Problem 69 - Chemistry and Chemical Reactivity

Question

Determine the \(\mathrm{N}-\) O bond order in the nitrate ion, \(\mathrm{NO}_{3}^{-}\).

Step-by-Step Solution

Answer

The N–O bond order in \(\mathrm{NO}_3^-\) is \(\frac{4}{3}\).

Step 1: Draw the Lewis Structure

First, draw the Lewis structure for the nitrate ion, \(\mathrm{NO}_3^-\). The central nitrogen atom is surrounded by three oxygen atoms. Connect the nitrogen to each oxygen with a single bond, and add one more double bond to one of the oxygen atoms. Add brackets and a negative sign to indicate the overall charge.

Step 2: Count the Total Number of Bonds

In the Lewis structure, count the total number of N-O bonds. With resonance, we have one double bond and two single bonds in any single resonance structure, totaling four bonds.

Step 3: Determine the Number of Resonance Structures

Recognize that there are three equivalent resonance structures for \(\mathrm{NO}_3^-\), each with its double bond between nitrogen and a different oxygen atom.

Step 4: Calculate Bond Order

Finally, calculate the bond order by dividing the total number of N-O bonds observed in all resonance forms by the number of oxygen atoms. Total bonds: 4 bonds. Number of resonance structures: 3. Bond order = \(\frac{4}{3}\).

Key Concepts

Lewis Structure

When representing a molecule like the nitrate ion (\(\mathrm{NO}_3^-\)), Lewis structures are a valuable tool in visualizing how atoms bond and distribute electrons. For the nitrate ion, nitrogen is the central atom bonded to three oxygen atoms.
To draw the Lewis structure correctly, start by placing nitrogen in the center and forming single bonds with each of the three oxygens. Since the nitrate ion carries a negative charge, ensure the structure reflects this by adding brackets and a negative sign outside.
Importantly, distribute \(24\) valence electrons around the structure: nitrogen contributes five, each oxygen contributes six, and one more electron accounts for the negative charge. Use the electrons to complete the octet for each oxygen, and form a double bond between nitrogen and one oxygen atom to minimize formal charges for stability.

Resonance Structures

Resonance structures are a concept used to represent the different possible arrangements of electrons in a molecule. These structures help in understanding that the actual distribution is a hybrid, or average, of the resonance forms.
For \(\mathrm{NO}_3^-\), you can draw three valid resonance structures. Each structure features a double bond between the nitrogen atom and a different one of the three oxygen atoms.
Switching the position of the double bond provides these distinct forms. Importantly, resonance structures imply delocalization of electrons across the molecule, which leads to a stabilization much akin to an electron cloud rather than fixed positions. These resonance delocalizations must be accounted for when analyzing molecular properties like bond order.

Nitrate Ion

The nitrate ion, \(\mathrm{NO}_3^-\), is a symmetric molecule important in different chemical and biological processes.
In the context of molecular bonding and electron distribution, the nitrate ion exhibits characteristics that can perplex students. It’s a classic example of resonance, having resonance-stabilized bonds.
Analyzing its \(\mathrm{N}-\mathrm{O}\) bonds, using resonance theory helps in clarifying its bond order. The bond order represents the average number of conventional bonds (like single or double bonds) between two atoms.

• Total \(\mathrm{N}-\mathrm{O}\) bonds, counting across all resonance structures, amount to four.
• Distributing these four bonds across the three resonance structures divides evenly to reflect a bond order of \(\frac{4}{3}\).

This fractional bond order indicates not all bonds are equivalent, yet it integrates the resonance into a stabilized form.