The Lewis Structure for the Nitrate Ion These three structures are called resonance structures. A double-headed arrow is used to indicate resonance structures. Each resonance structure is a legitimate Lewis structure. When we draw resonance structures, we leave the atoms in place and change the representation for how the electrons are arranged. The best description of the structure of the molecule is taken to be the average of the resonance structures, sometimes called a resonance hybrid. For the resonance structures of nitrate shown above, the N-O bond order is the same for each bond. The properties of the bond are somewhere in between a single and double bond. Experimental measurements show that all three N bonds in NO3- are identical. Critical Thinking Questions: 1. How many valence electrons does one nitrogen atom have? 2. How many valence electrons do three oxygen atoms have? 3. How many valence electrons does one NO2 molecule have? 4. How many valence electrons does one NO2-ion have? 5. How is the number of electrons used for each resonance structure shown in Model 3? 6. Why is the resonance hybrid representation of NO3- better than just a single structure? 7. How was the N bond order of NO3- calculated
The Correct Answer and Explanation is:
1. How many valence electrons does one nitrogen atom have?
Answer: 5 valence electrons
Explanation:
Nitrogen is in Group 15 of the periodic table. Elements in this group have 5 valence electrons. These are the electrons in the outermost shell that are involved in bonding.
2. How many valence electrons do three oxygen atoms have?
Answer: 18 valence electrons
Explanation:
Each oxygen atom has 6 valence electrons (Group 16 of the periodic table). For three oxygen atoms:
6 × 3 = 18 valence electrons.
3. How many valence electrons does one NO₂ molecule have?
Answer: 17 valence electrons
Explanation:
Nitrogen has 5 valence electrons, and two oxygen atoms contribute 6 electrons each:
5 + 6 + 6 = 17 electrons.
Note: This is for the neutral NO₂ molecule, not the nitrate ion.
4. How many valence electrons does one NO₃⁻ ion have?
Answer: 24 valence electrons
Explanation:
From above:
Nitrogen = 5
Oxygen (3 atoms) = 18
Extra electron due to the negative charge = 1
Total = 5 + 18 + 1 = 24 valence electrons.
5. How is the number of electrons used for each resonance structure shown in Model 3?
Answer: Each resonance structure uses all 24 valence electrons
Explanation:
Each Lewis structure must show the correct total number of electrons (24) distributed among atoms. These electrons are placed to satisfy the octet rule and to show formal charges accurately. The distribution changes between resonance structures, but the total number of electrons remains the same.
6. Why is the resonance hybrid representation of NO₃⁻ better than just a single structure?
Answer: It reflects the true nature of bonding in NO₃⁻ more accurately
Explanation:
No single resonance structure fully explains the bonding in the nitrate ion. Experimental data show all N–O bonds are identical in length and strength. The resonance hybrid represents this reality by averaging the electron distribution, showing partial double bond character for each N–O bond. This is more accurate than choosing one fixed double bond and two single bonds.
7. How was the N bond order of NO₃⁻ calculated?
Answer: By averaging the total bond types over the number of N–O bonds
Explanation:
There are three N–O bonds. Across the three resonance structures, there are three N=O double bonds total (one in each structure). Each bond shares this double bond character.
Bond order = total number of bonding pairs / number of bonds
= 4 (total bonding pairs: 1.33 per bond × 3 bonds) ÷ 3
= 1.33
So, each N–O bond has a bond order of 1.33, which is between a single and double bond.
