Draw the Lewis structures and determine which of these molecules has a central atom that violates the octet rule. Please note that NO3 has an odd number of electrons. But you must decide where the missing electron will be. Does the central atom violate the octet rule or do the oxygen atoms? Use form..
The Correct Answer and Explanation is:
Lewis Structure and Octet Rule Analysis: Nitrate Radical (NO₃)
Step 1: Count Valence Electrons
- Nitrogen (N): 5 valence electrons
- Oxygen (O): 6 valence electrons × 3 = 18
- Total: 5 + 18 = 23 electrons
- Since NO₃ is a neutral radical, it has 23 valence electrons, which is an odd number. This indicates that the molecule must have one unpaired electron, meaning it is a free radical.
Lewis Structure of NO₃ (Radical Form)
- Central Atom: Nitrogen (least electronegative, and only one atom)
- Connect nitrogen to three oxygen atoms using single bonds (3 bonds × 2 e⁻ = 6 e⁻).
- Remaining electrons: 23 – 6 = 17 electrons.
- Distribute electrons to outer atoms (oxygen) to fulfill their octet:
- Place 6 electrons on each O (18 needed, but we only have 17), so at least one atom will not complete its octet.
- Form a double bond with one oxygen to stabilize the molecule (reducing one lone pair from that oxygen and forming a π bond with N).
- You will end up with:
- One double-bonded O (2 bonds + 2 lone pairs = 8 e⁻),
- Two single-bonded O atoms with 3 lone pairs each,
- One unpaired electron either on nitrogen or an oxygen.
Octet Rule Violation
Nitrogen in the NO₃ radical is bonded to three oxygens and shares an unpaired electron, totaling 9 electrons around nitrogen (2 from each of three bonds + 1 unpaired). Therefore:
- Nitrogen violates the octet rule by having 9 electrons—it is hypervalent in this radical structure.
- Oxygen atoms can all satisfy the octet if the unpaired electron is placed on nitrogen.
Conclusion
The molecule NO₃ radical violates the octet rule because the nitrogen atom has 9 electrons around it due to the need to stabilize the radical structure. Oxygen atoms follow the octet rule if the unpaired electron is assigned to nitrogen. This type of octet rule violation is common in odd-electron molecules like free radicals.
