A Level Chemistry Revision "Electron Configuration"

Introduction

This tutorial focuses on electron configuration, a fundamental concept in A Level Chemistry. Understanding how electrons are assigned to atomic orbitals is crucial for grasping chemical behavior and bonding. This guide outlines the three key rules for electron configuration and provides examples to solidify your understanding.

Step 1: Understand the Three Key Rules

To correctly assign electrons to atomic orbitals, familiarize yourself with the following rules:

1. Aufbau Principle

   • Electrons occupy the lowest energy orbitals first before filling higher energy levels.
   • Visualize this as building a structure from the ground up.

2. Pauli Exclusion Principle

   • No two electrons in an atom can have the same set of four quantum numbers.
   • This means each orbital can hold a maximum of two electrons with opposite spins.

3. Hund’s Rule

   • Electrons will fill degenerate orbitals (orbitals of the same energy) singly before pairing up.
   • This helps to minimize electron-electron repulsion and stabilizes the atom.

Step 2: Assign Electrons to Orbitals

Now that you understand the rules, follow these steps to assign electrons:

1. Identify the Atomic Number

   • The atomic number of an element indicates the number of electrons.

2. Use the Aufbau Diagram

   • Refer to an Aufbau diagram to see the order of orbital filling:
     • 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.

3. Follow the Three Rules

   • Begin filling the orbitals according to the Aufbau principle.
   • Ensure that you respect the Pauli Exclusion Principle.
   • Apply Hund’s Rule when filling p, d, and f orbitals.

Example 1: Electron Configuration of Oxygen

1. Identify Atomic Number: Oxygen has an atomic number of 8.
2. Fill Orbitals:
   • 1s² (2 electrons)
   • 2s² (2 electrons)
   • 2p⁴ (4 electrons)
3. Final Configuration: The electron configuration is 1s² 2s² 2p⁴.

Example 2: Electron Configuration of Sodium

1. Identify Atomic Number: Sodium has an atomic number of 11.
2. Fill Orbitals:
   • 1s² (2 electrons)
   • 2s² (2 electrons)
   • 2p⁶ (6 electrons)
   • 3s¹ (1 electron)
3. Final Configuration: The electron configuration is 1s² 2s² 2p⁶ 3s¹.

Step 3: Practice and Assess Understanding

To reinforce your learning, try assigning the electron configuration for a different element.

Example Task: Electron Configuration of Chlorine

1. Identify Atomic Number: Chlorine has an atomic number of 17.
2. Fill Orbitals and Write Configuration:
   • 1s²
   • 2s²
   • 2p⁶
   • 3s²
   • 3p⁵
3. Final Configuration: The electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁵.

Conclusion

Understanding electron configuration is essential for mastering various concepts in chemistry. By following the three key rules and practicing with different elements, you can develop a strong grasp of this topic. As a next step, consider exploring how electron configurations influence chemical properties and bonding in more complex molecules.