Explain Hund’s rule with an example.

Hund’s rule is a fundamental principle in quantum chemistry that helps us understand how electrons are distributed in atomic orbitals. It states that when electrons occupy degenerate orbitals—those that have the same energy—they will fill each orbital singly before pairing up. This behavior minimizes electron-electron repulsion and leads to a more stable arrangement of electrons. Let’s break this down further with an example.

The Basics of Electron Configuration

To grasp Hund’s rule, it’s essential to know a bit about electron configuration. Electrons are arranged in orbitals around the nucleus of an atom, and these orbitals can hold a maximum of two electrons. When multiple orbitals are available at the same energy level, such as the three p orbitals in the second energy level (2p), Hund’s rule comes into play.

Understanding Degenerate Orbitals

Degenerate orbitals are orbitals that have the same energy. For instance, in the case of the 2p subshell, there are three degenerate orbitals: 2p_x, 2p_y, and 2p_z. According to Hund’s rule, if we have multiple electrons to place in these orbitals, we will first place one electron in each orbital before any orbital gets a second electron.

Example: The Nitrogen Atom

Let’s consider the nitrogen atom, which has an atomic number of 7. This means it has 7 electrons. The electron configuration for nitrogen is 1s² 2s² 2p³. Here’s how we apply Hund’s rule:

• The first two electrons fill the 1s orbital: 1s².
• The next two electrons fill the 2s orbital: 2s².
• Now, we have three electrons left to place in the 2p orbitals.

According to Hund’s rule, we will place one electron in each of the three 2p orbitals (2p_x, 2p_y, and 2p_z) before any of them gets a second electron. This results in the following arrangement:

• 2p_x: ↑
• 2p_y: ↑
• 2p_z: ↑

Each arrow represents an electron with its spin direction. By filling each orbital singly first, nitrogen achieves a more stable configuration due to reduced repulsion between the electrons.

Why It Matters

Hund’s rule is crucial for predicting the chemical properties of elements. The way electrons are arranged influences how atoms interact with each other, which is foundational for understanding chemical bonding and reactivity. For example, elements with unpaired electrons, like nitrogen, tend to be more reactive than those with fully paired electrons.

In summary, Hund’s rule is a key concept in electron configuration that helps explain the stability of atoms and their behavior in chemical reactions. By ensuring that electrons occupy degenerate orbitals singly before pairing, we can predict and understand the properties of various elements in the periodic table.