What is Hund's Rule in Chemistry?

📚 What is Hund's Rule?

Hund's Rule, named after German physicist Friedrich Hund, governs how electrons fill orbitals within a subshell. Simply put, it states that electrons will individually occupy each orbital within a subshell before doubling up in any one orbital. Furthermore, these unpaired electrons will have the same spin (either all spin up or all spin down) to maximize the total spin.

📜 History and Background

Friedrich Hund introduced this rule in 1925, as part of his work on atomic spectroscopy. It's rooted in quantum mechanics and helps predict the ground state electron configuration of atoms and molecules. Hund's Rule is essential for understanding chemical bonding, magnetic properties, and a variety of other chemical phenomena.

🔑 Key Principles of Hund's Rule

• ⚛️ Maximize Multiplicity: The term with the greatest spin multiplicity (number of unpaired electrons + 1) will have the lowest energy.
• orbital before any orbital is doubly occupied.
• ⬆️⬇️ Maximize Total Spin: If two or more orbitals have the same energy (degenerate orbitals), electrons will individually occupy them with parallel spins before pairing up. This minimizes electron-electron repulsion.

🧪 Real-World Examples

Let's look at some examples to solidify your understanding:

Example 1: Nitrogen (N)

Nitrogen has an electron configuration of $1s^22s^22p^3$. The $2p$ subshell has three orbitals ($2p_x$, $2p_y$, $2p_z$). According to Hund's Rule, each $2p$ orbital will first get one electron before any orbital gets a second electron. Therefore, the electron configuration looks like this:

• ✨ $2p_x$: ↑
• 💫 $2p_y$: ↑
• 🌟 $2p_z$: ↑

Example 2: Oxygen (O)

Oxygen has an electron configuration of $1s^22s^22p^4$. The first three electrons will fill the $2p$ orbitals individually like nitrogen. The fourth electron will then pair up with one of the existing electrons:

• 💡 $2p_x$: ↑↓
• 🔥 $2p_y$: ↑
• ⚡ $2p_z$: ↑

🧲 Hund's Rule and Magnetism

Hund's Rule also helps explain the magnetic properties of atoms and ions. Substances with unpaired electrons are paramagnetic (attracted to magnetic fields), while those with all paired electrons are diamagnetic (repelled by magnetic fields). The more unpaired electrons, the stronger the paramagnetic effect.

📝 Conclusion

Hund's Rule is a cornerstone of understanding electron configurations and the behavior of atoms and molecules. By understanding and applying Hund's Rule, you can predict the ground state electron configuration, magnetic properties, and chemical behavior of elements and compounds.