The Molecular Orbital Theory (MOT) is a scientific idea that helps us understand how molecules stick together. It was created by scientists Hund and Mulliken in the 20th century.
Before MOT, there was another theory called the valence-bond theory. But it couldn’t explain some molecules that have bonds between being weak (like a single bond) and strong (like a double bond). These kinds of bonds are seen in molecules that can change their shape, like in some special molecules.
MOT is better because it can explain these situations. It tells us where we’re likely to find electrons around a molecule and matches the shapes of the molecules themselves. This helps us understand how molecules connect and act.
In simpler terms, Molecular Orbital Theory tells us that when atoms join together to make molecules, they create new spaces where electrons can be found. These electrons are shared among the atoms in different ways throughout the molecule.
This theory has been really important because it helped us understand how molecules stick together. We think of these molecular orbitals as being made up of atomic orbitals from the individual atoms. We use math and models like the Hartree-Fock or density functional theory to figure out exactly how these orbitals behave based on the Schrödinger equation.
The idea that molecular orbitals in molecules are like mixtures of atomic orbitals can be explained like this:
Video Lesson – Molecular Orbital Theory
Think of molecular orbitals as invisible spaces around a molecule where we are most likely to find electrons. These spaces are like special shapes that show where electrons hang out in a molecule.
Molecular orbitals are made by mixing the electron clouds of the atoms that make up the molecule. They help us understand how atoms stick together in molecules.
According to this theory, there are three main kinds of molecular orbitals:
When two electron clouds come together, they can either work together (like clapping hands) or cancel each other out (like 2 waves crashing). If they work together, they form a molecular orbital, showing us where electrons are most likely to be around the molecule.
Case 1: When two waves from different atoms match up nicely, they add together, just like when two friends both push a swing at the same time, making it swing higher. This combination is represented as Φ = ΨA + ΨB.
Case 2: When the two waves from different atoms don’t match up, they actually work against each other. It’s like if one friend pushes the swing forward while the other tries to pull it back. This makes a new wave with less amplitude, and we write it as Φ´ = ΨA – ΨB.
Why Are Anti-bonding Orbitals Higher in Energy?
The energy levels of bonding molecular orbitals are always lower because electrons in these orbitals are attracted to the nuclei of the atoms, which brings the atoms closer together. In contrast, anti-bonding molecular orbitals have higher energy levels because electrons in these orbitals experience repulsion between the nuclei of the atoms, pushing the atoms apart. This repulsion results in higher energy and less stability.
Difference between Bonding and Antibonding Molecular Orbitals:
| Molecular Orbital Theory | |
| Bonding Molecular Orbitals | Anti-Bonding Molecular Orbitals |
| Molecular orbitals formed by the additive effect of the atomic orbitals are called bonding molecular orbitals. | Molecular orbitals formed by the subtractive effect of atomic are called anti-bonding molecular orbitals. |
| The probability of finding the electrons is more in the case of bonding molecular orbitals. | The probability of finding electrons is less in antibonding molecular orbitals. There is also a node between the anti-bonding molecular orbital between two nuclei where the electron density is zero. |
| These are formed by the combination of + and + and – with – part of the electron waves | These are formed by the overlap of + with – part. |
| The electron density in the bonding molecular orbital in the internuclear region is high. As a result, the nuclei are shielded from each other and hence the repulsion is very less. | The electron density in the antibonding molecular orbital in the internuclear region is very low, so the nuclei are directly exposed to each other. Therefore, the nuclei are less shielded from each other. |
| The bonding molecular orbitals are represented by σ, π, δ. | The corresponding anti-bonding molecular orbitals are represented by σ∗ , π∗, δ∗. |
Stabilization and Destabilization Energy:
Paramagnetic and Diamagnetic:
Now, let’s see which category these molecules fall into:
Features of Molecular Orbital Theory:
F. Hund and R. S. Mulliken established the Molecular Orbital Theory (commonly abbreviated to MOT) to describe the structure and behaviour of various molecules at the beginning of the twentieth century. The valence-bond theory failed to explain how certain molecules, such as those in resonance-stabilized compounds, possess two or more analogous bonds with bond orders that fall between those of a single bond against those of a double bond
The molecular orbital function can be used to compute the space in a molecule where the likelihood of finding an electron is highest. The wave behaviour of electrons in a specific molecule is described by molecular orbitals, which are mathematical functions.
By approximating the states of bonded electrons—the molecular orbitals—as linear combinations of atomic orbitals, molecular orbital theory transformed the understanding of chemical bonding (LCAO).
