Science Class 11 - Classification Of-Elements-And-Periodicity-In-Properties Notes

Comprehensive study notes for Class 11 - Classification Of-Elements-And-Periodicity-In-Properties olympiad preparation

Classification of Elements and Periodicity in Properties

Welcome to the chapter on Classification of Elements and Periodicity in Properties for Class 11. In this chapter, you will learn how elements are classified, the development of the periodic table, and how the properties of elements show periodic trends. By the end of this chapter, you will understand the basis of the modern periodic table and the trends in properties of elements.

Introduction

There are more than 100 elements known today. To study them easily, scientists have classified elements based on their properties. The periodic table is a systematic arrangement of elements that shows repeating patterns, or periodicity, in their properties.

Early Attempts at Classification

  • Dobereiner’s Triads: Groups of three elements with similar properties. The atomic mass of the middle element was nearly the average of the other two.
  • Newlands’ Law of Octaves: Every eighth element had properties similar to the first, like musical notes.
  • Mendeleev’s Periodic Table: Arranged elements in order of increasing atomic mass. Left gaps for undiscovered elements.

Modern Periodic Table

  • Arranged by increasing atomic number (number of protons).
  • Elements with similar properties are placed in the same group (vertical column).
  • Periods are horizontal rows in the periodic table.

Periodic Law

The modern periodic law states: The physical and chemical properties of elements are periodic functions of their atomic numbers.

Periodic Trends in Properties

  • Atomic Radius: Decreases across a period, increases down a group.
  • Ionization Enthalpy: Increases across a period, decreases down a group.
  • Electron Gain Enthalpy: Becomes more negative across a period.
  • Electronegativity: Increases across a period, decreases down a group.
  • Valency: Shows periodicity based on group number.

Applications of the Periodic Table

  • Predicting properties of elements.
  • Understanding chemical reactivity and trends.
  • Classifying new elements.

Practice Questions

  1. What is the basis of classification in the modern periodic table?
  2. State the modern periodic law.
  3. How does atomic radius change across a period?
  4. Name two elements that belong to the same group and have similar properties.
  5. Why did Mendeleev leave gaps in his periodic table?

Challenge Yourself

  • Draw the first 20 elements in the modern periodic table and mark the groups and periods.
  • Explain why fluorine is more electronegative than lithium using periodic trends.

Did You Know?

  • The periodic table is sometimes called the "chemist’s calendar".
  • Mendeleev predicted the properties of elements that were not discovered in his time!

Glossary

  • Atomic Number: Number of protons in an atom.
  • Periodicity: Repeating patterns in properties of elements.
  • Group: Vertical column in the periodic table.
  • Period: Horizontal row in the periodic table.

Answers to Practice Questions

  1. Increasing atomic number.
  2. The physical and chemical properties of elements are periodic functions of their atomic numbers.
  3. It decreases from left to right across a period.
  4. Sodium and potassium (Group 1); chlorine and bromine (Group 17).
  5. To accommodate elements that were not yet discovered but whose properties he predicted.

Understanding the periodic table helps you predict the behavior of elements and their compounds!