Resonance Structures

Resonance structures, may be defined as a representation of molecules by more than one valid Lewis structure. Resonance structures come into being when a single Lewis structure is inadequate to fully capture the electron distribution of the molecule, mostly due to electron delocalization across different atoms. Consequently, this goes on to give a resonance hybrid that better explains the actual electronic structure of the molecule.

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Resonance Structures
Types of Resonance Structures
Practical Applications of Resonance Structures
Some Solved Examples
Summary

Resonance Structures - Lewis Dot, Examples, Rules, Structure, FAQs

Resonance structures are a specific set of two or more Lewis structures all of which describe the electronic bonding in each atom of a molecule or polyatomic ion. Because of electron delocalization, most molecules cannot be adequately explained by a single Lewis structure. This is observed in rather quite a few molecules, for example, benzene, where electrons are not restricted between atoms, but distributed within the molecule.

Bond Order

Bond order is defined as the number of bonds between the two atoms in a molecule. For example, in an H-H or H₂ molecule, there is only a single bond present thus its bond order is 1. Further, in O=O or O₂, the bond order is 2 as it has 2 bonds between the oxygen atoms.

Resonance

The nitrite anion can have two possible structures with the atoms in the same positions. The electrons involved in the N–O double bond, however, are in different positions as shown below.

If nitrite ions do indeed contain a single and a double bond, we would expect the two bond lengths to be different. A double bond between two atoms is shorter than a single bond between the same two atoms. Experiments show, however, that both N–O bonds in NO²⁻ have the same strength and length, and are identical in all other properties.

It is not possible to write a single Lewis structure for NO²⁻ in which nitrogen has an octet and both bonds are equivalent. Instead, we use the concept of resonance: if two or more Lewis structures with the same arrangement of atoms can be written for a molecule or ion, the actual distribution of electrons is an average of that shown by the various Lewis structures. The actual distribution of electrons in each of the nitrogen-oxygen bonds in NO²⁻ is the average of a double bond and a single bond. We call the individual Lewis structures resonance forms. The actual electronic structure of the molecule is called a resonance hybrid of the individual resonance forms. A double-headed arrow between Lewis structures indicates that they are resonance forms. Thus, the electronic structure of the NO²⁻ ion is shown as:

Resonance Hybrid
It is the average of the resonance forms shown by the individual Lewis structures or canonical structures.

Types of Resonance Structures

Resonance structures may be characterized by some main points which have to be addressed.

1. Equivalent and Non-equivalent Resonance Structures: Resonance structures that contribute to the same amount in the resonance hybrid are called equivalent resonance structures. Those that don't contribute equally are non-equivalent structures. The nitrate ions all have equivalent resonance structures, so the average of the nitrogen-oxygen bond order is 1.33. In ozone, the two resonance structures are not equivalent, so one will predominate, or be more prevalent, in the resonance hybrid.

2. Stability of Resonance Structures Not all resonance structures are equal. Some are more stable than others due to formal charge, electronegativity, and the octet rule. Structures that result in the least formal charges possible while still maintaining full octets for all atoms are generally more stable and, therefore, contribute more significantly to the resonance hybrid.

3. Fractional Bond Orders: It is extremely common for the resonance structure to be fractional, thus indicating that the bonds are neither purely single nor double bonds. In the case of benzene, electron delocalization between carbon atoms gives a bond order of 1.5.

These characteristics of resonance structures enable an understanding of how electron delocalization could cause changes in molecular geometry and reactivity.

Practical Applications of Resonance Structures

The concept of resonance structures stretches far into related fields of chemistry and beyond.

1. Organic Chemistry: Resonance forms the basis for the stability and reactivity of an organic molecule. Using benzene, resonance stabilization will make it less reactive than an alkene because delocalized electrons increase its stability. This resonance is quite critical in the prediction of products that are expected to occur in a reaction.

2. Pharmaceuticals: An example, in the case of drug design, is how resonance structures can modulate the activity of compounds at hand. Resonance stabilization is responsible, for instance, for the improved binding capacity of some functional groups present in drugs to their respective targets. Indeed, the role of resonance goes as far as medicinal chemistry itself.

3. Materials Science: Resonance structures can be used to explain such diverse properties of a material, for example, polymers and nanomaterials. In most conjugated systems, electron delocalization provides special optical or electronic properties used in designing new materials.

4. Biochemistry: Many biological molecules, such as enzymes and nucleic acids, are in resonance. Knowledge such as this about resonance structures of these molecules gives insight into the type of function they might provide or interaction type within a biological system.

Tying it together, resonance structures are useful theoretical devices; they also have applications across the disciplines in science. Resonance allows a scientist to make more accurate predictions concerning the behavior of molecules, design more potent drugs, and invent new materials in general.

Some Solved Examples

Example 1
Question:

Which of the following resonating structures is not correct for CO₂?

3) (correct)

Solution

The Oxygen atom forming a triple bond cannot have a negative charge on it as Oxygen will then have 10 electrons in the valence shell which is not possible

Hence, the answer is Option (3)

Example 2
Question: The bond order of CO and O₂ is:

1) (correct) 3 and 2

2) 3 and 2.5

3) 3 and 1.3

4) 3 and 3.5

Solution: Bond order is the number of chemical bonds between a pair of atoms in a molecule. For CO, the bond order is 3, indicating a triple bond between carbon and oxygen. For O₂, the bond order is 2, indicating a double bond between oxygen atoms. Hence, Option (1) is correct.

Example 3
Question: Which of the following conditions is not correct for resonating structures?

1) The contributing structures must have the same number of unpaired electrons.

2) (correct) The contributing structures should be so written that unlike charges reside on atoms that are far apart.

3) The contributing structures should have similar energies.

4) The positive charge should be present on the electropositive element and the negative charge on the electronegative element.

Solution: Resonance structures with similar charges on adjacent atoms are insignificant due to electrostatic repulsion and are thus unstable. There is no rule that positive and negative charges on atoms should be far apart. Hence, the correct option is (2).

Example 4
Question: The bond order and the magnetic characteristics of CN^- are:

1)3, paramagnetic
2) 212 $\begin{array}{r}1 \\ 2- \\ 2\end{array}$diamagnetic
3) (correct)3, diamagnetic
4) 212 $\begin{array}{r}1 \\ 2- \\ 2\end{array}$paramagnetic

Solution: The bond order of CN⁻ is 3, as there is a triple bond between carbon and nitrogen. All the electrons are paired in this ion, making it diamagnetic. Hence, the answer is Option (3).

Example 5
Question: Point out an incorrect statement about resonance

1)Resonance structures should have equal energy

2)In resonance structures, the constituent atoms should be in the same position

3) (correct)In resonance structures, there should not be the same number of electron pairs

4)Resonance structures should differ only in the location of electrons around the constituent atoms

Solution

We know these facts-

Resonance structures should have equal energy.
In resonance structures, the constituent atoms should be in the same position.
In resonance structures, there should be the same number of electron pairs.
Resonance structures should differ only in the location of electrons around the constituent atoms

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Thus the incorrect statement about the resonance structure is C.

Hence, the answer is the option (3).

Summary

Resonance structures are an important feature in chemistry to try and indicate the delocalization of electrons within a molecule or even polyatomic ions. The concept of resonance shows that there will be more than one valid Lewis structure that details the bond and stability of these compounds—something a single structure cannot fully explain. Some other types of resonance structures, like equivalent and nonequivalent types, carry immense information regarding the behavioral history of a molecule.

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