Resonance

A single Lewis formula cannot describe delocalised electrons within molecules, or polyatomic ions that a single Lewis formula cannot clarify are called resonance. Several resonance structures depict a molecule or ion with such delocalised electrons. When resonance structures differ, it’s crucial to determine which one(s) best describes the actual bonding. The formal charge can predict which resonance structures are most likely to be preferred. The situation is with ozone (O₃), an oxygen allotrope with a V-shaped design and a 117.5° O–O–O angle.

When two or more hybrid structures represent a molecule and those structures differ in the position of electrons but not in the location of atoms, the structure is called a resonating structure, and the phenomenon is known as resonance.

Nuclear magnetic resonance is based on many nuclei having spun, and all nuclei are electrically charged. When an external magnetic field is applied, energy can be transferred to a higher energy level from the base energy.

What are Resonance Structures?

The Lewis structure sets that describe the electron’s delocalisation in a molecule or a polyatomic ion are known as resonance structures.

In most circumstances, a single Lewis structure has consistently failed to explain the bonding in either a polyatomic or molecular ion because of fractional bonds and partial charges. Chemical bonding can be described using resonance structures in such circumstances.

Characteristics of Resonance

• It doesn’t exist in the actual world.

• The bond length in identical resonating structures is the same.

• The resonance hybrid has the least amount of energy hence most stability.

• The more resonance and resonance energy there is, the more stable the structure becomes.

• Resonance is a theoretical idea that has yet to be proven experimentally.

Rules for Delocalization and Resonance Structures

• Resonance structures should contain the same number of electrons; no electrons should be added or subtracted. (Count the number of electrons to see how many there are.)

• Each resonance structure follows the Lewis Structures writing rules.

• The structure’s hybridisation must remain constant.

• The structure’s skeleton cannot be altered (only the electrons move).

• The number of lone pairs in a resonance structure must be the same.

Identifying Viable Resonance Structures Using Formal Charges

While every resonance structure adds to the molecule’s entire electronic structure, their contributions may not be equal. One way of determining the viability of a resonance structure and its relative significance among other structures is to assign formal charges to atoms in molecules. Use the following formula to find the standard charge on a particular atom in a covalent species:

Formal Charge=(number of valence electrons in the free orbital)−(number of lone-pair electrons)− ½ ( number bond pair electrons)

Rules for estimating resonant structure stability

• The more covalent bonds there are, the more stable the system is since more atoms will have complete octets.

• The structure with the fewest formal charges is more stable.

• The structure with the least formal charge separation is the most stable.

• Positive charges on the most electropositive (least electronegative) atom are more durable.

• Equivalent resonance forms have the same level of stability and contribute equally (e.g., benzene)

Examples-

NO₂⁻ Ion Resonance Structures

In the case of the nitrate ion(NO₂⁻), both nitrogen-oxygen links have the same bond length. The Lewis dot structures of the NO₂⁻ ion, on the other hand, indicate that the bond order of the two N-O bonds in it has changed. Furthermore, the equal bond lengths are explained using the resonance hybrid of this polyatomic ion, which is derived from its multiple resonance configurations. The NO₂⁻ – ion’s resonance hybrid shows that every oxygen atom has a partial charge of magnitude -½. The length of the N-O bonds can be specified as 125 pm.

O₃ Resonance Structures

The core oxygen atom of the ozone (O₃) molecule is singly connected to one oxygen atom and doubly bound to another. Although this molecule has no net charge, the Lewis structures reveal a +1 charge on the central oxygen and a -1 charge on the singly bonded oxygen. The oxygen at the centre of the resonance hybrid of ozone has a +1 charge, while the other oxygen atoms have a partial charge of -(½).

Carbonate Ion (CO₃²⁻) Resonance Structures 

Limestone, baking powder, and baking soda contain a polyatomic ion’s carbonate ion. When acid is added to the carbonate ion, carbonic acid is formed, quickly decomposing into water and carbon dioxide. The carbon dioxide generated during baking allows the bread to rise and has a lighter texture.

Carbonate has 24 electrons, two of which are responsible for the -2 charge. Electrons from calcium(Ca), sodium(Na), or any other salt that formed a cation that delivered electrons to the carbonate anion are most likely to be responsible.

To satisfy the octet rule, each carbon atom in a pure structure must share electrons. Using the formal charges on the atoms, we could reorganise our electrons to participate in a double bond with the carbon. The carbon atom’s positive charge has dissipated, and all valences have been filled; the octet rule has been met. The total of the formal charges is equal to the charge on the carbonate ion.

Nitrobenzene Resonance Structures

Due to an electron-withdrawing group with a double bond close to the phenyl ring, the aromatic ring of nitrobenzene has a lower electron density than benzene, as indicated by the resonance structures of nitrobenzene.

As a result, nitrobenzene’s phenyl ring is less nucleophilic than benzene. Ortho and parasites are both positive in resonance structures. As a result, the electrophile will react at the meta position rather than these locations in an electrophilic aromatic substitution method. When a double bond is in conjugation with the phenyl ring, the electrophilic aromatic substitution product seems to be the meta substituted product.

Conclusion

A single Lewis formula cannot describe delocalised electrons within molecules, or polyatomic ions that a single Lewis formula cannot clarify are called resonance. Several resonance structures depict a molecule or ion with such delocalised electrons. When resonance structures differ, it’s crucial to determine which one(s) best describes the actual bonding. The formal charge can predict which resonance structures are most likely to be preferred. We have studied the resonance structure of several molecules and polyatomic ions in detail.