Resonance Structures: Videos & Practice Problems

Resonance structures are multiple valid Lewis dot structures for polyatomic ions with pi bonds, illustrating different possible arrangements of electrons. These structures are connected by double-sided arrows, signifying their equivalency in depicting the molecule's structure. However, not all resonance structures are actually equivalent; some contribute more to the actual structure of the molecule than others, which becomes more apparent in advanced chemistry studies. The true representation of the molecule is the resonance hybrid, an average of all significant resonance structures. It is depicted by placing dotted lines where pi bonds may exist, as seen in the example of the nitrite ion. This concept is crucial for understanding the delocalization of electrons within certain molecules, which impacts their chemical behavior and stability.

Resonance Structures

Resonance structures are essential in understanding the behavior of molecules with pi bonds, particularly in polyatomic ions. They consist of two or more valid Lewis structures that depict the same arrangement of atoms but differ in the placement of electrons, specifically pi bonds or lone pairs. For instance, the nitride ion can be represented by two resonance structures where the pi bond is alternately placed on the left or right oxygen atom. Both representations are valid and illustrate the concept of resonance.

To indicate these resonance structures, double-sided arrows are used, signifying that the structures are equivalent. At an introductory level, it is acceptable to consider these resonance structures as equal; however, in more advanced studies, such as organic chemistry, it becomes clear that not all resonance structures hold the same significance or stability.

The actual structure of the molecule is best represented by a resonance hybrid, which is a composite of all major resonance structures. This hybrid reflects an average of the contributing structures. To illustrate the resonance hybrid, dotted lines are employed to indicate the presence of pi bonds that are delocalized across the molecule. For the nitride ion, dotted lines would be drawn where the pi bonds were originally located, effectively capturing the essence of the resonance hybrid.

Resonance Structures Example 1

The carbonate ion, represented as \( \text{CO}_3^{2-} \), exhibits resonance, which means it can be depicted in multiple valid structures that differ only in the placement of electrons. In the most common resonance structures, one oxygen atom is double bonded to the carbon atom, while the other two oxygen atoms are single bonded. The single-bonded oxygens each carry a formal charge of -1, while the double-bonded oxygen has no formal charge.

To visualize the resonance, consider the following structures:

1. In the first resonance structure, the double bond is placed between the carbon and the top oxygen. The two single-bonded oxygens each have three lone pairs and a formal charge of -1.

2. In the second resonance structure, the double bond shifts to the left oxygen. Here, the left oxygen has two lone pairs, while the other two single-bonded oxygens maintain their three lone pairs and -1 formal charge.

3. In the third resonance structure, the double bond is now with the right oxygen. Similar to the previous structures, the right oxygen has two lone pairs, and the other two single-bonded oxygens each have three lone pairs and a formal charge of -1.

These resonance structures are connected by double-headed arrows, indicating that they are all valid representations of the carbonate ion. The actual structure of the carbonate ion is a hybrid of these resonance forms, which helps to stabilize the ion by delocalizing the negative charge across the three oxygen atoms.

Resonance Structures Example 2

In the context of resonance structures, the average charge of an atom can be determined by analyzing the formal charges across all possible structures. For the phosphate ion, which consists of a phosphorus atom bonded to four oxygen atoms—one with a double bond and three with single bonds—there are four resonance structures to consider.

To calculate the average charge of the oxygen atoms, we first need to understand the concept of formal charge, which is defined by the formula:

$FC=\mathrm{Group}-\mathrm{Bonds}+\mathrm{Non-bonding}Electrons$

In this case, we can label the double-bonded oxygen as oxygen A and the single-bonded oxygens as oxygen B. Oxygen A, which is in group 6A, forms two bonds and has four non-bonding electrons. Thus, its formal charge is:

$FC=6-2+4=0$

For the single-bonded oxygens (oxygen B), each is also in group 6A. One of them forms one bond and has six non-bonding electrons, resulting in a formal charge of:

$FC=6-1+6=1$

The other two single-bonded oxygens each have a formal charge of:

$FC=6-1+6=1$

Now, summing the formal charges gives:

$0+\left(-1\right)+\left(-1\right)+\left(-1\right)=-3$

The overall charge of the phosphate ion is thus -3. To find the average charge per oxygen atom, we divide the total charge by the number of oxygen atoms:

$AverageCharge=-34=-34$

Therefore, the average charge of each oxygen atom in the phosphate ion is -3/4. This method of calculating average charges through formal charge analysis is essential for understanding the distribution of charge in polyatomic ions.

Determine the average charge of the oxygen atoms within the chlorite ion, ClO₂^–.

• A. +1/2
• B. -1/2
• C. +1
• D. -1

Answer: (B)

Determine which of the following drawings would be the best structure for the N₂O molecule.

• A. a
• B. b
• C. c
• D. All are equally stable

Answer: (A)

Which of the following phosphate, PO₄^3- Lewis structures is the best, most valid resonance structure?

• A.
• B.
• C.
• D.
• E.

