Lecture on Valence and Core Electrons

Key Points

• Valence Electrons vs Core Electrons
  • Valence Electrons: Outermost electrons involved in chemical reactions.
  • Core Electrons: Inner electrons, less involved in chemical reactions.

Purpose of Electron Configurations

• Provide insight into how an element is likely to react with other atoms.

Example: Oxygen

• Electron Configuration: 1s² 2s² 2p⁴

  • 8 electrons total: 2 in the first shell, 6 in the second shell.
  • Valence Electrons: 6 (in the 2nd shell).
  • Core Electrons: 2 (in the 1st shell).

• Stability Concept:

  • Atoms are more stable with filled outer shells (typically 8 electrons in s and p subshells).
  • Oxygen needs 2 more electrons to fill its outer shell to make it stable.
  • Often gains electrons from other atoms.

• Lewis Structure of Oxygen:

  • Shows 6 valence electrons as dots around the O symbol.

Example: Calcium

• Electron Configuration: [Ar] 4s²

  • 20 electrons total: Configuration up to Argon, then 2 in the 4th shell.
  • Valence Electrons: 2 (in the 4th shell).
  • Core Electrons: 18 (config. same as Argon).

• Stability Concept:

  • Easier for calcium to lose 2 electrons than gain 6 to fill outer shell.
  • Calcium tends to lose electrons and become ionized (Ca²⁺).

Valence Electrons by Group in Periodic Table

• s and p Block Elements:
  • Group 1: 1 valence electron.
  • Group 2: 2 valence electrons.
  • Group 13: 3 valence electrons.
  • Group 14: 4 valence electrons.
  • Group 15: 5 valence electrons.
  • Group 16: 6 valence electrons.
  • Group 17: 7 valence electrons.
  • Group 18 (Noble Gases): 8 valence electrons (very stable, unreactive).

Summary

• Valence Electrons: Key players in reactions, typically from outermost shell.
• Stability: Elements aim for full valence shells through gaining, losing, or sharing electrons.
• Periodic Table Insight: Valence electron count predicted by group number.