Why does NO3- (nitrate ion) bear three formal charges instead of four?
The correct answer and explanation is:
The nitrate ion, NO₃⁻, has three formal charges because of how the valence electrons are distributed between the nitrogen and oxygen atoms. The overall charge on the ion is -1, and the distribution of electrons among the atoms results in formal charges on the atoms in a way that minimizes the overall energy of the molecule.
To understand the formal charges, let’s look at the electron configuration. Nitrogen, which is the central atom, typically has five valence electrons, and each oxygen atom typically has six valence electrons. In the nitrate ion, there are three oxygen atoms, and each one is bonded to the nitrogen atom. The structure of the ion is resonance-based, meaning there are multiple equivalent ways to arrange the electrons.
In a typical resonance structure for the nitrate ion, two of the oxygen atoms will form double bonds with nitrogen, while one oxygen will form a single bond with nitrogen and bear the extra negative charge. The nitrogen atom will have no formal charge.
Now, for the formal charge calculation:
- Nitrogen has five valence electrons, and it forms three bonds (one single and two double bonds). According to formal charge formula, nitrogen will have a formal charge of zero.
- For each of the oxygen atoms, the one with a double bond will have no formal charge, as it has six valence electrons, four of which are involved in bonding, and two electrons remain as lone pairs.
- The oxygen with a single bond has one extra electron, giving it a -1 charge.
Since the nitrate ion as a whole has a -1 charge, and the formal charges of the atoms add up to -1, the distribution of formal charges is minimized with this resonance structure. This explains why the ion bears three formal charges instead of four. The resonance structure, with its three equivalent arrangements, ensures that the ion’s overall charge is distributed efficiently across the atoms.