1.3 Atomic Structure: Electron Configurations

Introduction

• The ground-state electron configuration of an atom describes the orbitals occupied by its electrons.
• The configuration can be predicted using three key rules.

Rules for Electron Configuration

Rule 1: Aufbau Principle

• Electrons fill the lowest-energy orbitals first.
• Order of filling: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, etc.
• The 4s orbital is filled before the 3d orbital.

Rule 2: Pauli Exclusion Principle

• Only two electrons can occupy an orbital and they must have opposite spins (up and down).

Rule 3: Hund's Rule

• Electrons occupy empty orbitals of equal energy with parallel spins before pairing up.

Examples of Electron Configurations

• Hydrogen (H): 1 electron, configuration: 1s¹
• Carbon (C): 6 electrons, configuration: 1s² 2s² 2pₓ¹ 2pᵧ¹
• Phosphorus (P): 15 electrons, configuration includes paired electrons in 1s, 2s, 2p, and 3s; three unpaired electrons in 3p.

Problem Examples

• Problem 1-1: Find the ground-state electron configuration for:

  • Oxygen
  • Nitrogen
  • Sulfur

• Problem 1-2: Determine the number of electrons in the outermost shell of:

  • Magnesium
  • Cobalt
  • Selenium

Conclusion

• Understanding electron configurations is essential for determining how atoms interact in chemical reactions.
• The rules provide a systematic way to predict the arrangement of electrons in atoms.