Answer: (C)

Here’s what students ask on this topic:

What are resonance structures?

Resonance structures are a way of representing the delocalization of electrons within certain molecules or ions where the chemical connectivity is the same but the electrons are distributed differently around the structure. This occurs in systems that can't be accurately represented by a single Lewis structure. Instead, multiple Lewis structures, called resonance structures, are used to model the molecule's electronic structure more accurately.

The actual molecule is a hybrid of these different structures, meaning it has characteristics of all the resonance forms rather than existing as any one of them. The electrons are not static; they are spread out or resonate across all the bonds, which can lead to greater stability for the molecule. Resonance is an important concept in understanding the behavior of molecules, especially in organic chemistry, where the delocalization of electrons can significantly affect a molecule's reactivity and properties.

How can resonance structures be drawn?

To draw resonance structures for a molecule or ion, you should follow these steps:

1. Identify the compound: Ensure you know the molecular formula and the connectivity of the atoms within the molecule.
2. Locate electrons: Identify all the valence electrons associated with the atoms in the molecule. Pay special attention to pi electrons and lone pairs that can be delocalized.
3. Determine the need for resonance: Look for atoms that can have multiple bonding arrangements without changing the positions of the atoms themselves. Common indicators include conjugated pi systems, atoms with lone pairs adjacent to pi bonds, or atoms that can have expanded octets.
4. Move electrons, not atoms: Draw resonance structures by moving electrons, not atoms. You can shift pi electrons or lone pairs to form new double bonds, and you can convert lone pairs on adjacent atoms into bonds.
5. Use curved arrows: When drawing, use curved arrows to show the movement of electrons from one location to another. The tail of the arrow starts where the electron pair is currently located, and the head points to where the electrons are moving to.
6. Keep the skeleton the same: The connectivity of the atoms should not change between resonance structures, only the placement of electrons.
7. Ensure valid structures: Each resonance structure must be a valid Lewis structure; that is, atoms

Why can't Lewis structures accurately depict molecules that exhibit resonance?

Lewis structures are a way to represent molecules by showing how electrons are distributed among atoms. However, they have limitations when it comes to depicting resonance. Resonance occurs when a molecule has multiple valid Lewis structures that contribute to the actual structure of the molecule. These structures, known as resonance structures, are like snapshots of the various ways electrons can be arranged within the molecule's framework of atomic nuclei.

The reason Lewis structures fall short in accurately depicting molecules with resonance is that they are static representations. They can show the different possible arrangements, but they cannot convey that the actual molecule is a hybrid of these structures. In reality, the electrons are delocalized and not confined to one position or bonding pattern as a Lewis structure might suggest. The molecule exists as a weighted average of the resonance structures, with the electrons spread over the atoms in a way that cannot be fully captured by a single, static Lewis structure. This delocalization often leads to greater stability and is a key feature of molecules like benzene, where the electrons are shared over several atoms.

What is a resonance hybrid?

A resonance hybrid is a concept from chemistry, specifically in the context of molecular structure and bonding. It represents the overall structure of a molecule that cannot be adequately described by a single Lewis structure. In molecules where resonance occurs, there are multiple valid Lewis structures, known as resonance structures, that can depict the possible distributions of electrons in the molecule. These structures differ only in the position of electrons, not the position of atoms.

The resonance hybrid is essentially a blend or an average of these resonance structures, where the actual molecule is thought to have a structure that is intermediate between them. It incorporates the delocalization of electrons across the molecule, which is a more accurate depiction of the electron distribution. The resonance hybrid is depicted by drawing the common structure and using dashed lines to indicate partial bonds or delocalized electrons, showing that the electrons are not localized to a single atom or pair of atoms but are spread out over the entire molecule or a portion of it. This concept helps to explain the stability and properties of certain molecules that cannot be captured by a single static structure.

How can one draw a resonance hybrid?

To draw a resonance hybrid, you must first identify a molecule that has resonance structures. These are molecules that have the same arrangement of atoms but different arrangements of electrons. Here's how to draw a resonance hybrid:

1. Identify Resonance Structures: Determine all the possible resonance structures for the molecule. These are valid Lewis structures that differ only in the position of the electrons, not the atoms.
2. Draw Lewis Structures: Sketch each resonance structure using Lewis dot structures. Make sure to show the movement of electrons with arrow notation.
3. Overlap Structures: Overlay the resonance structures on top of each other. This can be done mentally or by physically drawing one structure on a transparent sheet and placing it over another.
4. Combine Electron Features: In the resonance hybrid, show shared electron features, such as bonds, as solid lines. For electrons that are delocalized (those that move between structures), use dashed lines or a circle to indicate that the electrons are spread out over several atoms.
5. Indicate Partial Charges: If applicable, indicate partial charges on atoms that gain or lose electron density in the different resonance structures.

The final resonance hybrid is a representation that averages the resonance structures, reflecting the delocalization of electrons within the molecule. Remember, a resonance hybrid is more stable than any individual resonance structure and better represents the true electron distribution.