**Delocalisation Energy**

The delocalisation energy, also known as the resonance energy, is a concept in the Huckel method of molecular orbital theory. It refers to the extra stability that arises when electrons are distributed over multiple atoms in a molecule, rather than being localized on individual atoms or bonds.

**Huckel Method**

The Huckel method is a simplified approach to molecular orbital theory that is particularly useful for conjugated systems, such as polyenes and aromatic compounds. It allows us to determine the electronic structure and properties of these systems by considering only the π-electrons and their interactions.

**Localized vs Delocalized Electrons**

In a molecule, electrons can be either localized or delocalized. Localized electrons are confined to a specific atom or a bond between two atoms. On the other hand, delocalized electrons are spread out over multiple atoms, forming a π-electron cloud.

**Resonance Energy**

The delocalisation energy, or resonance energy, quantifies the extra stability that arises when electrons are delocalized. It is defined as the difference in energy between the actual molecule and a hypothetical molecule where all the electrons are localized.

The delocalisation energy is a measure of the extent of π-electron delocalization in a molecule. The greater the delocalisation energy, the more stable the molecule. On the other hand, a molecule with low or zero delocalisation energy would have localized electrons and be less stable.

**Importance of Delocalisation Energy**

The concept of delocalisation energy is important in understanding the stability and reactivity of conjugated systems. The extra stability provided by delocalized electrons can influence various molecular properties, such as bond lengths, bond strengths, and reactivity.

Delocalisation energy also plays a crucial role in determining the aromaticity of compounds. Aromatic compounds have a particularly high delocalisation energy due to the presence of a fully conjugated π-electron system. The delocalisation energy in aromatic compounds is responsible for their unique stability and reactivity patterns.

In conclusion, the delocalisation energy, or resonance energy, is a measure of the extra stability that arises when electrons are delocalized in a molecule. It is an important concept in the Huckel method of molecular orbital theory and is used to understand the stability, reactivity, and aromaticity of conjugated systems.