Question

Abnormality high heat of formation and shortening of bond length are criteria of: (a) Hybridisation (b) Resonance (c) Electron delocalization (d) Ionization

Short answer

The correct answer is (c) Electron delocalization.

Step-by-step solution

Understand the Terms

Before solving the question, let's understand the terms given in the options: 1. **Hybridization:** It is the mixing of atomic orbitals to form new hybrid orbitals suitable for the pairing of electrons. 2. **Resonance:** It is a way of describing delocalized electrons within certain molecules where the bonding cannot be expressed by one single Lewis structure. 3. **Electron Delocalization:** It is the distribution of electrons across several atoms, which generally results in increased stability. 4. **Ionization:** It is the process of removing electrons from an atom or molecule, which results in the formation of ions.

Analyze the Question

The question is asking about the criteria related to „abnormally high heat of formation‟ and the „shortening of bond length.‟ These phenomena typically suggest certain processes or structural features in chemistry.

Link the Phenomena to the Options

- **Abnormally High Heat of Formation:** This often indicates high stability due to electron delocalization. When electrons are delocalized, the energy is lowered, giving rise to a high heat of formation. - **Shortening of Bond Length:** Shorter bond lengths can result from increased electron density between nuclei, which is also a result of electron delocalization. - Both of these phenomena are characteristic of **Resonance** and **Electron Delocalization**.

Conclusion

When examining both phenomena - an abnormally high heat of formation and the shortening of bond length - both are attributed specifically to electron delocalization rather than resonance because they indicate the actual physical characteristics resulting from delocalized electrons. Thus, the correct answer is (c) Electron delocalization.

Key concepts

Hybridization

Hybridization is a fundamental concept in chemistry that involves the mixing of atomic orbitals to create new hybrid orbitals. These hybrid orbitals are formed to facilitate the pairing of electrons during chemical bonding. This mixing results in orbitals that have characteristics that differ from the original atomic orbitals. For example, in carbon, the \( sp^3 \) hybridization involves the mixing of one s orbital and three p orbitals to form four equivalent \( sp^3 \) hybrid orbitals, allowing carbon to form four single bonds, as seen in methane (\( CH_4 \)). This process is crucial for understanding molecular geometry and bonding properties. Hybridization explains why molecules have specific shapes and bond orientations that maximize bond strength and molecular stability.

Resonance

Resonance is a powerful tool in chemistry that helps explain the structure of molecules where electron distribution cannot be represented by a single Lewis structure. Instead of one fixed structure, a molecule like benzene can be described using multiple resonance structures. These structures show different arrangements of electrons but the same arrangement of nuclei. Resonance provides a more accurate depiction of real molecular structures because electrons are delocalized over several atoms. This concept helps in understanding the energy stability of molecules, as the different resonance forms contribute to lowering the overall energy, making the structure more stable than any individual resonance form could suggest alone.

Electron Delocalization

Electron delocalization involves the distribution of electrons across multiple atoms within a molecule. This spreading of electrons results in greater stability for the molecule. When electrons are not confined between two atoms, they can reduce the potential energy of the structure. This increased stability is often reflected as a high heat of formation. In molecules like benzene, delocalization causes all the carbon-carbon bonds to have the same length, shorter than a single bond but longer than a double bond, indicating equal sharing of electrons or resonance.

Ionization

Ionization is the process where electrons are removed from atoms or molecules, resulting in the formation of ions. This process can either be endothermic or exothermic depending on the ions involved. First ionization energy refers to the energy required to remove one mole of electrons from one mole of gaseous atoms to form cations. For example, when sodium loses an electron, it forms a cation \( Na^+ \) with a positive charge. Ionization is a critical process in forming ionic compounds. Understanding ionization energies is essential for predicting how different elements will react during chemical reactions.

Bond Length

Bond length is the average distance between the nuclei of two bonded atoms. It is a crucial parameter in chemistry as it affects the strength of the bond and the energy of the molecule. Shorter bond lengths usually indicate stronger bonds, as there are more shared electrons holding the atoms close together. In molecules with electron delocalization, average bond lengths can be shorter than expected due to the increased electron density between the bonding nuclei, contributing to overall molecular stability. Measuring bond lengths helps in identifying the nature of bonds—whether they are single, double, or triple.

Heat of Formation

Heat of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. It serves as an indicator of stability in compounds. A highly negative heat of formation suggests that a compound is more stable, as it releases more energy upon formation. The relationship between electron delocalization and heat of formation is crucial. Delocalization often leads to highly stable molecules, which is reflected in an abnormally high heat of formation. Understanding heat of formation allows chemists to predict reaction spontaneity and stability of resultant compounds.