Understanding Molecular Orbitals and Magnetism

Molecular orbitals, or MOs, are important for understanding how different substances behave, especially when it comes to magnetism.

In Year 12 Chemistry, when we learn about Molecular Orbital Theory, we discover that these orbitals play a big role in deciding if a compound will be magnetic or not. We can classify compounds as either diamagnetic or paramagnetic based on their properties.

What Are Molecular Orbitals?

First, let’s break down what molecular orbitals are.

MOs are created when atomic orbitals from different atoms combine to form molecules. There are two types of MOs:

• Bonding orbitals help stabilize the molecule.
• Antibonding orbitals can make the molecule less stable.

Electrons fill these orbitals following specific rules like:

• Aufbau principle: Electrons fill the lowest energy levels first.
• Hund’s rule: Electrons will occupy separate orbitals before pairing up.
• Pauli exclusion principle: No two electrons can have the same set of quantum numbers.

How Electrons Affect Magnetism

The way electrons are arranged in molecular orbitals determines whether a substance is magnetic.

1. Diamagnetic Substances:

   • These compounds have all their electrons paired. This means there is no overall magnetic effect.
   • Examples include noble gases and some molecules like oxygen ($O_2$) when all electrons are paired.
   • If every electron in a molecule is either in bonding orbitals or paired in antibonding orbitals, the substance will be diamagnetic.

2. Paramagnetic Substances:

   • In contrast, paramagnetic substances have unpaired electrons. These unpaired electrons create a magnetic effect, making the substance attracted to magnets.
   • A well-known example is molecular oxygen ($O_2$), which has two unpaired electrons in its highest occupied molecular orbital (HOMO).
   • For oxygen ($O_2$), the electrons fill the orbitals like this:
     • $\sigma(1s)^2$,
     • $\sigma^*(1s)^2$,
     • $\sigma(2s)^2$,
     • $\sigma^*(2s)^2$,
     • $\sigma(2p_z)^2$,
     • $\pi(2p_x)^2$,
     • $\pi(2p_y)^1$,
     • $\pi^*(2p_x)^1$.
   • The two unpaired electrons in the $\pi^*(2p_x)$ and $\pi^*(2p_y)$ orbitals make it paramagnetic.

The Importance of Energy Levels

The energy levels of MOs are also very important. When we look at MOs, we can see that the energy of bonding orbitals is lower than that of antibonding orbitals.

This difference helps us understand how electrons will fill these orbitals.

• If a compound has unpaired electrons in orbitals that have the same energy (called degenerate orbitals), the electrons will fill separate orbitals before pairing up, following Hund’s rule.

Conclusion

To sum it up, molecular orbitals act like a map for predicting how a substance will behave in a magnetic field.

By looking at how the electrons are arranged in these orbitals, we can easily decide if a substance is diamagnetic or paramagnetic.

This knowledge is not just for fun; it helps us understand why some substances attract magnets while others do not.

In my own studies, it’s been incredible to see how the arrangement of electrons in molecular orbitals connects to the physical properties we observe in different substances. This connection really highlights the beauty of chemistry and the interactions between atoms!