Lecture Notes: Determining Valence Electrons in Main Group Elements

Introduction

• Focus on determining valence electrons for elements in the main groups.
• Main groups are classified using the one through eight system.
• Groups 3 through 12 are ignored in this context.
• Valence electrons are those in the outermost shell.

Determining Valence Electrons

• Valence electrons can be determined by the group number for main group elements.

Sodium Example

• Electron Configuration for Sodium:
  • 1s² 2s² 2p⁶ 3s¹
• Valence Electrons in Sodium:
  • Sodium has one valence electron in the third energy level.
  • Sodium is in group 1, which corresponds to its one valence electron.

Chlorine Example

• Electron Configuration for Chlorine:
  • 1s² 2s² 2p⁶ 3s² 3p⁵
• Valence Electrons in Chlorine:
  • Chlorine has seven valence electrons in the third energy level.
  • Chlorine is in group 7, which corresponds to its seven valence electrons.

Reaction of Sodium and Chlorine

• Sodium loses one electron:
  • Becomes a sodium cation (Na⁺) with a stable electron configuration like neon.
• Chlorine gains one electron:
  • Becomes a chloride anion (Cl⁻) with a stable electron configuration like argon.
• Ionic Bond Formation:
  • Attraction between Na⁺ and Cl⁻ forms an ionic bond.

Reactivity of Groups

• Group 1 (Alkali Metals):
  • Highly reactive due to having one valence electron.
• Group 7 (Halogens):
  • Highly reactive as gaining one electron gives them a noble gas configuration.
• General Rule:
  • Group number equals number of valence electrons for main group elements.

Additional Example: Oxygen

• Oxygen Group:
  • Group 6, so oxygen has six valence electrons.

Conclusion

• Understanding electron configurations and valence electrons explains the reactivity of elements.
• Main group elements' reactivity is linked to their desire to achieve stable electron configurations similar to noble gases.