Summary of Class 10 Science Chapter 5: Periodic Classification of Elements

The discovery and classification of elements revolutionized the understanding of chemistry. Chapter 5 discusses how elements are organized in the Periodic Table and their properties.

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Why Classify Elements?

As the number of elements discovered increased, scientists felt the need to organize them systematically to study their properties efficiently. The classification led to the development of the Periodic Table, which is the backbone of modern chemistry.

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Early Attempts at Classification

1. Dobereiner’s Triads (1817)
   Elements with similar properties were grouped into sets of three. The atomic mass of the middle element was the average of the other two.

   • Example: Lithium ($Li$), Sodium ($Na$), Potassium ($K$).

   Limitation: Could not include all elements.

   
   
   
   
   (Caption: Dobereiner’s triads were an early attempt to classify elements.)

2. Newlands’ Law of Octaves (1866)
   Elements were arranged in increasing atomic mass, and every eighth element had properties similar to the first.

   • Limitation: Worked only for lighter elements.

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The Modern Periodic Table

1. Mendeleev’s Periodic Table (1869)
   Mendeleev arranged elements in increasing atomic mass and left gaps for undiscovered elements, predicting their properties.

   • Example: Gallium and Germanium were discovered later, validating his predictions.
   • Limitation: Some elements were placed out of order to fit the periodicity of properties.
   
   
   
   
   (Caption: Mendeleev’s periodic table laid the foundation for modern classification.)

2. Modern Periodic Table
   The current table is based on the atomic number rather than atomic mass. It organizes elements into Groups (Vertical Columns) and Periods (Horizontal Rows).

   Features:

   • Groups: Indicate similar chemical properties.
   • Periods: Show a gradation in properties.

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Trends in the Periodic Table

1. Atomic Size

   • Across a Period: Decreases due to increasing nuclear charge.
   • Down a Group: Increases due to the addition of new shells.

2. Metallic and Non-Metallic Properties

   • Metals: Found on the left, lose electrons to form positive ions.
   • Non-Metals: Found on the right, gain electrons to form negative ions.
   
   
   
   
   (Caption: Metallic and non-metallic properties show distinct trends in the periodic table.)

3. Valency

   • Determined by the number of electrons in the outermost shell.
   • Remains the same down a group but changes across a period.

4. Reactivity

   • Metals: Reactivity increases down a group and decreases across a period.
   • Non-Metals: Reactivity decreases down a group and increases across a period.

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Significance of the Periodic Table

1. Systematic Study: The table organizes elements for easier study.
2. Prediction: Properties of undiscovered elements can be predicted.
3. Relationship Understanding: Shows relationships between various elements and their properties.

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Conclusion

The Periodic Table is a masterpiece of scientific organization. It bridges the gap between known and unknown elements, helping scientists understand the properties of matter more deeply.

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Did You Know?

• Hydrogen is placed separately in the periodic table because it shows properties of both metals and non-metals.
• The heaviest element currently known is Oganesson (Og), with an atomic number of 118.

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Image Sources

• Triads table and Metallic trends: Unsplash, Pixabay.
• Mendeleev’s Table: Pexels.

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