Antibonding Molecular Orbital: Definition & Overview

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

In this lesson, we will be learning about what an anti-bonding orbital is in terms of molecular orbital theory and getting a brief overview of its role in molecular systems.

The Ties that Bind

Have you ever wondered what actually holds two atoms together in a molecule? What if we took two marbles and connected them with a short piece of rope? They would be held together by the rope that's tied to each marble. It's kind of the same for atoms that are 'tied' together, only they're obviously not held together by rope. Atoms are held together by chemical bonds. But what does that actually mean? What goes into making a chemical bond? Although the answer to that question is not necessarily trivial, there are some simple ways we can look at chemical bonding so that we can all have a basic understanding of what's going on.

One of the most popular ways to think about chemical bonding is known as molecular orbital theory. An important part of molecular orbital theory is a concept called an anti-bonding molecular orbital, and that is actually the main point of discussion in our lesson. We will first get a brief review of molecular orbital theory and then introduce what an anti-bonding molecular orbital is and its consequences on molecular systems.

Molecular Orbital Theory Crash Course

Molecular orbital (MO) theory is a method to describe chemical bonding that uses mathematics to explore the consequences of atomic orbital overlap. One thing that's very important to understand is the difference between atomic orbitals and molecular orbitals. Although both kinds of orbitals are regions of space where electrons reside, an atomic orbital is a region of space associated with a single atom and a molecular orbital is a region of space that is attributed to the molecule as a whole.

When atomic orbitals are mathematically combined to produce molecular orbitals, the result of constructive interference of the two original atomic orbitals gives us a bonding MO that is lower in energy. The result of destructive interference of the two original atomic orbitals gives us an anti-bonding MO that is always higher in energy than the bonding MO. Since nature likes low energy 'stuff', electrons would always prefer to occupy the bonding MO since it's of lower energy.

MO theory treats electron orbitals as waves. When we're talking about constructive interference, that's when two waves come together and when they interact with one another, they combine in a productive fashion to give an overall wave that's taller than the original two waves.

Example of constructive interference

When two waves come together and interact with one another and they cancel one another, the result is called destructive interference.

Example of destructive interference

When Electrons Go Where They Shouldn't

Let's look at the MO diagram for the hydrogen molecule. When the two individual hydrogen atoms come together to form the H2 molecule, each atom brings one electron to contribute towards the bond. Notice that both electrons go into the bonding MO. Why would this be the case? It's because that's the lowest energy scenario! Electrons would always prefer to be in the lowest possible energy state, which is the bonding MO in this case.

Molecular orbital diagram for the H 2 molecule

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