Problem 20 - Chemistry An Atoms First Approach

Question

The three NO bonds in \(\mathrm{NO}_{3}^{-}\) are all equivalent in length and strength. How is this explained even though any valid Lewis structure for \(\mathrm{NO}_{3}^{-}\) has one double bond and two single bonds to nitrogen?

Step-by-Step Solution

Answer

The three NO bonds in the nitrate ion (\(\mathrm{NO}_{3}^{-}\)) are all equivalent in length and strength due to resonance. The actual structure is a hybrid of three possible resonance structures, where the double bond can be on any of the three oxygen atoms. These resonance structures contribute equally to the overall properties of the ion, resulting in the bond lengths and strengths being averaged. This leads to all three NO bonds having the same intermediate length and strength between a single bond and a double bond.

Step 1: Draw the Lewis structure for the nitrate ion (\(\mathrm{NO}_{3}^{-}\))

First, let's draw the Lewis structure for the \(\mathrm{NO}_{3}^{-}\) ion. To do this, we will: 1. Determine the total number of valence electrons. 2. Arrange atoms in a skeleton structure. 3. Distribute electrons to satisfy octets. - Nitrogen has 5 valence electrons, each oxygen atom has 6 valence electrons, and there's an extra electron from the negative charge. So, the total number of valence electrons is \(5 + (3 \times 6) + 1 = 24\). - Place nitrogen in the center and the three oxygen atoms around it. - Distribute the electrons as follows: one double bond to one oxygen and single bonds to the other two oxygen atoms. Complete octets around each atom by adding lone pairs. The resulting Lewis structure will have one double bond and two single bonds to nitrogen.

Step 2: Draw the resonance structures for the nitrate ion (\(\mathrm{NO}_{3}^{-}\))

The actual structure of the nitrate ion is a hybrid of the three different possible resonance structures where the double bond can be on any of the three oxygen atoms. We can draw these structures by moving the double bond to each of the oxygen atoms in turn: - Double bond between N and O1, single bonds between N and O2, N and O3. - Double bond between N and O2, single bonds between N and O1, N and O3. - Double bond between N and O3, single bonds between N and O1, N and O2. Remember to use the double-headed arrows to signify resonance between these structures.

Step 3: Explain the bond lengths and strengths

The three resonance structures contribute equally to the overall properties of the \(\mathrm{NO}_{3}^{-}\) ion. As a result, the bond lengths and bond strengths are averaged, meaning all three NO bonds have the same length and strength. This is consistent with the observed properties of the nitrate ion, where all NO bonds are equivalent in length and strength. The single and double bond character is distributed among the three N-O bonds, resulting in bonds that are intermediate in strength and length between a single bond and a double bond.