Chemistry Lecture Notes: Classification of Elements and Periodicity in Properties

Overview

• Covered basic concepts from Chapters 1 & 2 in Class 11: Basic Concepts of Chemistry & Structure of Atom.
• Importance of concept clarity for competitive exams (e.g. NEET, JEE).
• Chapter focus: Classification of Elements and Periodicity in Properties (Chapter 3).
• Understanding this chapter is crucial for grasping future chapters in Class 11 & 12.

Key Learning Points

1. Elements Overview

• Approximately 120 elements known today:
  • 118 or 116 elements shown in some textbooks.
  • Includes both naturally occurring and artificially made elements.

2. The Modern Periodic Table

• Arrangement of elements in a systematic way:
  • Convenience in studying chemistry through group properties.
• Essential to understand the history of periodic tables before the modern version.

3. Historical Periodic Tables

• Focus on four major historical periodic tables:
  1. Dobereiner's Law of Triads
  2. Newlands' Law of Octaves
  3. Mendeleev's Periodic Table
  4. Lothar Meyer's Periodic Table

4. Dobereiner's Law of Triads

• Triads: Groups of three elements.
• Arranged by atomic mass.
• Key observation: The atomic mass of the middle element = average of the first and third elements.
• Examples:
  • Triad 1: Lithium (7), Sodium (23), Potassium (39)
  • Triad 2: Calcium, Strontium, Barium
• Limited to about 16 known elements at the time.

5. Newlands' Law of Octaves

• Arranged elements by increasing atomic mass.
• Observed that every eighth element is similar to the first.
• Example Sequence: Hydrogen, Lithium, Beryllium, etc.
• Limitations:
  • Principle applicable only up to Calcium.
  • Placed two elements in a single slot; no justification for this.
  • Did not leave gaps for newly discovered elements.

6. Lothar Meyer's Periodic Table

• Arranged elements based on a graph of atomic volume vs atomic mass.
• Key observations:
  • Alkali metals at the peaks.
  • Alkaline earth metals on a descending curve.
  • Halogens on an ascending curve.
  • Transition elements at the bottom peak.
• Limitations:
  • Similarity of properties cannot be solely determined by position on the graph.

7. Mendeleev's Periodic Table

• Arranged by atomic mass and introduced the concept of groups (vertical columns) and periods (horizontal rows).
• Mendeleev's Periodic Law:
  • Physical and chemical properties of elements depend on their atomic masses.
• Left gaps for undiscovered elements (e.g., Eka-boron for germanium).
• Limitations:
  • Uncertain position of Hydrogen.
  • No position for isotopes.
  • Misplacement of elements (e.g., Tellurium and Iodine).

Conclusion

• Understanding the historical context of periodic tables is essential for grasping the modern table.
• Next Lesson: Detailed overview of the modern periodic table and its classifications.

Study Tips

• Focus on understanding the patterns and principles behind each historical table.
• Revise the properties and examples of elements in triads and octaves.
• Familiarize with Mendeleev's contributions and limitations for better comprehension of modern concepts.

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