Question

Resonance structures can be used to explain the bonding in charged groups of atoms as well as in neutral molecules, such as ozone. The nitrate ion, \(\mathrm{NO}_{3}{ }^{-}\), has one additional electron plus the outer electrons contributed by nitrogen and oxygen atoms. That extra electron gives the ion its charge. Draw the resonance structures, verifying that each obeys the octet rule.

Short answer

The nitrate ion, \( \text{NO}_3^- \), has 3 resonance structures, each with one double bond between nitrogen and an oxygen, while the other oxygens carry negative charges.

Step-by-step solution

Count Total Valence Electrons

Nitrogen (N) has 5 valence electrons, and each Oxygen (O) has 6 valence electrons, contributing a total of 18 electrons from the three oxygen atoms. Including the additional electron due to the ion's charge, the total number of valence electrons for the nitrate ion \( \text{NO}_3^- \) is: \[ 5 + (3 \times 6) + 1 = 24 \] electrons.

Determine the Central Atom

The central atom is usually the least electronegative, which in this case is nitrogen (N). So, nitrogen will occupy the central position, surrounded by three oxygen atoms.

Create the Initial Structure

Place single bonds between the nitrogen and each of the three oxygen atoms. This uses up \(3 \times 2 = 6\) electrons. Then, distribute the remaining 18 valence electrons as lone pairs, starting on the outer atoms (oxygen atoms).

Verify and Adjust for Octet Requirement

Check if the central nitrogen atom has a complete octet. The current structure violates nitrogen's octet because it only has 6 electrons. Convert one pair of lone pair electrons from one of the oxygen atoms into a double bond with nitrogen. This forms a more stable structure for the nitrogen atom.

Create Resonance Structures

Create additional resonance structures by alternating which oxygen forms the double bond with nitrogen. Three structures are possible: each with one double bond between nitrogen and one of the three oxygen atoms. All structures have nitrogen with a complete octet and the oxygen atoms either have a double bond or carry a charge to complete their octets.

Verify the Octet Rule for All Atoms

Ensure each atom has 8 electrons around it in each resonance structure. The double-bonded oxygen has a complete octet. The singly bonded oxygens have 7 electrons and a formal charge of -1 to complete their octets. The central nitrogen has 8 electrons, satisfying the octet rule in all resonance forms.

Key concepts

Nitrate Ion

The nitrate ion, represented as \( \mathrm{NO}_3^- \), here serves as a fascinating example of how resonance structures illustrate chemical stability. In the world of chemistry, ions are atoms or molecules that carry an electric charge. The nitrate ion specifically carries a -1 charge. This imbalance stems from having more electrons than protons.
Understanding the arrangement of atoms and these extra electrons in the nitrate ion allows us to explore its bonding through resonance. For a nitrate ion, nitrogen (N) is the central atom, surrounded symmetrically by three oxygen (O) atoms. This structure is crucial, as it allows the resonance concept to manifest.
Through resonance, we can demonstrate how electrons are not confined to one position but rather "move" across the ion to form different, equally valid structures. These alternate forms stabilize the ion further by spreading the charge across different atoms. Hence, resonance is an essential tool in explaining the chemical behavior of complex ions like the nitrate ion.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a key role in chemical bonding. For the nitrate ion, calculating the total number of valence electrons is the first critical step in drawing its resonance structures.
Let's break down the counting of valence electrons for \( \mathrm{NO}_3^- \):

• Nitrogen (N) contributes 5 valence electrons.
• Each of the three oxygen atoms contributes 6 valence electrons, adding up to 18 valence electrons from oxygen.
• Additionally, the ion's negative charge indicates one extra electron, resulting in a total of 24 valence electrons.

These electrons are used to form bonds and occasionally as lone pairs on atoms. Knowing the quantity of these electrons is crucial for constructing accurate Lewis structures and understanding how atoms connect in the ion.
Carefully arranging these electrons to satisfy the octet rule and creating possible resonance structures underscores the importance of these valence electrons in chemical bonding.

Octet Rule

The octet rule is a fundamental concept in chemistry, stating that atoms tend to form bonds until they are surrounded by eight electrons in their valence shell. This configuration is similar to that of a noble gas, which is very stable.
For the nitrate ion, each atom's electron configuration must meet the octet rule to achieve stability. Initially, in our nitrate ion structure, nitrogen is bonded to three oxygens, using some valence electrons for bonding but not yet satisfying the octet requirement for nitrogen.
To complete nitrogen's octet, one lone pair of electrons from one oxygen atom forms a double bond with nitrogen. Consequently, we achieve an octet for nitrogen, and the structure becomes more stable. However, in this case, the other oxygens will not always get to maintain a full octet because of the way resonance is established.

• The double-bonded oxygen complies with the octet rule.
• The singly bonded oxygens have seven electrons, which can be adjusted and completed by considering their formal charges.

This dynamic enables the nitrate ion to form different resonant structures, stabilizing the ion by fulfilling the octet rule across the structures.

Formal Charge

Formal charge is an essential component in understanding more about how resonance structures depict reality. It helps determine the optimal Lewis structure by allowing chemists to identify the most stable form of a molecule or ion.
For the nitrate ion, we calculate formal charge as the difference between the number of valence electrons an atom possesses in the free state and the number assigned to it in a molecule.

• For nitrogen in some resonance forms, it maintains a formal charge of zero due to a full octet with bonds.
• For oxygen atoms that form a double bond with nitrogen, their formal charge becomes zero.
• The singly bonded oxygens traditionally carry a formal charge of -1 to fulfill their octet requirement by contributing an additional electron to their lone pair.

In forming resonance structures, achieving the lowest possible formal charges, especially on the central atom, enables chemists to sketch the most stable forms of ions and molecules. Hence, the formal charge is vital in assessing the feasibility and stability of each resonance form, thereby offering comprehensive insight into the electron distribution within a molecule like the nitrate ion.