A Level Chemistry Revision "Electron Configuration 2"

Introduction

This tutorial provides a step-by-step guide to understanding electron configuration, focusing specifically on the arrangement of electrons in atomic orbitals. It is particularly relevant for students preparing for A Level Chemistry, as it covers the nuances of electron configuration, including the handling of 3d and 4s subshells, as well as exceptions involving chromium and copper.

Step 1: Understand the Basics of Electron Configuration

• Electron configuration describes how electrons are distributed among the various atomic orbitals.
• Orbitals are organized by energy levels and are filled according to the Aufbau principle, which states that electrons occupy the lowest energy orbitals first.

Step 2: Follow the Order of Filling Orbitals

• Use the diagonal rule to determine the order in which orbitals are filled. The typical order is:
  1. 1s
  2. 2s
  3. 2p
  4. 3s
  5. 3p
  6. 4s
  7. 3d
  8. 4p
• Start filling from the 1s orbital and move through the sequence until the electron count reaches the total number of electrons in the atom.

Step 3: Pay Attention to 4s and 3d Subshells

• The 4s subshell is filled before the 3d subshell due to its lower energy level when these orbitals are empty.
• Remember that after filling the 4s subshell, the 3d subshell is filled next.

Step 4: Recognize Exceptions to the Rule

• Some elements, specifically chromium (Cr) and copper (Cu), exhibit exceptions in their electron configurations:
  • Chromium: Instead of the expected [Ar] 4s² 3d⁴, it is actually [Ar] 4s¹ 3d⁵. This occurs to achieve a more stable half-filled d subshell.
  • Copper: Instead of [Ar] 4s² 3d⁹, it is [Ar] 4s¹ 3d¹⁰. This configuration provides stability with a fully filled d subshell.
• Always verify these exceptions as they are critical for accurately predicting chemical behavior.

Step 5: Practice Writing Electron Configurations

• Start with simple elements and gradually work up to more complex ones.
• Use the periodic table to determine the total number of electrons in an element and apply the filling order.
• Example:
  • Oxygen (O, atomic number 8):
    • Configuration: 1s² 2s² 2p⁴
  • Iron (Fe, atomic number 26):
    • Configuration: [Ar] 4s² 3d⁶

Conclusion

Understanding electron configuration is essential for mastering concepts in chemistry. By following the filling order of orbitals and recognizing exceptions like chromium and copper, you can accurately determine the electron arrangements of elements. Practice writing configurations for various elements to enhance your skills. As a next step, explore related topics such as ionization energy and electron affinity, which build upon the principles of electron configuration.