Which is a valid Lewis structure for the nitrate ion ${\mathrm{NO}}_3^{-}$ ?

The nitrate ion (NO₃⁻) is a polyatomic ion that consists of one nitrogen atom and three oxygen atoms. To determine its correct Lewis structure, we must ensure that the octet rule is satisfied for all atoms in the molecule. This involves arranging the electrons so that every atom, except hydrogen, has a complete octet.

In the valid Lewis structure for the nitrate ion, nitrogen is centrally located and forms double bonds with one oxygen atom and single bonds with the other two oxygen atoms. The two oxygen atoms with single bonds carry a negative formal charge. Importantly, resonance structures are possible for the NO₃⁻ ion, where the double bond can shift among the three oxygen atoms. These resonance forms indicate delocalized electrons, providing additional stability to the molecule.

When analyzing the NO₃ structure, the valid Lewis structure ensures the octet rule is adhered to for all atoms:

• Nitrogen: Forms bonds that allow it to share eight electrons, fulfilling its octet.
• Oxygen: Each oxygen atom in the NO₃⁻ ion also completes its octet, either through bonding or through lone pairs of electrons.
• Charge distribution: The overall charge of −1 is appropriately distributed among the oxygen atoms, maintaining the ion's stability.

The NO₃ structure depicted in option (b) correctly follows the octet rule, making it the valid Lewis structure for the nitrate ion. The resonance forms further support the delocalization of electrons, which is a key feature of the NO₃⁻ ion.

In conclusion, when assessing the NO₃ structure, ensure that all atoms satisfy the octet rule and consider resonance for a comprehensive understanding of the ion's electron distribution.