Question

Write three reasonable resonance structures of the azide ion \(\mathrm{N}_{3}^{-}\) in which the atoms are arranged as NNN. Show formal charges.

Short answer

The resonance structures of azide ion show that the electrons are delocalized and that the ion as a whole is stabilized by resonance. Each structure involves transferring a pair of electrons from one atom to another, resulting in different formal charges for each nitrogen atom in the respective structures. The concept of resonance helps illustrate where the electron density in a molecule is located and how it may shift.

Step-by-step solution

Understand azide ion molecule.

First, draw the Lewis structure of the azide ion, \(N_{3}^{-}\). In this structure, it's clear that two nitrogen atoms share a triple bond, and one shares a single bond with one of them. The single-bonded nitrogen has a pair of electrons and is negatively charged.

Draw the first resonance structure

In the first resonance structure, convert one of the pairs of triple-bonded nitrogen electrons to the single-bonded nitrogen, changing it to double-bonded. As a result, the former triple-bonded nitrogen becomes positive, having lost electrons, and the former single-bonded nitrogen is neutral.

Draw the second resonance structure

Now, shift the pair of electrons from the double-bonded nitrogen back to the former triple-bonded nitrogen, which restores the triple bond. Meanwhile, one of the pairs of the other nitrogen atom forms a double bond with the former double-bonded atom. This forms a structure where each nitrogen atom has a formal charge: one is positive, one is negative, and one is neutral.

Draw the third and last resonance structure

Lastly, revert to the original structure but swap the positions of the nitrogen atoms. This exchange depicts that the nitrogen atom that was initially single-bonded is now sharing a triple bond with another nitrogen, while the initially triple-bonded nitrogen has a negative charge. The other nitrogen atom is neutral, having a positive charge.

Key concepts

Azide Ion

The azide ion, often represented as \(\mathrm{N}_{3}^{-}\), is an example of a polyatomic ion where three nitrogen atoms are consecutively bonded. It's ubiquitous in organic and inorganic chemistry, frequently appearing in chemical synthesis and as a preservative in healthcare. In discussing the azide ion, understanding its resonance forms is crucial because it reveals how the ion can exhibit different structures which stabilize the overall system.

Despite the varying structures, the actual azide ion is a hybrid of these resonance forms. It’s important to note that resonance doesn't imply that the ion is constantly changing between these states. Rather, this ion's electron distribution is a combination of all the possible structures, giving it unique chemical properties and a lower energy state as compared to any individual resonance form.

Lewis Structure

Drawing the Lewis structure is an essential skill in chemistry that helps visualize the valence electrons of atoms and how they're shared or paired to form molecules or polyatomic ions. For the azide ion, the Lewis structure is pivotal in comprehending the connectivity and electron arrangement.

To draw the Lewis structure, one identifies the total number of valence electrons (in this case, 16 for the three nitrogen atoms plus one additional electron for the negative charge), then deduces the most stable electron configuration. The Lewis structure for the azide ion introduces the concept of resonance structures, as no single drawing can accurately represent the full electron configuration. The steps to draw the resonance structures for the azide ion, as seen in the solution above, expose the versatility of the Lewis structure in illustrating molecules with multiple valid configurations.

Formal Charge

The formal charge concept plays a significant role in Lewis structures, especially when dealing with ions and resonance structures. It represents the charge an atom would have if all the bonding electrons were equally shared between the bonded atoms. To calculate the formal charge, use the formula:\(\text{Formal Charge} = \text{(Valence electrons)} - \text{(Non-bonding electrons)} - \frac{1}{2} \times \text{(Bonding electrons)}\).

For each of the azide ion's resonance structures, the individual atoms' formal charges ensure the overall charge adds up to the ion's total charge. When one attempts to draw the resonance structures, calculating formal charges helps determine the most plausible electron arrangements. For instance, a less stable structure might have a high formal charge on a single atom, while a more stable structure tends to minimize the formal charges across the ion.