Difference Between HOMO and LUMO

The key difference between Homo and Lumo is that the HOMO donates electrons whereas the LUMO receives electrons.

The terms HOMO and LUMO are under the subtopic “molecular orbital theory” in general chemistry. The term HOMO stands for “highest occupied molecular orbital” while the term LUMO stands for “lowest unoccupied molecular orbital”. We call them “frontier orbitals”. A molecular orbital gives the most probable location of an electron in an atom. Molecular orbitals form from the combination of atomic orbitals of two separate atoms in order to share their electrons. This electron sharing forms a covalent bond between the atoms. When forming these molecular orbitals, they split into two forms as HOMO and LUMO.

CONTENTS

1. Overview and Key Difference
2. What is Homo
3. What is Lumo
4. Side by Side Comparison – Homo vs Lumo in Tabular Form
5. Summary

What is Homo?

HOMO stands for highest occupied molecular orbital. The electrons in these molecular orbitals can be donated to the LUMO type molecular orbitals. This is because these molecular orbitals contain weakly attached electrons. These molecular orbitals are the most available form for covalent chemical bonding. The presence of these molecular orbitals is characteristic for nucleophilic substances.

HOMO has low energy. Therefore, electrons tend to hold in these molecular orbitals; because electrons try to fill into the low energy levels first. This is why we call them “occupied orbitals”. Moreover, in the presence of light, electron excitations can donate electrons from HOMO to LUMO.

What is Lumo?

LUMO stands for lowest unoccupied molecular orbital. These molecular orbitals can receive electrons from HOMO. As its name implies, these orbitals are unoccupied; thus, contains no electrons. This is because the energy of these orbitals is very high and electrons tend to occupy in low energy levels first. Apart from that, these molecular orbitals are characteristic for electrophilic substances.

Moreover, if we provide light energy, the electrons of HOMO can get excited and move to LUMO. That’s why we say that LUMO can receive electrons.

What is the Difference Between Homo and Lumo?

HOMO stands for highest occupied molecular orbital while the term LUMO stands for lowest unoccupied molecular orbital. Both these forms of orbitals are important in covalent chemical bonding, especially in pi bond formation. As the key difference between Homo and Lumo, we can say that the HOMO can donate electrons whereas the LUMO can receive electrons. Moreover, the presence of HOMO is characteristic for nucleophiles while the presence of LUMO is characteristic for electrophiles.

The below infographic presents more details on the difference between Homo and Lumo.

Summary -Homo vs Lumo

The frontier molecular orbital theory explains the formation of HOMO and LUMO type molecular orbitals. Though there are several differences between these two types, the key difference between Homo and Lumo is that Homo donates electrons whereas Lumo receives electrons.

Reference:

1. Hunt, Ian R. “Ch 10: Introduction to MO Theory.” Ch 13 – NMR Basics. Available here 
2. “Introduction to Molecular Orbital Theory.” ACID_BASE INDICATORS, Henry Rzepa. Available here 

Image Courtesy:

1.”Molecule HOMO-LUMO diagram”By Tomgally (Public Domain) via Commons Wikimedia