Question

What is the HOMO-LUMO gap and how does it change the properties of a substance?

Answer 1

Hint: We can say molecular orbital describes the location and wave-like nature of an electron present in a molecule. We can molecular orbital as a mathematical function. Using this function, we could calculate the physical properties and chemical properties of a molecule. We can say a molecular orbital is formed by a combination of atomic orbitals.

Complete answer:
We have to know that in an isolated atom, the orbital electrons' area is controlled by functions known as atomic orbitals. At the point when different atoms join synthetically into an atom, the electrons' areas are dictated by the molecules all in all, so the atomic orbitals consolidate to frame molecular orbitals. The electrons from the constituent atoms possess the molecular orbital.
We can abbreviate HOMO as "Highest Occupied Molecular Orbital", and LUMO as "Least/Lowest Unoccupied Molecular Orbital".
Of the orbitals that contain electrons, the HOMO is the most elevated in energy, and of the orbitals that don't, the LUMO is the least in energy. That implies they are nearest in energy out of all orbitals found in the molecule.
Because of the energies of these orbitals being the nearest of any orbitals of various energy levels, the HOMO-LUMO gap is the place where the most probable excitations can happen. Subsequently, it is the main energy gap to consider.
Excitations become simpler as the HOMO-LUMO gap joins, for example, this is the reason why transition metal complexes are colored and also for large aromatic systems.
When the gap between HOMO-LUMO is larger, the aromatic system would be larger.
At the point when we have a larger aromatic system specifically, little HOMO-LUMO gap leads to portable pi electrons since it is simple for the electron to leap to a higher energy level that is nearby in energy.
The more prominent the portability of the pi electrons in a huge conjugated pi orbital system, the more the conveyance of the energy all through the molecule, which gets stabilized consequently, when the HOMO-LUMO gaps are smaller, and the molecule could have more stability.
This versatility of the pi electrons further implies that large aromatic systems have great conductivity and make extraordinary semiconductors since their conduction band hole is little; the development of electrons is electrical flow.

Note:
In simple words, the difference in energy between HOMO and LUMO is known as HOMO-LUMO gap. Frontier orbitals are the other name of HOMO-LUMO orbitals. We can predict the strength and stability of transition metal complexes using the energy difference between two frontier orbitals. In the field of organometallic chemistry, the size of lobe of LUMO could be useful to predict the location where the pi ligands addition could occur.