Question

Abnormality high heat of formation and shortening of bond length are criteria of (a) hybridisation (b) resonance (c) electron delocalisation (d) ionization

Short answer

(b) resonance

Step-by-step solution

Understand the Concepts

First, let's define the terms given in the options. Hybridization involves the mixing of atomic orbitals to form new hybrid orbitals. Resonance is a concept used to describe the delocalization of electrons in molecules with conjugated bonds, where the actual structure is a blend of two or more structures. Electron delocalization refers to electrons being spread over several atoms, typically involving pi bonds. Ionization is the process of removing an electron from an atom or molecule.

Analyze the Criteria

The criteria mentioned are 'abnormal high heat of formation' and 'shortening of bond length.' High heat of formation can indicate a stable compound due to resonance stabilization, as resonance can lead to energy release when the resonance structures contribute to overall stabilization. Shortening of bond lengths often occurs in cases of resonance, where the bond order increases due to shared electrons.

Match Criteria to Concepts

We need to match these criteria with the given concepts. Hybridization does not primarily relate to either high heat of formation or shortening of bond lengths. Ionization deals with removing electrons, which doesn't fit the criteria either. Resonance involves electron delocalization, which can stabilize compounds (high heat of formation when compared to theoretical formations without resonance) and result in shorter bond lengths between atoms sharing electrons. Electron delocalization is often directly linked to resonance.

Select the Correct Option

Considering the criteria, both resonance and electron delocalization can explain high heat of formation and shortening of bond lengths. However, resonance is more specifically known for these effects as it directly describes the structural stabilization through these mechanisms. Therefore, the best answer is resonance.

Key concepts

Hybridization

Hybridization plays a pivotal role in the structure and geometry of molecules. It entails the blending of atomic orbitals, such as s, p, and sometimes d orbitals, into a set of equivalent hybrid orbitals. This process occurs when atoms form chemical bonds, affecting the molecular geometry and bond angles. For instance, in methane (H_4), the carbon atom undergoes sp³ hybridization, wherein one s orbital and three p orbitals merge. This results in four equivalent sp³ hybrid orbitals, directing the bonds in a tetrahedral geometry.

Hybridization explains the formation of sigma (σ) bonds that have a direct overlap between atomic orbitals. These bonds are crucial for the structural framework of molecules.

• sp hybridization forms linear shapes, found in molecules like acetylene.
• sp² hybridization results in trigonal planar geometries, as seen in ethylene.
• sp³ hybridization leads to tetrahedral arrangements, such as in methane.

Overall, hybridization does not typically result in significant changes in bond lengths directly related to energy stabilization, as seen in processes like resonance. Instead, it predominantly influences molecular shapes and angles.

Resonance

Resonance is a crucial concept to grasp the stability and electronic arrangement within certain molecules. It occurs when a molecule can be represented by two or more valid Lewis structures. These structures, called resonance forms or resonance contributors, describe different versions of electron arrangements while maintaining the same atomic arrangement.

A key feature of resonance is that it allows for the delocalization of pi (cc) electrons over a region of the molecule. This electron sharing across different atoms leads to greater stability.

• The true structure is a resonance hybrid, more stable than any individual resonance form.
• Resonance stabilization can lead to higher heat of formation. This is because the hybrid structure is more energetically favorable due to its electron delocalization.
• Bond lengths can appear shorter due to increased bond order, where electrons are shared across a bond, effectively strengthening it.

The concept of resonance is widely observed in aromatic compounds like benzene, where the electron cloud is distributed evenly across the ring, creating a resonance-stabilized structure.

Electron Delocalization

Electron delocalization is a broader term that encompasses the distribution of electrons across multiple atoms, which is a fundamental aspect of resonance. In chemical bonding, delocalization involves the spread of electron density beyond a single bond, enhancing the molecule's stability and altering its properties.

Electron delocalization primarily occurs in systems with conjugated bonds, where alternating single and double bonds enable pi electron sharing across a molecule. This process accounts for many unique characteristics in organic and inorganic molecules:

• Conjugated systems, such as polyenes and aromatic rings, exhibit electron delocalization that leads to improved stability.
• Delocalization often causes the emergence of special properties like color in dyes because of electronic transitions involving extended pi systems.
• Bond lengths in conjugated systems are shorter than expected because the delocalization increases the effective bond order.

Understanding electron delocalization helps explain why resonance can lead to specific molecular changes, such as increased heat of formation and bond length alterations, as stabilized electron sharing lowers the overall energy of the molecule.