Problem 167 - Chemistry The Science in Context

Question

A linear nitrogen anion, \(\mathrm{N}_{5}\), was isolated for the first time in 1999 a. Draw the Lewis structures for four resonance forms of linear \(\mathrm{N}_{5}^{-}\) b. Assign formal charges to the atoms in the structures in part (a), and identify the structures that contribute the most to the bonding in \(\mathrm{N}_{5}^{-}\). c. Compare the Lewis structures for \(\mathrm{N}_{5}^{-}\) and \(\mathrm{N}_{3}^{-} .\) In which ion do the nitrogen-nitrogen bonds have the higher average bond order?

Step-by-Step Solution

Answer

Answer: The N5- ion has a higher average bond order for nitrogen-nitrogen bonds as compared to the N3- ion.

Step 1: Draw Lewis structures

To draw the Lewis structures, remember that nitrogen has five valence electrons. For N5-, the ion will have a total of 5 * 5 + 1 = 26 electrons. Here are four resonance structures for linear N5-: 1.[--N=N=N=N^-=N--](-) 2. [--N=N=N^-=N=N--](-) 3. [--N=N^-=N=N=N--](-) 4. [-N^-=N=N=N=N--](-)

Step 2: Assign formal charges and determine the best structures

To assign formal charges to the nitrogen atoms, use the formula (valence electrons - (unbonded electrons + 1/2(bonded electrons))). Go through each nitrogen atom in each resonance structure and calculate their formal charge. Structure 1: -1, 0, +1, 0, 0 Structure 2: 0, +1, -1, +1, 0 Structure 3: 0, 0, +1, 0, -1 Structure 4: -1, 0, +1, 0, 0 The structures that contribute the most to the bonding in N5- are those with the least overall formal charge, which are structures 1 and 4.

Step 3: Compare N5- and N3- Lewis structures

In order to compare the Lewis structures of N5- and N3-, first draw the Lewis structure for N3-. The azide ion (N3-) has a resonance structure similar to the following: 1. [--N=N^-=N--](-) Now compare the bond orders in N5- and N3-. For N5-, the average bond order will be (2 + 1 + 2)/3 = 5/3 ≈ 1.67. For N3-, the average bond order is (2 + 1)/2 = 3/2 = 1.5. Since 1.67 is greater than 1.5, the nitrogen-nitrogen bonds in N5- have a higher average bond order compared to those in N3-.

Key Concepts

Nitrogen Anion

A nitrogen anion is essentially a negatively charged nitrogen-containing ion. Here, we focus on the \(\mathrm{N}_{5}^{-}\) anion, which is a linear ion made up of five nitrogen atoms bonded together. The anionic (negative) charge arises from an extra electron that is added to the structure, resulting in a total of 26 valence electrons across the whole ion. Understanding nitrogen anions is crucial as it helps explain how nitrogen can form different ions by gaining electrons to achieve stability, following the octet rule.
Nitrogen anions play important roles in various chemical environments, serving as key intermediates and components in different reactions and states.

Resonance Structures

Resonance structures are a tool chemists use to describe the delocalization of electrons within a molecule that cannot be accurately represented by a single Lewis structure. For \(\mathrm{N}_{5}^{-}\), resonance structures help explain how the electrons are spread out across the molecule, influencing stability and reactivity.
In the case of \(\mathrm{N}_{5}^{-}\), we can draw four different resonance structures:

• --N=N=N=N^- =N--
• --N=N=N^- =N=N--
• --N=N^- =N=N=N--
• -N^- =N=N=N=N--

Each resonance structure shows different arrangements of double bonds and the position of the negative charge, distributing it across the various nitrogen atoms. The true electronic structure of the molecule is a hybrid of these forms, providing insight into which parts of the molecule are most reactive or stable.

Formal Charge

Formal charge is a concept used to estimate the distribution of electrons among the atoms in a molecule. Calculating the formal charge helps chemists determine the most balanced and stable electron configuration within the molecule.
The formula to calculate formal charge is: \[\text{Formal Charge} = (\text{valence electrons}) - (\text{unbonded electrons} + 0.5 \times \text{bonded electrons})\]For each nitrogen atom in the resonance structures of \(\mathrm{N}_{5}^{-}\), you can find the formal charge by applying this formula. Structures with minimal formal charge differences across the molecule are typically the most stable. In the case of \(\mathrm{N}_{5}^{-}\), structures 1 and 4 with the allocation of -1, 0, +1, 0, 0 charges are considered as the most significant contributors because they have the least formal charge variance.

Bond Order

Bond order is a measure of the number of chemical bonds between a pair of atoms. It can affect the stability of a molecule, with higher bond orders generally indicating stronger, more stable bonds.
To compare bond orders in different molecules, let's consider \(\mathrm{N}_{5}^{-}\) and the azide ion \(\mathrm{N}_{3}^{-}\). The average bond order for \(\mathrm{N}_{5}^{-}\) is calculated as 1.67, while for \(\mathrm{N}_{3}^{-}\) it's 1.5. This shows that, on average, the bonds in \(\mathrm{N}_{5}^{-}\) are stronger than those in \(\mathrm{N}_{3}^{-}\). Bond orders help chemists predict reactivity and properties of substances, serving as a guide to understand how molecules might behave in different chemical environments.

Nitride Ion

The nitride ion is known as \(\mathrm{N}^{3-}\), consisting of a nitrogen atom with three extra electrons, resulting in a full octet and a 3- charge. While the \(\mathrm{N}_{5}^{-}\) ion is a complex structure, nitride ions are simpler and often act as basic, negatively charged species in ionic compounds.
Nitride ions are quite reactive due to their high negative charge and are often found in compounds like metal nitrides, where they contribute to unique properties such as hardness and high melting points. Understanding nitride ions is crucial for material science and chemistry where nitrides play a key role in creating materials with specific desired properties.