Electron Configuration: Aufbau Principle, Hund's Rule, and Pauli Exclusion Principle

Understanding Electron Configuration

Electron configuration describes the arrangement of electrons in an atom's orbitals. This arrangement follows specific rules and principles to ensure electrons are placed in the most stable configuration possible.

Key Principles

• Aufbau Principle: Electrons fill orbitals starting from the lowest energy level to the highest. Think of it as filling seats in a theater from the front row to the back.
• Hund's Rule: Electrons will fill an unoccupied orbital before they pair up in an occupied one. This is to maximize the number of unpaired electrons, which leads to greater stability.
• Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers. In simple terms, an orbital can hold a maximum of two electrons with opposite spins.

Sub-shells: s, p, d

Electrons are arranged in sub-shells, which are designated as s, p, and d:

• s-sub-shell: Can hold up to 2 electrons.
• p-sub-shell: Can hold up to 6 electrons.
• d-sub-shell: Can hold up to 10 electrons.

Electron Configuration Notation

Electron configuration is written using the sub-shell notation, indicating the number of electrons in each sub-shell. For example, the electron configuration of oxygen is written as:

$$1s^2 2s^2 2p^4$$

This means oxygen has 2 electrons in the 1s sub-shell, 2 electrons in the 2s sub-shell, and 4 electrons in the 2p sub-shell.

Worked Example