Lecture Notes: The Development of the Periodic Table

Introduction

• Objective: Understand early attempts to arrange elements in a periodic table.
• Key Figures: Johan Döbereiner, John Newlands, Dimitri Mendeleev.
• Modern Periodic Table Features: Groups, transition elements, atomic numbers.

The Modern Periodic Table

• Structure:
  • Arranged into columns called groups (e.g., Group 1, Group 2, Transition Elements, Groups 3-7, Group 0).
  • Elements with similar properties occur at regular intervals.
• Examples:
  • Group 1: Highly reactive metals (e.g., lithium, sodium, potassium).
  • Group 7: Highly reactive nonmetals (e.g., fluorine, chlorine, bromine).
• Reason for Similar Properties:
  • Elements in a group have the same number of electrons in their outer energy level.

Early Attempts to Order Elements

• Johan Döbereiner's Triads:
  • Elements with similar properties often occurred in groups of three (e.g., lithium, sodium, potassium; chlorine, bromine, iodine).
• John Newlands' Law of Octaves:
  • Arranged elements by increasing atomic weight.
  • Every eighth element had similar reactivity.
  • Problems: Sticking to atomic weight led to grouping elements with different properties; not widely accepted.

Dimitri Mendeleev’s Periodic Table

• Innovations:
  • Arranged elements by increasing atomic weight.
  • Switched order to fit patterns in groups.
  • Left gaps for undiscovered elements, predicting their properties.
• Impact:
  • Predictions confirmed with the discovery of missing elements; gained acceptance.

Differences in the Modern Periodic Table

• Ordering:
  • Modern: By atomic number (number of protons).
  • Mendeleev: By atomic weight (protons not yet discovered).
  • Problem with atomic weight: Isotopes can misplace elements in groups.
• Example: Thorium and iodine misplacement corrected by atomic number ordering.
• Group Zero:
  • Noble gases added (not fully discovered during Mendeleev’s time).

Additional Resources

• Revision Workbook: Available for further practice and questions.

This summary captures the development of the periodic table and highlights the contributions of significant scientists in making the periodic table a useful tool in chemistry.