CHEM104 Lecture Chapter 2 Overview: Valence Electrons

Introduction to Valence Electrons

• Valence electrons are crucial to chemists as they are involved in chemical bonding.
• They are located on the outermost surface of an atom.
• These electrons occupy the highest energy level of the atom.

Understanding Electron Configurations

• Potassium Example:
  • Electron configuration: [Ar] 4s^1
  • Highest energy level: 4
  • Valence electrons: 1
• Calcium Example:
  • Electron configuration: [Ar] 4s^2
  • Highest energy level: 4
  • Valence electrons: 2
• Scandium Example:
  • Electron configuration: [Ar] 4s^2 3d^1
  • Highest energy level: 4 but 3d electrons are not counted as they are from a lower energy level.
  • Valence electrons: 2
• Gallium Example:
  • Electron configuration: [Ar] 4s^2 3d^10 4p^1
  • Highest energy levels: 4s and 4p
  • Valence electrons: 3

Valence Electrons and Periodic Table Structure

• Main group elements (S and P block): Valence electron count often matches group number.
• Using Periodic Table Systems:
  • Old system (e.g., Group 3A for Gallium)
  • New system (e.g., Group 13 for Gallium)
  • Subtract 10 from the new system number to find valence electrons.

Group Characteristics on the Periodic Table

• Alkali Metals & Hydrogen:
  • 1 valence electron
• Alkaline Earth Metals:
  • 2 valence electrons
• Boron Group Elements:
  • 3 valence electrons
• Carbon Group Elements:
  • 4 valence electrons
• Nitrogen Group Elements:
  • 5 valence electrons
• Oxygen Group Elements:
  • 6 valence electrons
• Halogens:
  • 7 valence electrons
• Noble Gases:
  • 8 valence electrons, except Helium which has 2

D and F Block Elements

• Generally have either one or two valence electrons.
• For class purposes, assume two valence electrons.
• Exceptions exist but are not required knowledge for this class.

Summary

• Valence electrons play a critical role in determining an element's chemical properties and reactivity.
• Understanding the electron configuration and periodic table structure helps predict valence electrons.
• This knowledge is essential for understanding chemical bonding and reactions.