Incomplete Lewis structures for the nitrous acid molecule, HNO2, and the nitrite ion, NO2-, are shown below. (a) Complete each Lewis structure by adding electron pairs as needed. (b) Is the formal charge on N the same or different in these two species? (c) Would either HNO2 or NO2 – be expected to exhibit resonance? (d) Would you expect the N = O bond in HNO2 to be longer, shorter, or the same length as the N − O bonds in NO2 _? Explain
The Correct Answer and Explanation is :
To answer your questions step by step:
(a) Completing the Lewis structures:
- HNO2 (Nitrous acid): Nitrous acid consists of a nitrogen atom, an oxygen atom, a hydroxyl group (-OH), and a second oxygen atom double-bonded to nitrogen. The correct Lewis structure will have:
- Nitrogen at the center.
- One oxygen atom double-bonded to nitrogen.
- A second oxygen atom single-bonded to nitrogen and also bonded to a hydrogen atom (the hydroxyl group).
- One lone pair of electrons on nitrogen.
- Lone pairs on oxygen atoms to complete their octets.
- NO2- (Nitrite ion): The nitrite ion has two resonance structures. The Lewis structure will have:
- Nitrogen at the center, bonded to two oxygen atoms.
- One oxygen atom single-bonded to nitrogen, carrying a negative charge.
- The other oxygen atom double-bonded to nitrogen.
- The negative charge resides on the singly bonded oxygen.
- Lone pairs on each oxygen to complete their octets.
(b) Formal charge on nitrogen:
- In HNO2, the formal charge on nitrogen is 0 (since nitrogen has 5 valence electrons, shares 3 electrons through bonds, and has 1 lone pair).
- In NO2-, the formal charge on nitrogen is also 0. In this case, nitrogen shares 4 electrons in bonds and has no lone pairs.
Thus, the formal charge on nitrogen in both species is the same (zero).
(c) Resonance:
- Both HNO2 and NO2- exhibit resonance. The nitrite ion (NO2-) has two resonance structures where the double bond can shift between the two oxygen atoms.
- HNO2 also exhibits resonance between the oxygen atoms double-bonded to nitrogen, where the structure can shift, with one of the oxygen atoms having a partial negative charge and the other having a partial positive charge.
(d) N=O bond length comparison:
- The N=O bond in HNO2 would be shorter than the N−O bonds in NO2-. In HNO2, the N=O bond is a double bond, which is shorter and stronger than the single N−O bond. In NO2-, there is resonance between the two O atoms, leading to bond lengths between a pure single bond and a pure double bond, making these bonds slightly longer than in HNO2.
I will now generate the image for the Lewis structures of HNO2 and NO2-.
Here are the completed Lewis structures for HNO2 (nitrous acid) and NO2- (nitrite ion). The diagram shows the bonding and lone pairs for each species.
Feel free to ask if you need further clarification or have more questions!
