrons, which is essential for the correct determination of formal charge.