illustrate clearly, using Lewis dot structures, why it is considered an exception. Discuss whether resonance structures can account for its stability. A: NO3 is an molecule that does not satisfy the octet rule. The Lewis dot structure for nitrogen trioxide is :O.N.:O: It does satisfy the octet rule because each atom in the molecule only has access to 7 electrons, instead of the eight required, as shown by the circled atoms. O·NO:
The molecule shown in the image is incorrectly labeled and explained. The species NO₃ mentioned in the text is actually NO₃⁻ (nitrate ion), which does follow the octet rule through resonance. However, the structure in the image corresponds to NO₂ (nitrogen dioxide) or more likely NO (nitric oxide) derivatives, not NO₃⁻. Based on the image and the description of each atom having only 7 electrons, the molecule that actually displays an exception to the octet rule is NO (nitric oxide), not NO₃⁻. Let’s correct and explain the concept clearly:
Correct Answer:
Nitric oxide (NO) is a molecule that does not satisfy the octet rule.
Lewis Dot Structure for NO:
:N≡O:•
In this structure:
- Nitrogen forms a triple bond with oxygen and has one lone electron (an odd number of electrons).
- This results in 7 valence electrons around nitrogen and 8 around oxygen, violating the octet rule for nitrogen.
Explanation
The octet rule states that atoms tend to form molecules in which each atom has eight electrons in its valence shell, achieving a noble gas configuration. However, nitric oxide (NO) is a well-known exception. It is a free radical, meaning it contains an unpaired electron, which results in one of its atoms (nitrogen) having only seven valence electrons.
The Lewis structure of NO shows a nitrogen atom triple-bonded to an oxygen atom, with one unpaired electron residing on nitrogen. This configuration provides oxygen with a full octet but leaves nitrogen with only seven electrons, making NO an odd-electron molecule. This makes NO inherently reactive, but it does exist stably due to delocalization of electrons and molecular orbital stabilization.
Unlike molecules that obey the octet rule, NO cannot form a resonance structure that distributes the extra electron to give both atoms a full octet. However, molecular orbital theory explains NO’s stability through delocalized bonding. The half-filled π* antibonding orbital reduces the bond order slightly, but gives some stability to the structure.
In contrast, NO₃⁻ (nitrate ion) does obey the octet rule and has three equivalent resonance structures, where the double bond rotates between the three oxygen atoms, delocalizing the negative charge and enhancing stability.
In conclusion, NO is a true exception to the octet rule, and its stability cannot be fully explained using simple Lewis structures or resonance but rather through molecular orbital theory.
